Endocrinology Basic and Clinical Principles - part 10 ppsx

The granulosa cells andtheca cells of the selected follicle show a high rate of cell Fig.. Because estradiol is a key mediator of follicle selection and theca cell steroidogenesis is ess

Chapter 26 / Endocrinology of the Ovary 393

mRNA and protein synthesis in the oocyte, and it begins

to increase in size

The signal for follicle recruitment is unknown It is

known that recruitment can occur in

hypophysecto-mized animals, indicating that recruitment is not

depen-dent on luteinizing hormone (LH) or FSH There is

evidence that the rate of recruitment can be modulated

by intraovarian and environmental factors The rate of

recruitment is related to the total number of primordial

follicles in the ovaries, indicating that intraovarian

mechanisms are important for regulating recruitment

Evidence from experiments in rodents indicates that

recruitment can be attenuated by neonatal thymectomy,

starvation, or administration of exogenous opioid

pep-tides, suggesting that there may be endocrine signals

capable of modulating the rate of recruitment

2.2 Selection of Dominant Follicle

The selection of the dominant follicle is one of the

final steps in the year-long program of follicle

develop-ment In women, the follicle that will ovulate is selected

in the early follicular phase of the menstrual cycle At

that time, each ovary contains a cohort of rapidly

grow-ing follicles 2–5 mm in diameter These small antral

follicles contain a fully grown oocyte, approx 1 million

granulosa cells, and several layers of theca cells From

this cohort, the follicle most advanced in the

develop-mental program is selected to become dominant Once

it reaches a size of 6–8 mm in the early follicular phase,changes occur, possibly in the structure of the basallamina, that permit FSH to enter the follicle and begin

to stimulate the granulosa cells The granulosa cells andtheca cells of the selected follicle show a high rate of cell

Fig 2 Morphology of ovarian follicle.

Fig 3 Ultrastructure of ovarian steroid-secreting cells The

spe-cialized ultrastructure of steroid-producing cells includes chondria (M) with vesicular cristae, abundant agranular ER, and numerous lipid vesicles (L) containing cholesteryl esters N = nucleus (magnification: ×21,000)

mito-394 Part IV / Hypothalamic–Pituitary

proliferation, whereas mitosis stops in the cells of other

cohort follicles The ability to sustain a high capacity for

rapid cell division is a characteristic feature seen only in

dominant follicles The smaller follicles in the cohort

with slower growth inevitably undergo atresia

When biologically active FSH first enters the

fol-licle at about 6 to 7 mm, the granulosa cells begin to

express the aromatase enzyme and to secrete estradiol

In addition, the granulosa cells begin to secrete

increas-ing amounts of inhibin B Together, these hormones

cause a small but significant and progressive

decre-ment in the circulating FSH concentration owing to

their inhibitory effects on pituitary secretion The lack

of FSH support to the cohort follicles causes

develop-mental failure and certain atresia Counteracting the

FSH withdrawal by administration of exogenous FSH

is the basis for ovulation induction protocols that are

used clinically to develop multiple preovulatory

fol-licles for assisted reproduction techniques

In contrast to the cohort follicles, the dominant

fol-licle preferentially sequesters FSH in the follicular

fluid, thus enabling it to maintain adequate FSH

sup-port even though circulating FSH concentrations

decline Another important mechanism that confers a

developmental advantage to the dominant follicle is

sensitization of the follicle cells to FSH The granulosa

cells of the dominant follicle produce growth and

dif-ferentiation factors, such as insulin-like growth factors

(IGFs) and inhibin, that augment the stimulatory

effects of FSH By virtue of these mechanisms, the

dominant follicle can continue to grow and thrive

while the cohort follicles die By simply changing

the concentration of FSH during the follicular phase

of the cycle, the number of preovulatory follicles can

be determined

The theca cells do not respond to FSH but are

regu-lated by LH The mean circulating concentrations of

LH do not change appreciably during the follicular

phase of the menstrual cycle At the time theca cells

first appear in secondary follicles, they have

ste-roidogenic capacity, but the stimulatory effects of

LH are attenuated by granulosa cell–secreted factors

Because estradiol is a key mediator of follicle selection

and theca cell steroidogenesis is essential for the

fol-licle to secrete estradiol, it is important for thecal

ste-roidogenesis to increase in dominant follicles It is

likely that the same factors that sensitize the granulosa

cells to FSH also augment the stimulatory effects of LH

on theca cell steroidogenesis Thus, theca cell

steroido-genesis is enhanced only when the granulosa cells have

expressed the aromatase enzyme In women, the

capac-ity to secrete large amounts of estrogen is the exclusive

property of dominant follicles

2.3 Atresia

Greater than 99% of the follicles present in the ries die by atresia Atresia occurs in both preantral andantral follicles and is not exclusively related to the fail-ure of a follicle to become dominant Indeed, approx95% of the follicles become atretic prior to the firstovulation

ova-The process of follicle atresia occurs by apoptosis.The granulosa cells undergo nuclear and cytoplasmiccondensation, plasma membrane blebbing, and therelease of apoptotic bodies containing cellular orga-nelles The nuclear DNA undergoes internucleosomalcleavage, and the cellular fragments are removed fromthe ovary by phagocytosis It is clear that removal ofFSH support from follicles in the gonadotropin-depen-dent stages of follicle development will trigger atresia,but the causes of apoptosis in preantral follicles are lesscertain

3 STEROID HORMONE PRODUCTION 3.1 Two-Cell, Two-Gonadotropin Concept

of Follicle Estrogen Production

The production of large quantities of estradiol is one

of the most important endocrine functions of the nant follicle It is through estradiol concentrations thatthe state of follicle development is communicated to thehypothalamus and pituitary such that the midcycle ovu-latory surge of LH is timed appropriately Another keyfunction of estradiol is to prepare the endometrium forimplantation of the embryo

domi-Experiments conducted during the 1950s strated that both the theca interna and the granulosa com-partments of the ovarian follicle are required forestradiol production In addition, both LH and FSHstimulation are required for estradiol production tooccur These observations have been confirmed manytimes in a variety of mammalian species, and the molec-ular basis for the two-cell, two-gonadotropin conceptfor follicle estrogen biosynthesis has been established(Fig 4)

demon-From the time the theca cells first differentiate intoendocrine cells, they contain LH receptors and the keysteroidogenic enzymes required for androgen biosyn-thesis from cholesterol: cholesterol side-chain cleav-age cytochrome P450 (CYP11A), 3β-hydroxysteroiddehydrogenase (3β-HSD), and 17α-hydroxylase/C17–

20 lyase cytochrome P450 (CYP17) Thus, the thecacells are endowed with the capacity to synthesize

androgens from cholesterol de novo under the control

of LH Although the principal androgen secreted bythe theca cells is androstenedione, the human CYP17enzyme is extremely inefficient at converting 17β-

Chapter 26 / Endocrinology of the Ovary 395

hydroxyprogesterone to androstenedione Consequently,

steroidogenesis proceeds via the delta 5 pathway, where

17β-hydroxypregnenolone is metabolized into

dehydro-epiandrosterone (DHEA) by the CYP17 enzyme, and

then DHEA is converted into androstenedione by the

3β-HSD The theca cells in women do not express

aromatase CYP19 and, hence, cannot produce

estra-diol In certain species, notably the horse and pig,

theca cells do express low levels of CYP19 and can

produce small amounts of estrogen; however,

coop-eration with the granulosa cells is still required to

secrete high concentrations of estradiol

In contrast to the theca cells, the granulosa cells are

incapable of de novo steroidogenesis in the follicular

phase of the menstrual cycle It is not until the

peri-ovulatory period that the granulosa cells express LH

receptors and CYP11A as they begin to luteinize

There-fore, in the follicular phase of the cycle, the granulosa

cells cannot produce the androgen substrate required

by the CYP19 enzyme When a follicle is selected to

become dominant, the granulosa cells express high

lev-els of CYP19 and 17β-hydroxysteroid dehydrogenase

(17β-HSD) under the control of FSH This enables the

granulosa cells to metabolize the androstenedione

pro-duced by the theca cells to estradiol Thus, it takes twocells, theca and granulosa, and two gonadotropins, LHand FSH, for the ovarian follicle to produce estradiol

3.2 Intracellular Compartmentalization

of Steroidogenic Enzymes

The regulation of steroid hormone production occurs

in two ways Acute regulation of the rate of esis takes place by controlling the rate of cholesterolaccess to the CYP11A enzyme This is possible becausethe CYP11A is localized in the inner leaflet of the innermitochondrial membrane (Fig 5) Because cholesterol

steroidogen-is sparingly soluble in water, diffusion from the outer tothe inner mitochondrial membrane is very slow Acutestimulation with LH causes production of the steroido-genesis acute regulatory protein (StAR) that facilitatesthe transport of cholesterol across the mitochondrialmembranes StAR is thought to function by bringing theouter and inner mitochondrial membranes into contact

at focal points, thus facilitating the movement of terol from the outer to the inner membrane The activity

choles-of the StAR protein is rapidly terminated by proteolyticcleavage When cholesterol is present in the inner mito-chondrial membrane, the CYP11A enzyme readilymetabolizes it to pregnenolone Pregnenolone is able todiffuse out of the mitochondria, where it is metabolized

to other steroids that, in the ovary, are localized in themicrosomes

3.3 Hormonal Regulation

of Cellular Differentiation

The second means for regulating steroid hormonebiosynthesis is to control cellular differentiation by

Fig 4 Two-cell, two-gonadotropin concept of follicle estrogen

production LH stimulates the theca cells to differentiate and

produce androstenedione from cholesterol FSH stimulates the

differentiation of the granulosa cells The androstenedione

dif-fuses across the basal lamina and is metabolized to estradiol in

the granulosa cells Gs = stimulatory G-protein; AC = adenylate

cyclase, ATP = adenosine triphosphate; cAMP = cyclic

adenos-ine monophosphate.

Fig 5 Compartmentalization of steroidogenic enzymes

Diffu-sion of cholesterol across the mitochondrial membranes is tated by StAR (S) The function of StAR is terminated by proteolysis Cholesterol in the mitochondria is converted into pregnenolone by CYP11A Pregnenolone diffuses out of the mitochondria and is metabolized to other steroids in the mi- crosomes In the ovary, depending on the cell type, the final product may be progesterone or androgen.

facili-396 Part IV / Hypothalamic–Pituitary

altering the concentrations of the various steroidogenic

enzymes expressed in the cells Changes in the

concen-trations of steroidogenic enzymes occur over more

pro-longed and developmentally regulated time frames on

the order of days or longer, whereas the acute regulation

of steroidogenesis occurs on the order of minutes

The signal initiating granulosa cell growth and

dif-ferentiation has not been fully defined It is clear that

gonadotropins are not involved because the granulosa

cells in primordial follicles do not express FSH or LH

receptors Evidence is beginning to emerge indicating

that proteins secreted by the oocyte such as growth

dif-ferentiation factor-9, a member of the transforming

growth factor-β (TGF-β) superfamily, play an essential

role in initiating follicle development Prior to the

selec-tion of the dominant follicle, the granulosa cells do not

express CYP19 and therefore do not contribute to

estra-diol production

When preantral follicles contain approximately two

layers of granulosa cells, the granulosa cells secrete

proteins into the stroma that cause undifferentiated

mesenchymal cells to differentiate into theca cells The

signals have not been fully defined, but it appears that

several small molecular weight proteins potentially

including IGF-1 and stem cell factor or kit ligand may

be components of the differentiation signal When the

theca cells first differentiate, they contain LH receptors,

StAR, CYP11A, 3β-HSD, and CYP17 Thus, they are

capable of producing androstenedione at the preantral

stage of follicle development

Excessive androgens can have detrimental effects onovarian function; therefore, it is beneficial to ensure thatandrogens do not accumulate before CYP19 is expressed

in the granulosa cells The granulosa cells secrete eral factors that inhibit the stimulatory actions of LH ontheca cell steroidogenic enzyme gene expression andandrogen production including activin and TGF-β(Table 1)

sev-If a follicle becomes selected, the inhibitory signalfrom the granulosa cells changes to one in which thestimulatory effects of LH are enhanced Many of thesame molecules both enhance the effects of LH on thecacell differentiation and sensitize the granulosa cells tothe stimulatory effects of FSH It is through the enhance-ment of LH and FSH action by factors such as IGF-1 andinhibin family members (Table 1) that expression ofsteroidogenic enzymes in the theca cells and CYP19 inthe granulosa cells is increased even though the concen-trations of LH and FSH do not increase in the circula-tion Although the nature of the signals is not fullyunderstood, it is clear that there is a detailed system ofcommunication among the oocyte, granulosa cells, andtheca cells that ensures that the differentiation and func-tion of the follicle cells are coordinated Successfulcompletion of this developmental program results in apreovulatory follicle ready to ovulate

4 OVULATION

Ovulation is the end process of a series of eventsinitiated by the gonadotropin surge and resulting in the

Table 1 Autocrine/Paracrine Factors Regulating Ovarian Steroid Hormone Production

Cellular Effect on LH-dependent Effect on FSH-dependent Factor a origin androgen production in vitro b estrogen production in vitro b

aIGF-1 = insulin-like growth factor-1; TGF- β = transforming growth factor-β; bFGF = basic fibroblast growth factor; NGF = nerve growth factor; GDF-4 = growth differentiation factor-9; HGF = hepatocyte growth factor; KGF = keratinocyte growth factor; TNF- α = tumor necrosis factor- α; IL-1β = interleukin-1β.

b+, augments; –, inhibits; ?, unknown.

Chapter 26 / Endocrinology of the Ovary 397

release of a mature fertilizable oocyte from a Graafian

follicle During the second half of the follicular phase

and as follicles grow, plasma estradiol concentrations

begin to rise About 24–48 h after plasma estradiol

lev-els reach a peak, the midcycle LH surge takes place

This preovulatory LH surge occurs at around d 14 of a

28-d cycle, with a total duration of approx 48 h

Ovula-tion occurs 36 h after the onset of the LH surge

Proges-terone and FSH levels remain low in the follicular phase

until just before ovulation At this time, a small FSH

surge accompanies the greater LH surge, and

progester-one levels rise slightly just before ovulation

The precise hormonal regulation mechanisms

oper-ating during ovulation are not fully elucidated

How-ever, it is well known that the gonadotropin surge at the

end of the follicular phase is essential for ovulation

The midcycle LH surge results from activation of

posi-tive estradiol feedback at the level of both the pituitary

and hypothalamus The increasing amounts of

estra-diol secreted by the dominant follicle trigger the

hypo-thalamic gonadotropin-releasing hormone (GnRH)

surge The administration of a GnRH antagonist in

women prevents the surge or interrupts it if it has

already started This suggests that GnRH is necessary

not only for the surge to occur but also for the

mainte-nance of the surge Additionally, the pituitary LH surge

is facilitated by an increased responsiveness of

gonad-otrope cells to GnRH observed following exposure to

rising estradiol and by an increase in GnRH receptor

number The feedback signal to terminate the LH surge

is unknown The decline in LH may be owing to the

loss of the positive feedback effect of estrogen,

result-ing from the increasresult-ing inhibitory feedback effect of

progesterone, or owing to a depletion of LH content of

the pituitary from downregulation of GnRH receptors

The rise in progesterone concentrations may lead to a

negative feedback loop and inhibit pituitary LH

secre-tion by decreasing GnRH pulse frequency Moreover,

LH downregulates its own receptors just before

ovula-tion, resulting in decreased estrogen production

The LH surge stimulates resumption of meiosis I

in the oocyte with release of the first polar body The

oocyte nucleus or germinal vesicle undergoes a series of

changes that involve germinal vesicle breakdown and

the progression of meiosis to the second meiotic

metaphase or first polar body stage It has been

sug-gested that the LH surge overcomes the arrest of meiosis

by inhibiting the oocyte maturation inhibitor (OMI)

secretion This inhibitor is produced by granulosa cells

and leads to the arrest of meiosis during folliculogenesis

It appears that OMI exerts its inhibitory action on

meio-sis, not directly on the oocyte, but acts to increase the

concentrations of cAMP in the cumulus cells, which

then passes via gap junctions into oocyte and halts otic maturation The LH surge, by inhibiting OMI secre-tion and thereby decreasing cAMP, allows theresumption of meiosis The second meiotic division iscompleted at the time of fertilization, if it occurs, yield-ing the ovum with the haploid number of chromosomesand the second polar body that is released

mei-With the LH surge, the production of antral fluid inthe dominant follicle increases, and the follicle enlargesmarkedly This results in a relatively thin peripheralrim of granulosa cells and regressing thecal cells towhich the oocyte, with its associated cumulus cells, isattached only by a tenuous and thinning stalk of granu-losa cells The increasing size of the follicle and itsposition in the cortex of the ovarian stroma cause it tobulge out from the ovarian surface, leaving only a thinlayer of epithelial cells between the follicular wall andthe peritoneal cavity At one site on its surface, thefollicle wall becomes even thinner and avascular; thecells in this area dissociate and then appear to degen-erate and the wall balloons outward The follicle thenruptures at this site, the stigma, causing the fluid toflow out on the surface of the ovary, carrying with itthe oocyte and its surrounding mass of cumulus cells.Follicle rupture and oocyte extrusion are evoked by

LH and progesterone-induced expression of teolytic enzymes such as collagenases Enzymatic deg-radation of the follicle wall is a primary hypothesis toexplain the rupture Increased prostaglandin (PG) syn-thesis also appears to play a role in the extrusion of theoocyte PGs probably contribute to the process of ovu-lation through various pathways, such as affecting thecontractility of the smooth muscle cells on the ovaryand activating proteolytic enzymes, especially thoseassociated with collagen degradation

pro-5 LUTEINIZATION

Luteinization is the process that transforms the losa and theca cells into luteal cells This process istriggered by the surge of LH at midcycle, once the granu-losa cells have acquired receptors for LH, and does notnecessarily signify that ovulation has occurred The LHsurge causes profound morphologic changes in the fol-licle that becomes corpus luteum These include acqui-

granu-sition by the granulosa cells of the capacity of de novo

synthesis of steroids (mainly progesterone and gen) and invasion of the previously avascular granulosacell layer by a vascular supply

estro-After ovulation and expulsion of the unfertilized egg,the granulosa cells continue to enlarge, become vacu-olated in appearance, and begin to accumulate a yellowpigment called lutein, and they are now called as granu-losa lutein cells Luteinization of granulosa cell involves

398 Part IV / Hypothalamic–Pituitary

the appearance of lipid droplets in the cytoplasm,

devel-opment by the mitochondria of a dense matrix with

tubular cristae, hypertrophy of the ER and enlargement

of the granulosa cell into the “large luteal cell.” Thecal

cells are also luteinized (theca-lutein cells) and make up

the outer portion of the corpus luteum These “small

luteal cells” are much less active in steroidogenesis and

have no secretory granules The basal lamina of the

fol-licle dissolves, and capillaries invade into the granulosa

layer of cells in response to secretion of angiogenic

fac-tors such as vascular endothelial growth factor by the

granulosa and thecal cells

The corpus luteum is a transient endocrine organ that

predominately secretes progesterone, and its primary

function is to prepare the estrogen-primed endometrium

for implantation of the fertilized ovum The

granulosa-lutein cells express cholesterol side-chain cleavage

enzyme and 3β-HSD, and, accordingly, they have a

high capacity to produce progesterone and estradiol

Blood vessels penetrating the follicle basal lamina

pro-vide these cells with low-density lipoproteins, the main

source of cholesterol as a substrate for progesterone

and estradiol synthesis in luteal cells Seven days after

ovulation, approximately around the time of expected

implantation, peak vascularization is achieved This

time also corresponds to peak serum levels of

proges-terone and estradiol The secretion of progesproges-terone and

estradiol is episodic and correlates with the LH pulses

During the process of luteinization, LH is required to

maintain steroidogenesis by granulosa-lutein cells The

role of other luteotropic factors such as prolactin (PRL),

oxytocin, inhibin, and relaxin remains unclear

Theca-lutein cells that express the enzymes in the androgen

biosynthetic pathway and produce androstenedione

are also involved in steroid biosynthesis

The life-span of the corpus luteum is 14 days after

ovulation and depends on continued LH support The

mechanism involved in maintaining the function of the

corpus luteum for 14 d and in precipitating the process

of luteolysis (programmed cell death) at the end of this

period is incompletely understood It is clear, however,

that LH maintains the functional and morphologic

integrity of the corpus luteum, yet it is insufficient to

prevent luteolysis Corpus luteum function declines by

the end of the luteal phase unless human chorionic

gona-dotropin (hCG) is produced by a pregnancy Luteolysis

can be viewed as a default response to lack of

stimula-tion by hCG If pregnancy does not occur, the corpus

luteum undergoes luteolysis under the influence of

luteolytic factors These factors include estradiol,

oxy-tocin, and PGs The luteolytic effect of both estrogen

and oxytocinappears to be mediated, at least in part, by

local formation of PGF PGF exerts its effects via

the synthesis of endothelin-1 (ET-1), which inhibitssteroidogenesis and stimulates the release of a cytokine,tumor necrosis factor-α (TNF-α), which induces cellapoptosis

Corpus luteum starts to undergo luteolysis approx 8

d after ovulation Luteolysis involves fibrosis of theluteinized cells, a dramatic decrease in the number ofsecretory granules with a parallel increase in lipiddroplets and cytoplasmic vacuoles, and a decrease invascularization The luteal cells become necrotic,progesterone secretion ceases, and the corpus luteum isinvaded by macrophages and then by fibroblasts Endo-crine function is rapidly lost, and the corpus luteum isreplaced by a scarlike tissue, the corpus albicans

6 DEFECTS IN OVULATORY FUNCTION

Ovulatory defects can be classified into three groupsbased on the World Health Organization (WHO) defini-tion These classes suggest different etiologies and,consequently, different optimal treatment approaches

1 Group I: hypogonadotropic hypogonadism: Patients withhypogonadotropic hypogonadism comprise 5–10% ofanovulatory women These patients have low serum FSHand estradiol levels This category includes women withhypothalamic amenorrhea (HA), stress-related amenor-rhea, anorexia nervosa, and Kallman syndrome Thesewomen will respond to gonadotropin therapy for ovula-tion induction

2 Group II: eugonadotropic hypogonadism: Patients areeugonadotropic, normoestrogenic, but anovulatory andconstitute the majority of anovulatory women evaluated(60–85%) They exhibit normal FSH and estradiol lev-els This category includes women with polycystic ovarysyndrome (PCOS), among other disorders These womenrespond to most ovulatory agents

3 Group III: hypergonadotropic hypogonadism: Patientswith hypergonadotropic hypogonadism account for 10–30% of women evaluated for anovulation These patientstend to be amenorrheic and hypoestrogenic, a categorythat includes all variants of premature ovarian failure(POF) and ovarian resistance syndromes These patientswill not respond to ovulation induction but are candi-dates for oocyte donation

Hyperprolactinemia accounts for 5–10% of womenwith anovulation, and these patients respond well tomedications that lower PRL secretion Although many

of these women have normal estrogen levels (i.e., areeuestrogenic) and therefore can be categorized as hav-ing a WHO Group II ovulatory defect, some of thesewomen may be hypoestrogenic and be more similar toGroup I patients Consequently, these patients are of-ten considered separately from those women meetingthe standard WHO classification of ovulatory disor-ders

Chapter 26 / Endocrinology of the Ovary 399

Following we discuss in some detail the

pathophysi-ology and clinical presentation of patients with HA

(WHO Group I), PCOS (WHO Group II), and POF

(WHO Group III)

6.1 Hypothalamic Amenorrhea

Amenorrhea with signs or symptoms of

hypoestro-genism and low gonadotropin levels with exclusion of

related disorders confirms a diagnosis of HA WHO

classifies HA as Group I anovulation Hypothalamic or

pituitary dysfunction may involve the amount of

prod-ucts (e.g., GnRH, FSH) secreted or the pulse frequency

of the products

A thorough history and physical examination can

help elucidate potential etiologies

Hyperprolactine-mia and hypo/hyperthyroidism should be ruled out in

all women with amenorrhea An imaging study of the

hypothalamus and pituitary is imperative to evaluate

for tumors The accuracy of the assays used for FSH

and LH is poor in the lower ranges Therefore, the “lab

results” for FSH and LH in patients with HA may be

“low” or “low normal.”

Anatomic or developmental lesions of the

hypo-thalamus or pituitary gland can lead to hypothalamic

amenorrhea Patients with Kallman syndrome have a

failure of migration of the GnRH neurons from the

nasal placode to the hypothalamus They present with

amenorrhea and anosmia Patients with idiopathic

hypogonadotropic hypogonadism present similar to

those with Kallman syndrome, but without anosmia

Treatment options include the GnRH pump or

gonado-tropin ovulation for infertility, and HRT for

osteoporo-sis prevention and estrogen replacement

Hypothalamic lesions, tumors, or space-occupying

lesions (e.g., sarcoidosis) can lead to HA

Craniophar-yngiomas are the most common tumor affecting the

reproductive function of the hypothalamus They are

treated surgically in combination with radiotherapy

Ia-trogenic HA may result from damage during surgery or

irradiation of the hypothalamus These patients should

be tested for insufficiency of all pituitary secretagogues

and replaced as indicated

HA from pituitary lesions can be the result of tumors(micro/macroadenomas), infarction (e.g., Sheehan syn-drome), empty sella syndrome, trauma (with transec-tion of the pituitary stalk), space-occupying lesions (e.g.,sarcoidosis), and lymphocytic hypophysitis

Prolactinomas are the most common type of mas found in the pituitary Initial treatment of PRL-secreting micro- or macroadenomas is with dopamineagonists, e.g., bromocriptine or cabergoline The effects

adeno-of dopamine analogs on PRL levels can be detectedwithin weeks Surgery is reserved for refractory cases.Other secretory products of adenomas include GH,adrenocorticotropic hormone, and FSH

Patients with empty sella syndrome may present withnormal, low, or elevated levels of pituitary hormones.Those with trauma or infarction may have aberrations ofvarious pituitary hormones, and assessment of patientsshould include the adrenal, thyroid, ovary, and GH.These women need to be treated on an individualizedbasis as indicated

Nonanatomic defects of the pituitary are guishable from hypothalamic lesions A GnRH stimu-lation test is not routinely used in clinical practice todifferentiate between pituitary and hypothalamic dys-function because of difficulties in interpretation of testresults and little effect on patient management.Functional lesions disrupting the hypothalamic pitu-itary axis can result from a variety of stressors Physicalstressors such as anorexia nervosa or excessive exerciselead to HA Diagnosis is based on history or findings ofsevere weight loss or cachexia with laboratory findingsconsistent with HA Treatment includes resolution ofthe stressor and HRT to prevent osteoporosis

indistin-6.2 Polycystic Ovary Syndrome

Androgens are C19 steroids secreted by the zonareticularis of the adrenal cortex and the theca and stroma

of the ovaries, produced through de novo synthesis from

cholesterol The ovarian theca is responsible for ing approx 25% of circulating testosterone, and for 50%

secret-of all androstenedione, the most important precursor secret-ofdihydrotestosterone and testosterone Androgen excess

Table 2 Etiologies of POF

Decrease in initial pool of oocytes Increase rate of loss of oocytes

Gonadal dysgenesis X-chromosome defects

Iatrogenic (surgical/chemotherapy) Enzymatic abnormalities

Infection/toxins

400 Part IV / Hypothalamic–Pituitary

or hyperandrogenism affects 5–10% of

reproductive-age women

A common feature of androgen excess disorders is

ovulatory dysfunction, which may arise from a

disrup-tion of gonadotropin secredisrup-tion or from direct ovarian

effects Androgens may directly alter the secretion of

gonadotropins in women However, the effect of

andro-gens on the hypothalamic-pituitary-ovarian axis appears

to be primarily dependent on their aromatization to

es-trogens Excessive androgen levels may also directly

inhibit follicle development at the ovarian level, which

may result in the accumulation of multiple small cysts

within the ovarian cortex, the so-called polycystic ovary

(Fig 6)

By far the most common cause of androgen excess is

the PCOS, accounting for approx 80–85% of patients

with androgen excess, and 4–6% of reproductive-age

women Although there is continuing debate regarding

the definition of PCOS, useful diagnostic criteria arose

from a 1990 National Institutes of Health (NIH)

confer-ence on the subject These criteria note that PCOS should

include, in order of importance, (1) clinical and/or

bio-chemical evidence of hyperandrogenism; (2) ovulatory

dysfunction; and (3) the exclusion of other causes of

androgen excess or ovulatory dysfunction, including

adrenal hyperplasia, hyperprolactinemia, thyroid

dys-function, and androgen-secreting neoplasms (ASNs)

The presence of polycystic ovaries on ultrasound was

not included as part of the definition arising from the

1990 NIH conference However, in approx 70% of

pa-tients with PCOS, the ovaries contain intermediate and

atretic follicles measuring 2–5 mm in diameter,

result-ing in a “polycystic” appearance at sonography (Fig 7)

Diagnostic criteria for PCOS using ovarian morphologic

features have been suggested However, note that cystic ovaries” on sonography or at pathology mightsimply be a sign of dysfunctional folliclar development.For example, this ovarian morphology is frequentlyseen in patients with other androgen excess disorders,including nonclassic and classic adrenal hyperplasia It

“poly-is also frequently observed in patients with prolactinemia, type 2 diabetes mellitus, and bulimianervosa, independent of the presence of hyperandro-genism Up to 25% of unselected women have poly-cystic ovaries on ultrasound, many of whom arenormoandrogenic regularly cycling Hence, we considerthe presence of polycystic ovaries to be only a sign,albeit nondiagnostic, of androgen excess or PCOS Arecent expert conference has suggested including thepresence of polycystic ovaries as part of the diagnosticscheme for PCOS (Rotterdam, 2004)

hyper-Classically, pathologic features of the ovaries inPCOS includes thickening and collagenization of thetunica albuginea, a paucity of corpus luteum, basalmembrane thickening, an increased number of follicles

in various stages of development and atresia, andstromal/thecal hyperplasia (hyperthecosis) (Fig 8A).Although the number of cysts measuring 4–6 mm isgreater than normal, the fact that most of these are invarious stages of atresia leads to a relative deficiency ingranulosa cells and/or predominance of theca/stromalcells Although we and others have reported that thetheca/stromal cells in PCOS frequently demonstrate

“luteinization” (Fig 8B), Green and Goldzieher (1965)did not observe any abnormality of the follicular or th-eca cells on light or electron microscopy

The presence of multiple follicular cysts typicallyresults in “polycystic”-appearing ovaries, which givethe syndrome its name However, note that patients withPCOS demonstrate a spectrum of histologic findings.Givens (1984) has described ovaries with an increasednumber of follicular cysts and minimal stromal hyper-plasia, classified as type I Alternatively, type IV ova-ries demonstrated a small number of follicular cysts,with marked stromal hyperplasia and “hyperthecosis.”Types I and IV ovaries appear to represent the twoextremes of a continuum Kim et al (1979) studied ninepatients with clinical evidence of androgen excess Four

of these patients demonstrated “polycystic” ovaries, andthe remaining five had histologically normal ovaries Inthese patients, adrenocortical suppression with dexam-ethasone (2 mg daily for 3 d) minimally suppressedandrogen levels in all patients, whereas an oral contra-ceptive administered for 21 d normalized androgens inboth groups of patients with androgen excess Thus,ovarian hyperandrogenism was present in patients withand without polycystic-appearing ovaries

Fig 6 Polycystic ovary bivalved during ovarian wedge

resec-tion Note the multiple follicular cysts measuring 2–6 mm in

diameter, and the increased stromal volume.

Chapter 26 / Endocrinology of the Ovary 401

In addition to the direct effects of androgens on

ova-rian function, hyperinsulinism and excess LH levels

ap-pear to contribute to the ovarian androgen excess present

in PCOS Many women with PCOS appear to be

uniquely insulin resistant, with compensatory

hyperinsulinemia, independent of body weight The

compensatory hyperinsulinemia, resulting from the

un-derlying insulin resistance, augments the stimulatory

action of LH on the growth and androgen secretion of

ovarian thecal cells, while inhibiting the hepatic

pro-duction of sex hormone–binding globulin Overall,

in-sulin resistance and secondary hyperinin-sulinemia affect

a large fraction of patients with PCOS and may cause oraugment ovarian androgen excess in these patients.The LH/FSH ratio is also elevated in 35–95% ofpatients with PCOS, although recent ovulation ap-pears to be associated with a transient normalization

in the ratio The use of insulin sensitizers to treatpatients with PCOS may result in lower circulatinglevels of LH, suggesting that insulin resistance or,more likely, hyperinsulinemia is in part responsiblefor the gonadotropic abnormalities observed in manywomen with PCOS although not all researchers agree.The excess LH present contributes to the stimulation

Fig 7 Transvaginal ultrasound visualization of a polycystic ovary Note the string of subcapsular follicles measuring 3–6 mm in

diameter, with increased central stroma mass.

Fig 8 Section of polycystic ovary Note (A) the markedly thickened ovarian capsule with multiple subcapsular Graafian follicles

(hematoxylin adn eosin [H&E] stain, ×2.5) and (B) the islands of luteinized stromal cells, characteristic of hyperthecosis (H&E

stain, ×10).

402 Part IV / Hypothalamic–Pituitary

of theca cell biosynthesis, further leading to the

excess ovarian secretion of androgens

Ovulatory dysfunction in PCOS frequently results

in oligoovulatory infertility As a general rule, women

with PCOS require ovulation induction with either

clo-miphene citrate or gonadotropins In this context,

women with PCOS are at especially increased risk of

developing the hyperstimulation syndrome, a

syn-drome of massive enlargement of the ovaries;

develop-ment of rapid and symptomatic ascites, intravascular

contraction, hypercoagulability, and systemic organ

dysfunction; and multiple gestations These

complica-tions occur generally following treatment with

gona-dotropins, although ovarian hyperstimulation has even

been reported in women with PCOS conceiving a

singleton pregnancy spontaneously or after the use of

clomiphene or pulsatile GnRH

6.3 Premature Ovarian Failure

Menopause occurring prior to 40 yr of age is termed

POF The diagnosis is based on findings of amenorrhea,

hypoestrogenism, and elevated gonadotropins In a

study of 15,253 women attending menopause clinics in

Italy, the Progetto Menopausa Italia Study Group found

that 1.8% of the women reported POF Coulam et al

(1986) reported a 1% risk incidence of POF in a group

of 1858 women in Rochester, Minnesota The

preva-lence of POF in women with primary amenorrhea is

estimated at 10–28% Women with secondary

amenor-rhea have a lower prevalence, at approx 4–18% Risk

factors for POF include nulliparity and lifelong

irregu-lar menses; however, age at menarche, oral

contracep-tive use, and smoking were not associated with the

condition

These findings may not imply irrevocable

quies-cence of follicular activity, because there are numerous

reports of reinitiation of menses in women previously

diagnosed with POF Because of the multiple possible

etiologies and the unresolved nature of the damage,

there are no predictors of remission Little is known

about these periods other than that they do occur

spon-taneously and can result in viable pregnancy Failure of

oocytes secondary to depletion or inhibition of their

function results in POF Histologic examination of the

ovaries reveals minimal follicular activity; dense

con-nective tissue and/or lymphocytic infiltrates may be

seen Nelson et al (1994) reported on follicular activity

in women with POF with normal karyotypes They

found that although almost half of the patients had

estradiol levels consistent with follicular activity, the

follicles were not functioning normally

Approximately 50% of POF is of an idiopathic

eti-ology Other causes include chromosomal anomalies

leading to gonadal dysgenesis, autoimmunity, therapeutics, ovarian surgery, inherited enzymaticdefects, and infections In these settings, POF mayresult from a reduction in the initial pool of follicles, or

chemo-an accelerated loss of oocytes (Table 2) Pure gonadaldysgenesis (46, XX) results in women born with ova-ries lacking oocytes More common is the increaseddestruction of oocytes Women with X-chromosomedefects (Turner syndrome, Turner mosaic, trans-locations, deletions, and heterozygote fragile X) showaccelerated loss of oocytes, which is clearly associatedwith POF Specifically, a critical region on Xq appears

to play a key role in ovarian function, and a disruption

of this region leads to premature activation of lar apoptosis The thymus is essential to ensure appro-priate number of oocytes at birth; therefore, thymicaplasia can also lead to POF

follicu-The association between autoimmunity and POF isclear POF was associated with other endocrine autoim-mune conditions including those of the adrenal (Addisondisease: 2.5%) and thyroid glands (hypothyroidism:27%), as well as diabetes mellitus, in a prospectiveanalysis of 120 women with POF and normal karyotype.Women with POF have increased prevalence of otherautoimmune diseases such as hypoparathyroidism,myasthenia gravis, pernicious anemia, and systemiclupus erythematosus Antibodies against various ova-rian antigens have been isolated in higher frequency inwomen with POF Antiovarian antibodies have beentargeted against oocytes, theca, granulosa, and gonado-tropin receptors Several investigators have found anti-bodies against steroid-producing antibodies andantibodies toward steroidogenic enzymes (CYP21,CYP17, CYP19, and 3β-HSD) in women with Addisondisease and POF

Surgical and chemotherapeutic treatments can lead

to POF Women undergo oophorectomy and ovariancystectomy for a variety of reasons including dermoids,endometriosis, persistent cysts, and cancer Womenwith multiple ovarian surgeries are at increased risk ofPOF Alkylating agents used in chemotherapy are asso-ciated with oocyte damage and POF The use of GnRHanalogs and ovarian autografts may prevent oocytedamage

Several enzymatic defects have been implicated asthe etiology for POF Women with defects in galactose-1-phosphate uridyl-transferase have a high prevalence

of POF Women with this disorder appear to have anadequate number of follicles but show evidence ofaccelerated loss prior to menarche 17α-Hydroxylasedeficiency has also been associated with POF

Infections with varicella, shigella, or malaria werefound in 3.5% of women with POF Exposure to envi-

Chapter 26 / Endocrinology of the Ovary 403

ronmental toxins has been associated with POF

Polcyclic aromatic hydrocarbons, from combustion of

fossil fuels or in cigarette smoke, can stimulate apoptosis

in oocytes leading to POF POF was found to be

second-ary to exposure to 2-bromopropane in a study of 16

women exposed to the cleaning solvent

The patient’s history and physical examination

should include assessment for the possible etiologies

just discussed Laboratory evaluation should include

levels of FSH, LH, estradiol, thyroid-stimulating

hor-mone, prolactin, fasting glucose, calcium, phosphate,

and electrolytes A chromosomal analysis should be

done on those under 35 yr of age

Treatment for POF should focus on supporting and

educating the patient, treating symptoms of

hypo-estrogenemia, and preventing osteoporosis Infertility

should be addressed by educating the patient regarding

possible remission with resumption of menses and

fer-tility, ovum donation, and adoption The patient may be

started on any regimen of hormone replacement therapy

(HRT) with estrogen and progesterone as indicated, with

appropriate counseling regarding the risks of

thrombo-sis and breast cancer Patients with POF should be

fol-lowed closely and evaluated for other endocrinopathies,

especially adrenal insufficiency, on an annual basis

There is little prospective evidence to support the use of

glucocorticoids in the treatment of POF, and this

man-agement has significant risks such as avascular necrosis

of the femoral head, and knee, as well as iatrogenic

Cushing syndrome Numerous case reports offer the

promise of potential therapies for POF to restore

ova-rian function, but these should be avoided until proven

with appropriate studies

REFERENCES

Coulam CB, Adamson SC, Annegers JF Incidence of premature

ovarian failure Obstet Gynecol 1986;67:604–606.

Givens JR Polycystic ovaries—a sign, not a diagnosis Semin

Reprod Endocrinol 1984;2:271–280.

Green JA, Goldzieher JW The polycystic ovary IV Light and

elec-tron microscope studies Am J Obstet Gynecol 1965;91:173–181.

Kim MH, Rosenfield RL, Hosseinian AH, Schneir HG Ovarian

hyperandrogenism with normal and abnormal histologic

find-ings of the ovaries Am J Obstet Gynecol 1979;134:445–452.

Nelson LM, Anasti JN, Kimzey LM, et al Development of ized graafian follicles in patients with karyotypically normal

lutein-spontaneous premature ovarian failure J Clin Endocrinol Metab

1994;79:1470–1475.

The Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus shop Group Revised 2003 consensus on diagnostic criteria and long-term health risks related to polycystic ovary syndrome.

Work-Fertil Steril 2004;81:19–25.

SUGGESTED READINGS

Chabbert-Buffet N, Bouchard P The normal human menstrual cycle.

Rev Endocr Metab Disord 2002;3:173–183.

Clayton RN, Ogden V, Hodgkinson J, Worswick L, Rodin DA, Dyer

S, Meade TW How common are polycystic ovaries in normal women and what is their significance for the fertility of the popu-

lation Clin Endocrinol 1992;37:127–134.

Goldzieher JW, Green JA.The polycystic ovary I Clinical and

his-tologic features J Clin Endocrinol Metab 1962;22:325–338.

Hillier SG Gonadotropic control of ovarian follicular growth and

development Mol Cell Endocrinol 2001;179:39–46.

Knochenhauer ES, Key TJ, Kahsar-Miller M, Waggoner W, Boots

LR, Azziz R Prevalence of the polycystic ovarian syndrome in unselected Black and White women of the Southeastern United

States: A prospective study J Clin Endocrinol Metab 1998;83:

3078–3082.

LaBarbera AR, Miller MM, Ober C, Rebar RW Autoimmune

etiol-ogy in premature ovarian failure Am J Reprod Immunol

Micro-biol 1988;16:115–122.

Laml T, Preyer O, Umek W, Hengstschlager M, Hanzal H Genetic

disorders in premature ovarian failure Hum Reprod Update

2002;8:483–491.

Marshall JC, Eagleson CA, McCartney CR Hypothalamic

dysfunc-tion Mol Cell Endocrinol 2001;183:29–32.

Polson DW, Wadsworth J, Adams J, Franks S Polycystic ovaries—

a common finding in normal women Lancet 1988;1:870–872.

Progetto Menopausa Italia Study Group Premature ovarian failure: frequency and risk factors among women attending a network of

menopause clinics in Italy Br J Obstet Gynaecol 2003;110:

59–63.

Rebar RW, Connoly HV Clinical features of young women with

hypergoandotropic amenorrhea Fertil Steril 1990:53:804–810.

Richards JS, Russell DL, Robker RL, Dajee M, Alliston TN.

Molecular mechanisms of ovulation and luteinization Mol Cell

Endocrinol 1998;145:47–54.

Yen SS, Rebar R, Vandenberg G, Judd H Hypothalamic

amenor-rhea and hypogonadotropinism: responses to synthetic LRF J

Clin Endocrinol Metab 1973;36:811–816.

Zawadzki JK, Dunaif A Diagnostic criteria for polycystic ovary syndrome: towards a rational approach In: Dunaif A, Givens JR,

Haseltine F, Merriam GR, eds Polycystic Ovary Syndrome.

Boston, MA: Blackwell Scientific 1992;377–384.

Chapter 27 / The Testis 405

405

From: Endocrinology: Basic and Clinical Principles, Second Edition

(S Melmed and P M Conn, eds.) © Humana Press Inc., Totowa, NJ

27

lar steroidogenesis The detailed description of steroidhormone action is discussed in another chapter

2.1 Regulation of Leydig Cell Function

2.1.1 G ONADOTROPIN -R ELEASING H ORMONE

The regulation of testicular function depends ongonadstropin-releasing hormone (GnRH) secretion bythe small numbers of GnRH neurons scattered in theanterior hypothalamus (Fig 2) GnRH is then trans-ported through axons to the median eminence, where itenters the capillaries of the hypothalamic portal blood

to the anterior pituitary GnRH secretion is affected bymany neurotransmitters including glutamate acting vianitric oxide, dopamine, γ-aminobutyric acid, neuropep-tide Y, opiates, galanin, and galanin-like peptide.GnRH is released into the portal blood in pulses, andthe pulse frequency is regulated by a pulse generator inthe mediobasal hypothalamus Changes in cell mem-brane potential may predispose the GnRH neurons tobursts of GnRH release that are in synchrony with LHsecretory bursts

GnRH binds and activates a G protein cell membranereceptor on the gonadotropes in the anterior pituitary.Binding of GnRH to its receptor activates the mem-brane-associated phospholipase C and increases intrac-ellular inositol phosphate Inositol triphosphatemobilizes intracellular calcium and opens the voltage-gated calcium channels, resulting in increases in intra-

LEYDIG CELLS AND STEROIDOGENESIS

SPERMATOGENESIS AND SERTOLI CELL FUNCTION

1 INTRODUCTION

The mammalian testis has two basic compartments:

the interstitial (intertubular) compartment and the

semi-niferous tubule compartment (Fig 1A) The interstitial

compartment is highly vascularized and contains Leydig

cells clustered near or around the vessels These cells

are responsive to luteinizing hormone (LH) and secrete

testosterone, which subsequently accumulates in the

interstitium and the seminiferous tubules at relatively

high concentrations The Leydig cell possesses

abun-dant smooth endoplasmic reticulum (SER) and

mito-chondria, both of which contain the enzymes associated

with steroid biosynthesis (Fig 1B) The seminiferous

tubule compartment contains Sertoli cells and

develop-ing and mature germ cells The formation of

spermato-zoa from stem spermatogonia (spermatogenesis)

includes mitotic and meiotic division, followed by

cel-lular differentiation (spermiogenesis) Thus, the two

major areas of activity within the testis center on

ste-roidogenesis and spermatogenesis A large body of

lit-erature provides evidence that LH (via stimulation of

testosterone) and follicle-stimulating hormone (FSH)

are the key regulators of spermatogenesis

2 LEYDIG CELLS AND STEROIDOGENESIS

In this section, we briefly review the endocrine and

paracrine regulation of Leydig cell function and

testicu-406 Part IV / Hypothalamic–Pituitary

cellular calcium Rises in intracellular calcium result in

the release of both LH and FSH GnRH also increases

the transcription of genes for the gonadotropins via

diacylglycerol, phosphokinase C, and

mitogen-acti-vated protein kinase/JNK pathways GnRH receptors

are upregulated by pulsatile GnRH On the other hand,

continuous GnRH results in desensitization of the GnRH

receptors followed by suppression of LH and FSH and

disturbance of gonadal function

2.1.2 G ONADOTROPINS

Both LH and FSH production are dependent on

GnRH Both gonadotropins are glycoproteins

consist-ing of a common α-subunit and a hormone-specific

β-subunit GnRH increases gene transcription of both the

LH and FSH β-subunit gene via specific transcriptionfactors (e.g., LH via SF-1, EGR-1, and SP1 and FSH via

fos and jun as well as androgen response elements) The

α-subunit is less rigorously regulated and both pulsatileand continuous GnRH increase gene expression Stud-ies in monkeys and humans have shown that bursts ofGnRH secretion are necessary for the pulsatile release

of LH In the human, LH pulses occur every 60–120min In pubertal boys, there is increased LH pulsatilesecretion during sleep With aging there are decreases inthe pulse amplitude of LH secretion The synthesis oftestosterone by the testis is under the regulation of LHthrough a G protein–associated transmembrane recep-tor LH binds to the receptors to initiate signalingthrough activation of Gs protein, adenylate cyclase,cycxlic adenosine monophosphate (cAMP), and proteinkinase A (PKA), stimulating testicular steroidogenesis.GnRH and LH/FSH secretion is regulated by negativefeedback mechanisms In primates including humans,testosterone suppresses LH synthesis and secretion pri-marily through its action on the GnRH neurons andpulse generator FSH secretion is also under the nega-tive feedback of testosterone In humans, it has beenshown that the nonaromatizable androgen 5α-dihy-drotestosterone decreases LH pulse frequency, sug-gesting that androgens act via the androgen receptor

Fig 1 (A) Light micrograph showing interstitial (intertubular)

and seminiferous tubular (ST) compartments of mouse testis.

The interstitial compartment contains Leydig cells (L) clustered

around the blood vessels (V) (B) Electron micrograph showing

interstitial (IT) and seminiferous tubular (ST) compartments of

mouse testis Leydig cells (L) are seen in the IT compartment A

portion of a Sertoli cell (S) with distinct nucleus is seen in the

seminiferous tubular compartment.

Fig 2 Regulation of hypothalamic-pituitary-testis The solid

lines represent stimulating effects and the dashed lines negative feedback actions.

Chapter 27 / The Testis 407

(AR) to regulate GnRH secretion Testosterone also

decreases LH secretion by gonadotropes in rodent

pitu-itary, but its role in the negative feedback of human

pituitary is less clear Estradiol (E2), acting

predomi-nantly through the estrogen receptor α (ERα), also

plays a role in the negative feedback of GnRH and

gonadotropin secretion When administered to men

estrogen antagonists (clomiphene) and aromatase

inhibitors (testolactone) result in elevation of both

LH and FSH Men with ERα gene mutation and

aromatase deficiency also have elevated FSH and LH

These pharmacologic manipulations and models in

nature indicate that E2plays a role in the negative

regu-lation of GnRH and gonadotropin secretion

2.1.3 A CTIVINS , I NHIBINS , AND F OLLISTATIN

Although FSH secretion is primarily regulated by

GnRH and gonadal steroids, there are other paracrine

and endocrine factors such as pituitary activin and

follistatin and testicular inhibin β that only regulate

FSH without affecting LH secretion and synthesis

Activin stimulates FSH β gene transcription through

activation of the Smad family of proteins Follistatin

binds to activin and inhibits its biologic activity The

testicular protein inhibin β, secreted by Sertoli cells,

competes with activin for binding to the activin

recep-tor, preventing the initiation of signaling of Smads and

thus decreasing FSH gene transcription

2.1.4 C LINICAL I MPLICATIONS

Serum FSH, LH, and inhibin β levels are useful in the

diagnosis of hypogonadism and infertility In men with

hypothalamic-pituitary dysfunction, serum LH and FSH

levels are low (hypogonadotropic hypogonadism),

whereas in men with primary testicular dysfunction,

serum LH levels are elevated if Leydig cell function is

compromised, and serum FSH is also elevated if Sertoli

cell function or seminiferous tubule damage occurs

(hypergonadotropic hypogonadism) Because inhibin

selectively suppresses FSH secretion, circulating

inhibinβ and FSH are inversely related in healthy men

and men with primary or testicular disease Circulating

inhibinβ reflects Sertoli cell function and is decreased

in men with seminiferous tubule dysfunction

2.2 Leydig Cell Function

2.2.1 L EYDIG C ELLS

The structure of the adult Leydig cell is shown in

Fig 1B The predominant cytoplasmic organelle is the

SER, which is characteristically more abundant in

ste-roidogenic cells Mitochondria and lipid droplets are

also numerous in Leydig cells, playing important roles

in steroidogenesis Leydig cells are believed to be

mesenchymal in origin though recent evidence gests that there may be a neural crest component In thehuman, fetal Leydig cells become apparent at 8 wk andmultiply to reach a maximum at 15 wk of gestation,coinciding with a rise in androgen concentration in tes-tis and blood, and then remain inactive for the rest ofgestation The number of Leydig cells increases at 2 to

sug-3 mo after birth, which is associated with the surge ofserum testosterone at this early age Leydig cells thenenter into a period of quiescence until puberty Duringpuberty the number of adult Leydig cells increases fur-ther and reaches a maximum of 500 million at about theage of 20 yr The increased number of cells and theirstimulation by increasing LH levels results in a peak ofserum testosterone in early adulthood The number ofLeydig cells remains stable between age 20 and 60, andthen gradually decreases after the age of 60 Thedecrease in the number of Leydig cells and functionmay be responsible for the androgen deficiency asso-ciated with aging in men

is converted by P450c17/C17 hydroxylase into 17pregnenolone and then into dehydroepiandrosterone(DHEA) by P450C17/C17, 20 lyase/desmolase This ∆5pregnenolone pathway is predominant over the ∆4progesterone pathway in the human testis AlthoughDHEA can then be converted by the 17β-hydroxy-steroid dehydrogenase (17β-HSD) via androstenediolinto testosterone, the dominant pathway in the humantestis is for DHEA to be converted into androstenedi-one by 3β-hydroxysteroid dehydrogenase (3β−HSD)and then into testosterone In the ∆4 pathway, predomi-nant in rodents, pregnenolone is converted into proges-terone by 3β−HSD Progesterone is then converted into

α-17α-hydroxyprogesterone and androstenedione viaP450C17 Androstenedione is converted into testoster-one by 17β-HSD

2.2.3 M ECHANISMS OF T ESTOSTERONE A CTIONS

Testosterone can act on the androgen receptor (AR)directly or as a prohormone (Discussion of the bind-ing of testosterone to the AR and the complexity ofregulation of the AR action is beyond the scope of thischapter.) Mutations of AR transcription events result

408 Part IV / Hypothalamic–Pituitary

in the syndrome of androgen insensitivity that spans

the clinical manifestations from male infertility to a

female phenotype The 5α-reductase enzyme converts

testosterone into 5α-dihydrotestosterone (5α-DHT),

an irreversible reaction 5α-DHT is then metabolized

to 5α-androstane-3α (and -3β), 17β-diols, and the

tri-ols 5α-DHT binds to the AR to exert its action There

are two 5α-reductase enzymes: 1 and 2 5α-Reductase

2 enzyme is expressed in male reproductive tissues

(prostate, testis, epididymis, seminal vesicles) from

the embryogenesis to adulthood, genital skin, hair

fol-licles, and liver 5α-Reductase 1 enzyme is present in

nongenital skin, sebaceous gland, and liver, and more

recently, it was found to be expressed in bone and brain

Genetic mutations of 5α-reductase enzyme 2 result in

males with ambiguous genitalia, small hypospadiac

phallus, and blind vagina These males may achieve

partial virilization at puberty owing to the surge of

serum testosterone allowing some conversion to 5

α-DHT Males with 5α-reductase 2 deficiency have small

prostates, decreased facial and body hair and relativelynormal bone mineral density (BMD)

Testosterone is also converted by the aromataseenzyme into E2 This conversion occurs in the Leydigcells and accounts for <10% of E2produced in the adultmale The majority of E2 in males is derived from theperipheral conversion of testosterone into E2or andros-tenedione into estrone and then into E2 E2acts via ERαand ER-β The action of E2in various tissues depends

on the balance of expression and transcription between

ERα and ERβ In the human, aromatization of osterone to E2appears to be important for achievingand maintaining bone mass and BMD In rodents, con-version of androgens into estrogens is important formale aggressive and sexual behavior In primates and

test-in humans, the requirement of conversion of one into estrogens for brain functions is much less clear.Recent development of knockout mouse models for

testoster-ERα and ERβ and reports of aromatase and ERα genemutations in human males allows better understanding

Fig 3 Regulation of testicular steroidogenesis by LH The black box represents the mitochondria and the heavy solid lines the

predominant steroidogenic pathway in humans STAR = steriodogenic acute regulatory protein.

Chapter 27 / The Testis 409

of the functions of these receptor subtypes There is

also recent evidence in mice suggesting that in the

pros-tate, 5αDHT is converted into 5α-androstane 3β,

17β-diol This steroid binds and exerts its effect via ERβ

rather than through the AR crosslinking the androgens

and estrogen actions in the male

2.2.4 R EGULATION OF L EYDIG C ELL S TEROIDOGENESIS

LH is the major regulator of Leydig cell function;

however, in the fetus, recent evidence suggests that

fac-tors such as pituitary adenylate cyclase–activating

polypeptide (PACAP) may regulate Leydig cell

func-tion LH elicits two types of responses in the Leydig

cells: acute or chronic LH binds to the transmembrane

G protein receptor and signals through the PKA-cAMP

pathway The acute response results in a rapid

produc-tion of testosterone within minutes and does not require

new transcription of mRNA In the acute response,

car-rier proteins deliver the substrate cholesterol for P450scc

enzyme complex in the mitochondria This

mitochon-drial transport of cholesterol is regulated by the

ste-roidogenic acute regulatory (StAR) protein Mutations

of the StAR gene result in lipoid congenital adrenal

hyperplasia in which steroidogenesis is absent in both

the adrenals and gonads owing to absence of shuttling of

cholesterol to P450scc complex within the

mitochon-dria Although StAR is important for cholesterol

shut-tling to the mitochondrial, other proteins such as PBR

may also be important for this trafficking

Chronic stimulation by LH has tropic effects on the

Leydig cells requiring both transcription and increased

translation of the proteins There is increased

expres-sion of the steroidogenic enzymes P450scc, P450c17, 3

β-HSD and 17β-HSD The steroidogenic organelle

including the mitochondrial membrane potential and

SER volume are both supported by LH

In addition to LH, local cell-to-cell interactions and

paracrine factors produced by Sertoli cells, germ cells,

peritubular cells, and macrophages may affect Leydig

cell function FSH receptors are located only in Sertoli

cells FSH can act via Sertoli cell secretory proteins to

regulate Leydig cells Using FSH β and FSH receptor

knockouts, recent studies showed that LH alone is

suf-ficient for normal postnatal development of Leydig cells

only when FSH receptors are present In the absence of

LH, FSH stimulates Leydig cell steroidogenesis Sertoli

cell factors such as insulin-like growth factor-1 increase

whereas transforming growth factor-β (TGF-β) and

interleukin-1 (IL-1) inhibit Leydig cell steroidogenesis

Other peptide hormones including PACAP, vasoactive

intestinal peptide, and arginine vasopressin have been

shown to regulate Leydig cell steroidogenesis in vitro,

but the significance of these findings is not clear

2.2.5 C LINICAL I MPLICATIONS

Decreased Leydig cell function is associated withdecreased production of testosterone and may mani-fest clinically with symptoms and signs of male hypo-gonadism Men with low serum testosterone levels maycomplain of decreased libido and erectile dysfunction,lack of energy, tiredness, mood changes, decreasedmuscle mass, and bone pain and fractures Physicalexamination and tests may show loss of body hair andregression of secondary sex characters, low lean bodymass and low BMD When intratesticular testoster-one decreases to a low level, spermatogenesis will beimpaired, resulting in infertility Except for the infer-tility, these clinical features ameliorate with testoster-one treatment Leydig cell numbers and volumedecrease with aging In addition the steroidogenicmachinery appears to be impaired with aging Leydigcell dysfunction associated with aging may result

in declining serum testosterone levels in older men.Androgen deficiency is treated by testosterone replace-ment therapy However, the benefits and risks must beconsidered especially in the treatment of older maleswith low serum testosterone levels

3 SPERMATOGENESIS AND SERTOLI CELL FUNCTION

Spermatogenesis is an elaborate process of cell ferentiation in which stem spermatogonia, through aseries of events, become mature spermatozoa andoccurs continuously during the reproductive lifetime ofthe individual Stem spermatogonia undergo mitosis toproduce two types of cells: regenerating stem cells anddifferentiating spermatogonia, which undergo rapid andsuccessive mitotic divisions to form primary spermato-cytes The spermatocytes then enter a lengthy meioticphase as preleptotene spermatocytes and proceedthrough two cell divisions (meiosis I and II) to give rise

dif-to haploid spermatids These in turn undergo a complexprocess of morphologic and functional differentiationresulting in the production of mature spermatozoa Theformation of spermatozoa takes place within the semi-niferous epithelium, consisting of germ cells at variousphases of development and supporting Sertoli cells Thedifferent generations of germ cells form associationswith fixed composition or stages, which constitute thecycle of seminiferous epithelium (12 in mouse, 14 inthe rat) When germ cell development is complete, themature spermatids are released from the Sertoli cellsinto the tubular lumen and proceed through the testicu-lar excurrent duct system, known as the rete testis, untilthey enter the epididymis via ductus efferens Duringpassage through the epididymis, the spermatids undergo

410 Part IV / Hypothalamic–Pituitary

a series of biochemical changes to become the motile

spermatozoa capable of fertilization

This review highlights the hormonal and genetic

control of spermatogenesis A brief overview of

testicu-lar organization, germ cell development, and cascade of

cell-cell interactions in the testis is also presented

3.1 Organization of Spermatogenesis

The general organization of spermatogenesis is

essen-tially the same in all animals and can be divided into

three main phases, each involving a class of germ cells

3.1.1 S PERMATOGONIAL P HASE

The initial phase (also known as

spermatocytogen-esis) is the proliferative or spermatogonial phase,

dur-ing which stem spermatogonia undergo mitosis to

pro-duce two types of cells: additional stem cells and

differentiating spermatogonia, which undergo rapid and

successive divisions to form preleptotene

spermato-cytes In both rat and mouse, there are three types of

spermatogonia: stem cell (Ais, or Aisolated), proliferative

(Apr, or Apairedand Aal, or Aalinged), and differentiating

[A1, A2, A3, A4, In (intermediate), and B]

spermatogo-nia The stem cells, Ais, divide sporadically to replicate

themselves as isolated entities and to produce pairs of

Aprspermatogonia The latter engage in a series of

syn-chronous divisions leading to the formation of chains of

Aalspermatogonia connected to each other by the

intra-cellular bridges The Aalspermatogonia do not divide

but, rather, differentiate into A1spermatogonia The type

A1 cells, however, divide to give rise to more

differen-tiating (A2, A3, A4, In, and B) cells In men, mostly three

different types of spermatogonia (the dark type A [Ad],

pale type A [Ap], and B type) have been identified The

Ap cells have the capacity to give rise to new Ap cells

as well as to the more differentiated B spermatogonia

and are considered to be the renewing stem cells The

Ad spermatogonia are reserve stem cells, which

nor-mally divide only rarely The precise mechanism by

which stem spermatogonia transform into

differentiat-ing spermatogonia and simultaneously renew their own

population is not known

3.1.2 M EIOTIC OR S PERMATOCYTE P HASE

The meiotic or spermatocyte phase leads to the

for-mation of haploid spermatids from young primary

sper-matocytes and is traditionally divided into five

sequential stages: leptotene, zygotene, pachytene,

diplo-tene, and diakinesis The meiotic phase involves DNA

synthesis in the youngest primary spermatocytes

(preleptotene) entering into the long meiotic prophase

and RNA synthesis in the diplotene stage Elaborate

morphologic changes occur in the chromosomes as they

pair (synapse) and then begin to unpair (desynapse)

during the first meiotic prophase These changes include(1) initiation of intimate chromosome synapsis at thezygotene stage, when the synaptonemal complex begins

to develop between the two sets of sister chromatids ineach bivalent; (2) completion of synapsis with fullyformed synaptonemal complex and occurrence of cross-ing over at the pachytene stage; and (3) dissipation ofthe synaptonemal complex and desynapsing (allowingthe chromosomal pairs to separate except at regionsknown as chiasmata) at the diplotene stage Followingthe long meiotic prophase, the primary spermatocytesrapidly complete their first meiotic division to form twosecondary spermatocytes, each containing duplicatedautosomal chromosomes and either a duplicated X or aduplicated Y chromosome These cells undergo a sec-ond maturation division, after a short interphase with noDNA synthesis, to produce four spermatids, each with

a haploid number of single chromosomes

Responding to unknown signals, type B nia divide to form young primary spermatocytes, thepreleptotene cells These cells are the last cells of thespermatogenic sequence to go through the S-phase ofthe cell cycle The morphology of preleptotene cells isvery similar to that of B cells except that the preleptotenecells are slightly smaller and have less chromatin alongthe nuclear envelope (Fig 4) The presence of leptotenecells signals the initiation of the meiotic prophase Dur-ing the leptotene phase, the chromosome appears assingle, randomly coiled threads, which thicken andcommence pairing during the zygotene phase throughthe formation of synaptonemal complex The longpachytene phase that occupies over a week in themouse commences with the completion of synapses and

spermatogo-is associated with further thickening and shortening ofthe chromosome During this phase, exchange of chro-mosomal material between maternal and paternal ho-mologous chromosomes occurs by a “crossing over,”with the chromosomes linked at such sites by chias-mata The pachytene phase is further characterized bynuclear and cytoplasmic growth, during which the celland its nucleus progressively increase in volume Asdesynapsis occurs during the next phase, known as thediplotene phase, the paired chromosomes partially sepa-rate but remain joined at their chiasmata The diplotenecells are the largest primary spermatocytes and also thelargest of any of the germ cell types Subsequently, inthe diakinetic phase, further shortening of the chromo-somes occurs, as they detach from the nuclear mem-brane Soon after this phase, the primary spermatocytesrapidly complete their first meiotic division, or meiosis

I, going through metaphase, anaphase, and telophase,during which the homologous chromosomes separateand migrate to the poles of the cell, which then splits to

Chapter 27 / The Testis 411

form two daughter cells called secondary

spermato-cytes

Thus, at the end of meiosis I, the chromosomal

complement has been reduced from tetraploid to

dip-loid and two secondary spermatocytes have formed

from one primary spermatocyte An electron

micro-graph of a secondary spermatocyte is shown in Fig 5

The mitochondria are round dispersed within the

cyto-plasm and often aggregated in small groups The Golgi

apparatus is extensive, but no proacrosomal granules

Fig 6 Portion of mouse stage XI tubule showing elongated

spermatids (ES) embedded deeply in the Sertoli cell (S) plasm.

cyto-Fig 5 Mouse secondary spermatocyte The mitochondria (M) are

round dispersed within the cytoplasm and often aggregated into

small groups The Golgi apparatus (G) is extensive, but no

proacrosomal granules characteristic of step 1 spermatids are seen.

Fig 4 A portion of the mouse stage VII tubule shows

preleptotene (PL) and pachytene (P) spermatocytes and step 7

(7) spermatids Two of the PL spermatocytes are connected by

intercellular bridges.

characteristic of step 1 spermatids are seen The ond meiotic division, or meiosis II, quickly follows,consisting of a transient interphase II with no chromo-some replication, followed by prophase II, metaphase

sec-II, anaphase sec-II, and telophase II Thus, at the end ofmeiosis II, each secondary spermatocyte gives rise totwo spermatids so that there are a total of four sperma-tids derived from the individual primary spermatocyte

3.1.3 S PERMIOGENESIS

The spermiogenic phase (spermiogenesis) involvesmorphologic and functional differentiation of newlyformed spermatids into mature spermatozoa Early inthis transformation, the Golgi apparatus packagesmaterial that initiates acrosome formation A flagellumforms from the centrioles and becomes associated withthe nucleus The nucleus progressively elongates as itschromatin condenses These elongated spermatids aredeeply embedded in the Sertoli cell cytoplasm (Fig 6).During spermiogenesis the genome is repackaged withprotamins rather than histones, which is necessary toreduce the volume of the genetic payload from the rela-tively bulky round spermatids to the streamlined sper-matozoa (compare the size of the elongated spermatids

412 Part IV / Hypothalamic–Pituitary

shown in Fig 6 with that of the round spermatids in

Fig 4) Late spermatids are released (spermiation)

almost simultaneously through the activity of the

Ser-toli cell The release of spermatids is associated with

the loss of the residual cytoplasm The process of

sper-miogenesis occurs without cell divisions, is one of the

most phenomenal cell transformations in the body, and

can be subdivided into many characteristic steps For

example, this process can be divided into 16 steps in the

mouse and 6 steps in the human

3.1.4 S TAGES OF THE S EMINIFEROUS E PITHELIUM

An intriguing feature of spermatogenesis is that the

developing germ cells form associations with fixed

composition or stages (Fig 7), which constitute the

cycle of the seminiferous epithelium (12 in the mouse

and 14 in the rat) Each stage lasts for a fixed period of

time at the end of which each germ cell type within that

stage will progress into the next stage For example, in

the Sprague-Dawley rat, the progression of stage VII to

stage VIII will take little more than 2 d The time val between the successive appearances of the samecell association at a given area of the tubule is known

inter-as the cycle of the seminiferous epithelium (which isabout 8.8 d in mice and 12.9 d in Sprague-Dawley rats).The duration of the seminiferous epithelial cycle in thehuman is about 16.0 d However in humans, unlikerodents, individual tubular profile almost always con-tains more than one cell association or stage Stages inhuman tubules may be mapped by drawing stageboundary lines among individual cell associations in across-sectioned tubule

3.1.5 S ERTOLI C ELLS

The Sertoli cells provide the fundamental tion and integrity of the seminiferous epithelium.These tall, irregularly columnar cells span the distancefrom the base of the tubule into the tubular lumen andare elaborately equipped to support spermatogenesis.The Sertoli cell nucleus is large, with its characteristic

organiza-Fig 7 Diagrammatic representation of seminiferous epithelial cycle in mouse The columns numbered with Roman numerals show

the various types of cells present at each cellular association, which are encountered in the various cross-sections of the seminiferous tubule Different types of A spermatogonia are not indicated in the cycle map mIn = dividing intermediate spermatogonium; B = B spermatogonium; Pl = preleptotene; L = leptotene; Z = zygotene; P = pachytene; Di = diakinesis; m2 0 m, dividing spermatocytes; 1–

16 = 16 steps of spermiogenesis (Reproduced from Russell et al., 1990.)

Chapter 27 / The Testis 413

tripartite nucleolus (Fig 8), and located within the

basal aspect of the cell, which rests on the basement

membrane Adjacent Sertoli cells form contacts (tight

junctions) with each other at their lateral surfaces and

near their base to effectively compartmentalize and

separate the two populations of germ cells Thus, at

each stage of the seminiferous epithelial cycle, the

germ cells are in intimate association with Sertoli cells

in a predictable fashion, with the more immature cells

(spermatogonia and young spermatocytes) located

near the basal compartment and the advanced (most

spermatocytes and spermatids) germ cells in the

adluminal compartment

The elaborate configuration and numerous processes

to encompass developing germ cells result in a much

greater surface of these cells In comparison to the rat

hepatocyte, e.g., the surface-to-volume ratio of the

Ser-toli cell is about 11 times greater than that of

hepato-cytes This high surface-to-volume ratio is reflective of

the extremely irregular shape and extensive surface

pro-cess of these cells Perhaps the most notable feature of

the Sertoli cell of many species is the

compartmental-ization of organelles within its cytoplasm This is

reflec-tive of regional functioning of the Sertoli cell in

relationship to the physiologic needs of various germ

cell types as well as polarized function/secretion of the

cell The SER is the most abundant organelle during

active spermatogenesis The rough endoplasmic

reticu-lum is relatively sparse in the Sertoli cell The

mito-chondria occupy about 6% of the Sertoli cell

cytoplasmic volume Compared with normal rats with

active spermatogenesis, Sertoli cells from the regressed

testes of hypophysectomized rats show a significant

reduction in the cell volume and surface area and

abso-lute volumes and surface areas of nearly all of their

orga-nelles

3.2 Hormonal Regulation

of Spermatogenesis

The hormonal control of spermatogenesis has been

studied for several decades since its dependence on

pitu-itary gonadotropins was first described This process is

thought to be primarily under the control of pituitary

gonadotropins, FSH, and LH (via the stimulation of

tes-tosterone), and on the interplay between Sertoli and

germ cells Sertoli cells possess receptors for both FSH

and androgen, and it is likely that these hormones exert

their stimulatory effects on Sertoli cells, which, in turn,

results in stimulation of intratubular factors for the

sur-vival of germ cells through a paracrine mechanism

However, despite the considerable attention that

hor-monal control of spermatogenesis has received to date,

the specific role and relative contribution of FSH and

testosterone on the regulation of spermatogenesis arestill debatable

3.2.1 G ONADOTROPINS AND A NDROGEN

R EGULATION OF S PERMATOGENESIS

The hormonal control of spermatogenesis has beenthe subject of numerous studies over many years Previ-ous studies have shown that quantitatively normalspermatogenesis (assessed by measurements of homog-enization-resistant advanced [steps 17–19] spermatids)can be restored by exogenous administration of test-osterone alone in adult rats made azoospermic by treat-ing them with implants of testosterone and estradiol or

by active immunization against either GnRH or LH Aseparate study reported that testosterone alone iscapable of maintaining advanced spermatid numbers inadult rats actively immunized against GnRH Theseresults of quantitative maintenance or restoration ofspermatogenesis by testosterone alone in rats in theabsence of both radioimmunoassayable LH and FSHsuggest that FSH has no effect on the regulation of sper-matogenesis in the adult rat Quantitative maintenance

of spermatogenesis has also been achieved in adultrats in which LH and FSH had been suppressed phar-macologically by a GnRH antagonist (GnRH-A) withtestosterone alone, when the testosterrone was adminis-tered at higher doses However, because testosteronesupplementation increases both the serum concentra-tions and pituitary content of FSH in GnRH-A-treatedrats, the observed quantitative maintenance of spermato-genesis in these rats cannot be attributed with certainty

to testosterone Others have further shown that matogenesis is not quantitatively restored in GnRH-immunized rats that received even the same larger

sper-Fig 8 Electron micrograph showing typical mouse Sertoli cell

nucleus (N) with its characteristic tripartite nucleolus (white N).

414 Part IV / Hypothalamic–Pituitary

amount of testosterone as used in the earlier studies and

further emphasized the need for both FSH and

testoster-one in the restoration of spermatogenesis In additional

studies, it was shown that cotreatment of testosterone

with an FSH antiserum to prevent T-induced restoration

of serum FSH levels in these GnRH-A-immunized rats

is not effective in restoring spermatogenesis This

implies the need of FSH for restoration of

sperma-togenesis in adult rats after chronic gonadotropic

sup-pression Supportive of this implication is the

demon-stration of the failure of quantitative restoration of

spermatogenesis in gonadotropin-deficient (hpg) mice

by androgens alone Similarly, in most studies of

hypophysectomized rats, spermatogenesis was not

quantitatively maintained or restored by exogenous

administration of testosterone, suggesting that FSH and/

or other pituitary hormones might be required for

com-plete regulation of spermatogenesis in this species

Clinical studies in men also suggest that both LH and

FSH are required to maintain quantitatively normal

spermatogenesis

A number of investigators have previously suggested

a definitive role of FSH on the regulation of

spermato-genesis in the adult rats under various experimental

situ-ations These studies were, however, of limited duration

(1 to 2 wk) Thus, stimulatory effects of FSH on

sper-matogenesis that are obvious after 1 or 2 wk of

gonado-tropin and/or testosterone deprivation might not become

so obvious after long-term treatment The most

defini-tive evidence, however, comes from an earlier study

that showed that replacement of recombinant human

FSH GnRH-A-treated rats fully attenuated the early

(1 wk) GnRH-A-induced reduction in germ cell

num-bers at stage VII as well as the number of advanced

(steps 17–19) spermatids and effectively prevented the

GnRH-A-induced reduction in the number of pachytene

and step 7 spermatids for 2 wk In addition, replacement

of FSH in GnRH-A-treated rats was able to increase the

number of B spermatogonia available for entry into

meiosis and maintain the number of preleptotene

spermatocytes throughout the treatment period The

ob-served beneficial effects of recombinant human FSH in

spermatogenesis in GnRH-A-treated rats are most likely

not owing to the stimulation of Leydig cell function (via

paracrine interaction between Sertoli and Leydig cells),

because the addition of FSH to GnRH-A had no

discern-ible effect on intratesticular or plasma testosterone

lev-els, accessory sex organ weight, and total volume of the

Leydig cells when compared with GnRH-A alone Mice

deficient in FSH β-subunit exhibited a striking decrease

in testis weight, seminiferous tubule volume, and

epid-idymal sperm number (up to 75%) compared with

litter-mate controls This 75% reduction in epididymal sperm

number is identical to the reported 76% decrease in thetransformation of round to elongated spermatids fol-lowing immunoneutralization of FSH in the adult rat.Thus, the reduction in epididymal sperm number inFSH-deficient mice is most likely attributed to adecrease in the number of elongated spermatids duringspermiogenesis However, the absence of any apparentfertility defect, despite a 75% reduction in the epididy-mal sperm number, in these mice suggest that there is farmore sperm produced in the adult mice than is required

to achieve fertility Moreover, in the FSH receptorknockout mice, the testes volume may be smaller but themice may still be fertile

The role of FSH in the regulation of esis in primates and humans has been documented.For example, administration of exogenous testosteroneimplants in adult macaque monkeys for 20 wk inducedazoospermia in some animals and variable degrees ofspermatogenic suppression in others Interestingly,such variability in testosterone-induced spermatoge-nic suppression was not associated with differences inresidual intratesticular androgens, LH, or inhibin B lev-els but, rather, was associated with differences in thedegree of FSH suppression between azoo- andnonazoospermic animals These results suggest thatFSH is a key factor in the maintenance of spermatoge-nesis in monkeys Testosterone treatment of healthymen suppressed gonadotropins, and intratesticular tes-tosterone also induced azoospermia in some individu-als and variable degrees of suppression in others Whenthese testosterone-treated men were supplemented with

spermatogen-LH or human chorionic gonadotropin (hCG), their matogenesis recovered qualitatively; the sperm countremained suppressed (25–50 million/mL) from the pre-treatment concentrations (75–100 million/mL) FSHsupplementation also stimulated spermatogenesis,though not quantitatively, in these men Spermatogen-esis of these men, however, was restored by the simul-taneous administration of both LH and FSH Datafrom men with a mutation in the gene encoding eitherthe FSH-R or the FSH β-subunit further provided anopportunity to evaluate the role of FSH in human sper-matogenesis Men homozygous for an inactivatingmutation of FSH receptor gene experienced variablesuppression of both spermatogenesis and fertility Menwith markedly impaired secretion of FSH caused by ahomozygous mutation in the gene for FSH β-subunithad azoospermia and severely reduced testis size Fromthese experiments of nature, it appears that FSH maynot be an absolute requirement for spermatogenesis inmen but may be necessary for quantitatively normalspermatogenesis Some other conditions/factors in ad-dition to FSH deficiency may cause the azoospermia,