In zebrafish, two bmp2 genes, bmp2a and bmp2b, bmp4 and bmp7 are expressed as early as 4 h after fertiliza-tion, in the sphere stage [3–5].. Zebrafish bmp2b and bmp4 are expressed in the v
Trang 1Bone morphogenetic proteins in the early development
of zebrafish
Mariko Kondo*
Department of Biological Sciences, The University of Tokyo, Japan
Introduction
Bone morphogenetic proteins (BMPs), now widely
known for their involvement in many biological
pro-cesses, were first described for their bone
morpho-genetic activity, and thus were given their names Four
proteins were initially identified, and one of them,
BMP1 is a metalloproteinase The other three (BMP2,
3 and 4) are members of the transforming growth
fac-tor b (TGF-b) superfamily of secreted signaling
mole-cules Subsequently, molecular cloning studies have
identified more than 20 members of the BMP
sub-group in the TGF-b family, from various species
Examples of these members are decapentaplegic (Dpp)
and 60A from Drosophila, Xenopus Vg1, and BMP5-7
Although it is not known whether all of the members
of this subgroup are involved in bone differentiation,
they control a wide range of biological processes in
various cell types, such as differentiation, cell
prolifer-ation, migrprolifer-ation, and apoptosis
The signaling cascade of BMPs has been
inten-sively studied, and the players have been identified
to a great extent (Fig 1) Signals from BMPs are mediated by BMP receptors, which also comprise a gene family, the TGF-b receptor family Functionally active BMPs form dimers, which are secreted and bind to the type I and type II receptors on the cell surface These receptors are serine⁄ threonine kinase receptors with a single transmembrane domain Bind-ing of the ligand to the receptor complex induces the type II receptor to phosphorylate the type I receptor, which then leads to activation of the type I receptor The signal is passed on to the substrates of the type I receptor kinase, receptor-activated Smad proteins (R-Smads, Smad1, -2, -3, -5 and -8), which, upon phosphorylation, are activated and bind to a common mediator Smad (Co-Smad, Smad4) The complexes move into the nucleus and act as regulators of transcription The activity of BMP is regulated by the binding of extracellular inhibitors The roles and functions of BMPs in embryogenesis, from insects to mammals, mostly during the early sta-ges, have attracted the interest of many scientists In this review, I mainly focus on the recent findings using
Keywords
bone morphogenetic protein (BMP);
dorsoventral patterning; embryogenesis;
zebrafish
Correspondence
M Kondo, Graduate School of Frontier
Sciences, The University of Tokyo,
Chiba, Japan
E-mail: konmari@biol.k.u-tokyo.ac.jp
(Received 30 November 2006, accepted
27 February 2007)
doi:10.1111/j.1742-4658.2007.05838.x
Bone morphogenetic proteins (BMPs) are known to be widely involved in various biological processes Many of the members of the BMP family, as well as related factors, receptors and molecules in the BMP signaling path-way, have been isolated, but their precise functions are still unclear In addition to the ‘classical’ model organism Xenopus, zebrafish, Danio rerio,
is now considered to be a suitable model organism to study the roles of the BMP signaling pathway during embryogenesis Mutagenesis screens have identified a number of mutants in the pathway Although they do not cover the entire members of the BMP signaling cascade that are currently known, they serve as a powerful tool to broaden our understanding of BMP func-tions, in combination with other experimental techniques
Abbreviations
ADMP, anti-dorsalizing morphogenetic protein; BMP, bone morphogenetic protein; TGF-b, transforming growth factor b.
*Correction added after online publication 22 May 2007: An author name has been removed at the request of the individual.
Trang 2zebrafish, Danio rerio Zebrafish has established the
position as a major model species for early
develop-mental studies during the past decade With zebrafish,
large-scale mutagenesis screens have been performed,
strains carrying mutation for genes involved in early
development have been identified, and much
know-ledge, mostly about the function of genes and the
rela-tionships among them, is being accumulated
Members of the BMP family
in zebrafish
The roles of BMP proteins in mesoderm induction and
dorsoventral patterning have been studied in detail
using Xenopus [1] Xenopus bmp2, bmp4 and bmp7
tran-scripts are maternally expressed bmp4 is expressed in
the ventral marginal zone during gastrulation but is
excluded from the organizer region, and is considered
to play the major role in specifying dorsal–ventral struc-tures Techniques such as animal cap assays and mRNA injection, and, recently, morpholino knock-downs, have been and are the major tools for identifica-tion and funcidentifica-tional assays of BMPs Knockdowns of bmp2, bmp4 and bmp7 by morpholino injections into embryos result in mild dorsalizations, and of these three BMP proteins, BMP4 seems to be the most effective [2] Members of the zebrafish BMP family involved in early embryogenesis have been likewise identified In zebrafish, two bmp2 genes, bmp2a and bmp2b, bmp4 and bmp7 are expressed as early as 4 h after fertiliza-tion, in the sphere stage [3–5] Expression patterns of bmp2 and bmp4 in general show high conservation to those homologs of mouse and Xenopus [6,7] Zebrafish bmp2b and bmp4 are expressed in the ventral regions
in gastrula embryos, indicating the involvement in regulation of dorsoventral patterning, whereas bmp2a cannot be detected by in situ hybridization To be more precise, bmp2b is expressed in the presumptive mesoderm region exclusive of the embryonic shield but, in contrast, bmp4 is expressed at a low level in the embryonic shield (Fig 2) Therefore it is likely that zebrafish bmp2b rather than bmp4 may be functionally homologous to Xenopus bmp4 Indeed, judging from the expression pattern in the shield, which corresponds
to the Xenopus organizer, and from further studies using zebrafish mutant strains, it has been proven that bmp2b possesses ventralizing activity [8], like bmp4 of mouse and Xenopus
Zebrafish bmp7 was identified through the analysis
of the mutant snailhouse (snh) (discussed below) bmp7
is expressed also like bmp2 and bmp4, from the sphere stage on [5,9] At the sphere stage, bmp7 is expressed
Fig 1 Schematic drawing of the BMP signal transduction pathway.
Extracellular BMP dimers bind to the type I and type II BMP receptor
complexes at the cell membrane The BMP type II receptor
phorylates the type I receptor, which transduces the signal by
phos-phorylating an R-Smad protein The phosphorylated R-Smad binds
with a Co-Smad, and the complex is translocated into the nucleus,
where it activates transcription of target genes P, phosphorylations.
Fig 2 Expression domains of bmp genes and genes involved in BMP signaling Expression of mRNA at shield stage is denoted by the shaded area The drawings are based on data from whole mount in situ hybridizations [9,13–15,19] All embryos are shown from the lateral view, with dorsal to the right.
Trang 3in the entire blastoderm except for the dorsalmost
regions where the organizer will form At shield stage,
a graded expression is observed, with a high expression
in the ventral half of the embryo (Fig 2)
Anti-dorsalizing morphogenetic protein (ADMP)
[10] is another member of the BMP family The admp
transcripts are, unlike other bmps, expressed dorsally
in the zebrafish organizer region Contradictory to its
dorsal expression, overexpression induces loss of dorsal
fates, and likely acts as an antagonist of organizer
function and inhibits head formation and promotes
trunk formation [10] Although admp may
cooper-ate with bmp2b or bmp7 in establishing dorsoventral
regionalization, ADMP appears to act through a
different signaling pathway because, although the
phenotypes from overexpression of these genes are
similar at early stages, they are completely different at
later stages [11]
BMP antagonists
Dorsal development in Xenopus embryos is carried out
by secreted peptides synthesized in the Spemann
organizer, namely Follistatin, Chordin and Noggin
[12] These proteins are antagonists of BMP and other
members of the TGF-b family, binding to and
inhibit-ing these signalinhibit-ing molecules from bindinhibit-ing to their
receptors in the extracellular space, thus inhibiting
ven-tralizing activities Of these proteins, Chordin has a
long-range effect In Xenopus, expression of the chordin
gene is localized to the dorsal marginal zone, but the
protein is considered to diffuse laterally and ventrally,
in a graded manner, hence producing a gradient in
BMP activity in the marginal zone
Similarly, zebrafish chordin [13], follistatin [14] and
noggin [15] genes have been isolated These genes,
when experimentally overexpressed in the embryo,
cause dorsalization of the embryo chordin, as in
Xen-opus embryos, is expressed in the dorsal embryonic
shield [13] (Fig 2), which corresponds to the organizer
region in frog In addition, the transcripts are detected
in additional regions, such as paraxial mesoderm and
ectoderm, which is different from what is observed in
Xenopus Three noggin homologs were identified [15],
named noggin1, 2 and 3, and show different expression
patterns, but have dorsalizing activities noggin1 is the
earliest of these noggin genes to be expressed in the
embryo, and its transcripts are found in the organizer
of gastrula embryos, later in the prechordal plate and
axial mesoderm (Fig 2) By contrast, noggin2
tran-scripts are detected at the end of gastrulation in the
axial mesoderm, and noggin3 transcripts are limited to
chondrogenic regions, and are not expressed in the
organizer Nevertheless, these three gene products have similar biological activities, being able to antagonize BMPs The expressed regions combined together, cor-respond to that of single-copy noggin homologs in other vertebrates, and could be explained as reflecting the additional genome duplication and subfunctionali-zation of the genes in teleosts follistatin displays an expression pattern clearly different from Xenopus: it is not expressed in the organizer [14] It is not detected at early gastrula stages but is detected first at 60% epiboly, in dispersed presumptive mesodermal cells located in the hypoblast (Fig 2), and then the paraxial expression domain expands, although excluded from anterior axial regions This indicates that follistatin is not involved in organizer activity, different from the Xenopus homolog, but resembles the mouse homolog, which is also not present in the organizer BMP inhib-iting activities in other domains or during later stages
of dorsoventral patterning is more likely to be the bio-logical function of zebrafish Follistatin
BMP mutations affecting dorsoventral patterning
Large-scale mutagenesis screens in zebrafish has been utilized for identification of genes essential for early embryogenesis [16], and a number of mutants with defects in dorsoventral patterning have been identified Figure 3 shows examples of mutants with either dor-salized (C1–C5) or ventralized (V1–V4) phenotypes [8]
Of those mutant strains isolated from mutant screens, swirl (swr) and snailhouse (snh) [17] were identified as being mutants for bmp2b [8] and bmp7 [5,9], respect-ively The swirl (bmp2b) mutant is severely dorsalized (C5 phenotype; Fig 3), and lacks ventral structures, but the paraxial mesoderm and the neuroectoderm are expanded This demonstrates that bmp2b is required for early dorsoventral patterning BMP2 functions to maintain its expression in ventral regions during gastr-ulation, and also maintains bmp4 expression, thereby
Fig 3 Morphologies of normal (N), dorsalized (C1–C5) and ventral-ized (V1–V4) embryos, adapted from Kishimoto et al [8] V1 and C1 embryos are weakly ventralized or dorsalized, respectively, and the strongest phenotypes are V4 and C5.
Trang 4the expression is autoregulatory snailhouse (bmp7)
mutants show slightly weaker dorsalization (C4
pheno-type) compared to swirl, although the snhty68mutant is
temperature sensitive and displays a stronger
pheno-type at 33C than at lower temperatures, and the aub
mutant is a stronger allele of snh and shows the C5
phenotype Interestingly, bmp2b and bmp7 are only
required during embryogenesis, as transient mutant
embryos rescued during embryogenesis develop into
adulthood and bred [5,18] swr, snh double mutants
are not stronger in phenotype compared to single
mutants, indicating that bmp2b and bmp7 function
in the same pathway, possibly as heterodimers [9],
because the coinjection of bmp2b and bmp7 into the
same blastomere, opposed to separate injections in
adjacent cells, results in stronger ventralization of the
embryo
BMP pathway analyses using zebrafish
mutants for bmp genes
The somitabun (sbn) mutant, which is strongly
dorsal-ized, is caused by the mutation in smad5, a signaling
molecule in the BMP pathway [19] The sbntc24
muta-tion is both dominant maternal (causing strong C4
dorsalization) and dominant zygotic (weak C1
dorsal-ization) The phenotype is rescued by smad5, bmp2b
and bmp4 The early expression of bmp2b in sbn is
not affected, but that in early gastrula is severely
reduced This expression also is rescued by smad5,
showing that the autoregulatory maintenance of
bmp2b expression is mediated by smad5 The sbn
phenotype is also rescued by human smad4 RNA
injection Thus, the interaction with Smad4 could
account for the sbn mutant phenotype In addition,
the loss of smad5 activity might lead to the inhibition
of other Smads, such as Smad1, involved in the
Smad signaling pathway, and also lead to the
pheno-type Overexpression of smad1 and smad5 by mRNA
injection into wild-type embryos resulted in
ventrali-zation (causing up to V4 phenotypes), with smad1
being more effective than smad5 [20] smad5 mRNA
injection rescues sbn and snh (bmp7), but not swr
(bmp2b) By contrast, smad1 rescues swr smad1 is
expressed ventrally in gastrula as is bmp2b, and the
expression is expanded dorsally and at a higher level
by bmp2b overexpression, but is lost in swr embryos,
and strongly reduced in sbn embryos There is a
dif-ference between smad1 and smad5 expression, which
may cause the difference in function Smad5 acts very
early in development, and its mRNA is supplied
maternally and expressed throughout the embryo at a
basal level (Fig 2), whereas smad1 transcripts are
only detected from 80% epiboly As sbn and swr [17], and sbn and snh [5], interact genetically, the bmp2b and bmp7 signals should converge at the level
of Smad5 [5] Based upon these observations, a model for dorsoventral patterning in three phases is suggested [5,19,20] In the first phase, likely by mater-nal components, the organizer is induced and the ini-tial dorsoventral pattern is set up bmp2b expression
is initiated The putative dorsoventral BMP gradient
is set up in the second phase, where bmp2b expres-sion is maintained through a positive feedback by BMP2B itself and Smad5, but BMP is inhibited by Chordin from the organizer During this phase, smad1 starts to be expressed smad1 is also positively autoregulated bmp7 is also required in this phase to establish the BMP gradient In the third phase, dorsoventral patterning is carried out by BMP signa-ling, which is mediated by Smad1 By contrast to the second phase, dorsoventral patterning is independent
of Smad5 and BMP7
The BMP morphogen gradient established along the ventrolateral axis is explained to induce different cell types [18] The morphogen concentration is high
in the ventral and lower in the dorsal domain The steepness of the gradient is determined by how much BMP there is available The different concentration induces differential gene expression according to the threshold needed for induction and leads to cell dif-ferentiation Depending on the threshold, the dorso-ventral position and quantity of a certain cell type is determined In swr embryos, no gradient develops due
to the lack of bmp2b (morphogen) sbn embryos have
a low, and snh embryos have a moderate, morphogen concentration Therefore, in these mutants, the morphogen gradient is less steep than wild-type embryos, and not sufficient to induce genes at the normal position and quantity in the embryo or can-not induce these genes at all, thus resulting in mutant phenotypes
There is only one report of a mutant strain of the BMP receptor The dorsalized mutant lost-a-fin (laf) (C2 phenotype) is a mutant of the alk8, the gene cod-ing a member of the ALK1 subgroup of BMP type I receptors [21,22] This mutant could not be rescued by overexpression of bmp2b or bmp7, but could be rescued by smad5 Moreover, alk8 mRNA injected wild-type embryos developed normally, whereas the constitutive active version of the receptor induced strong ventralization (V4 phenotype) [22], resembling the bmp2, bmp4 or bmp7 overexpressed embryos [4,5,9] These show that ALK8 acts as a BMP2B⁄ 7 receptor, and that Smad5 is positioned downstream of this pathway of dorsoventral patterning
Trang 5Mutations in BMP regulators
Mutation in the BMP binding protein Chordin causes
the dino mutant [23,24] dino mutants show a
ventral-ized phenotype (V2), with a small head and a large
tail Other ventral and posterior structures are also
enlarged and gene expression is altered: for example,
gata2, which is normally expressed in the ventral half
of the animal region, is expressed much broader and
stronger The generation of dino-swirl mutant [25]
showed that swirl (bmp2b) is epistatic to dino (chordin)
This is in agreement with the function of Chordin as a
suppressor of BMP
Tolloid is a metalloprotease that acts to cleave
Chordin, and to inhibit its activity In Xenopus, it is
considered to counteract the negative regulation of
BMP by Chordin The tolloid gene product of
zebra-fish was also shown to possess Chordin cleavage
activity [26] The transcripts are detected throughout
the early gastrula stage embryo but toward the end
of gastrulation, the accumulation of transcripts
around the blastopore and expression in the ectoderm
flanking the anterior neural plate is detected mini fin
(mfn), the mutant for tolloid-like 1 (tll1, renamed
from tolloid, tld ), show a very weak dorsalized
phe-notype (C1) [17], although the allele strengths differed
[27] Analyses using swr or din mutants revealed that
tld (tll1) gene expression is positively regulated by
bmp2b and negatively by chordin In some mfn
embryos, chordin expression is expanded posteriorly
and laterally, but not always in all mutant embryos
The wide range of mfn phenotype may be caused by
the degree of chordin expression Nevertheless, Tld
(Tll1) positively regulates bmp4 and tld (tll1) (itself),
and negatively regulates chordin Taken together with
the autoregulatory expression of bmp2b and bmp4,
these factors form a transcriptional feedback
regula-tory loop
Although Tld (Tll1) is involved in regulating BMP
expression and BMP activity, mini fin mutants show a
relatively weak phenotype, and no effects on
dorsoven-tral patterning could be observed This makes it
poss-ible to think that Chordin cleavage may not be so
important, but there is evidence for the redundancy in
cleavage enzymes, and Chordin is cleaved in mfn [28]
Moreover, subdorsalizing knockdown of Tsg (twisted
gastrulation, tsg) by a low dose of tsg-MO induces a
stronger phenotype of mfn (C3), indicating that Tld
(Tll1) also functions to pattern dorsoanterior
struc-tures [29]
Twisted gastrulation (Tsg), a BMP-binding protein,
initially found in Drosophila, is involved in dorsal–
ventral patterning, The role of twisted gastrulation
is not clearly understood because there are reports
on its functions in opposite directions in the BMP pathway
The function of Tsg as a BMP antagonist is repor-ted to be conserved in Xenopus and zebrafish [30–32] Injection of antisense morpholino against tsg1 blocking the Tsg function weakly ventralizes zebrafish embryos, whereas overexpression of tsg1 mRNA dorsalizes embryos [31] Tsg inhibits BMP activity synergistically with Chordin, as shown by the enhancement of ventral features caused by coinjection of subinhibitory doses
of morpholinos against these two factors [31] Accord-ing to Ross et al [31], there are three molecular func-tions of Tsg: (a) synergistic inhibition of BMP by forming a tripartite complex between Tsg, Chordin and BMP; (b) enhancement of the Tld⁄ BMP-1 medi-ated cleavage rate of Chordin, which may change the preference of site utilization; and (c) promotion of the dissociation of Chordin cysteine-rich (CR)-containing fragments from the ligand They suggest that the first
of these three is the primary function in zebrafish The function of Tsg as a BMP agonist has also been reported [29,33] Tsg is considered to ventralize Xeno-pusembryos by blocking the CR1 domain of the Chor-din protein that blocks BMP Tsg competes with CR-1, but not full-length Chordin, to bind BMP, and releases BMP, forming a complex between BMP and Tsg [33] In zebrafish, knockdown experiments with tsg-MO resulted in moderately to moderately strong dorsalization (C3–C4) [28,29] by contrast to the previ-ous study [31] that reports ventralization Quite sur-prisingly, with higher doses of Tsg, again, embryos are dorsalized (C1–C3) [28] In the low-dosage injected embryos, chordin expression is expanded laterally [29] The dual activities of tsg was also shown from injec-tion experiments into ventralized dino or sizzled mutants, where partial rescue of the phenotype was observed upon either tsg-MO or tsg mRNA [29]
In another study, chordin mRNA injection with
tsg-MO injection resulted in an increase of rescued dino embryos compared to without tsg-MO [28] Thus, Tsg decreases the effectiveness of Chordin as a BMP inhib-itor swr heterozygous embryos, which normally are phenotypically wild-type, are dorsalized (C1–C4) with
a subdorsalizing dose of tsg-MO (less that that required for dorsalizing wild-type embryo), suggesting the interaction of Tsg with BMP2B for dorsalization [29] The ventralizing activity of Tsg is not completely Chordin dependent because, as mentioned above, the ventralized phenotype of din mutants could be partially rescued by tsg-MO injection [29]
According to the model proposed by Larrain et al [34], based mainly on studies from Xenopus, Tsg acts
Trang 6in two steps, first by binding to BMP and forming a
ternary complex with full-length Chordin, thus
inhibit-ing BMP to bind to its receptor When this complex is
cleaved by Tolloid (in the case of Xenopus, Xolloid),
Tsg is able to free BMP from the Chordin complex
and to destabilize the Chordin proteolytic products
that still possess anti-BMP activity, thus functioning to
enhance BMP signaling [34] The involvement of other
unidentified factors in this process is also suggested
[28,29,35] The debate regarding the function of tsg in
the BMP pathway probably will remain unresolved for
some time
Other factors affecting the BMP
pathway
Factors inhibiting BMP2b, other than direct binding
proteins such as Noggin and Chordin, also affect
dorsoventral patterning One example is found from
the mutant bozozok (boz) with a mutation in the
homeodomain protein coding dharma⁄ nieuwkoid gene
[36] In wild-type embryos, bmp2b is first expressed
ubiquitously in the blastoderms but just before
gastru-lation, the transcripts disappear from the dorsal side
However, in boz mutants, this clearing of bmp2b
tran-scripts does not happen, and the expression is detected
in the dorsal margin and dorsal yolk syncytial layer
[37] Misexpression of dharma mRNA reduced the
level of bmp2b expression, thereby indicating that
dharma is responsible for the bmp2b down-regulation
in pregastrula embryos The level in which dharma
regulates bmp2b is not yet clear
sizzledis the gene responsible for the ogon (allelic to
mercedes and short tail) mutant, a ventralized mutant
(V1) [38,39] swr, snh and lost-a-fin are epistatic to
ogon [40,41] sizzled is expressed in late blastula to
24 hours after fertilization embryos, located in the
ventral side of the wild-type embryo, but the
expres-sion is expanded dorsally in ogon and din, and is
down-regulated in swr embryos [38,39] This suggests
that the expression of sizzled is regulated itself in a
BMP2B-dependent manner Sizzled is a homolog of
Xenopus Secreted Frizzled, which is a putative Xwnt8
antagonist, and is a secreted protein with similarity
with the Frizzled receptor, but lacks the
transmem-brane domain However, zebrafish Sizzled does not
inhibit the function of Wnt8 [39], but rather acts as a
BMP inhibitor Although overexpressed sizzled
dorsal-izes wild-type embryos, the din mutant is not rescued
Expression of chordin is induced by Sizzled in embryos
with Chordin, but not in the absence of Chordin
These results indicate that Chordin is required for
dor-salization through Sizzled
Recently, the mechanism by which Sizzled, with BMP2b, BMP7, Chordin, Tll1, and BMP1a (a new member of the Tolloid-family metalloproteinase), con-trols dorsoventral polarity, was proposed in zebrafish [42] and similarly in Xenopus [43] The dorsalizing activity of Sizzled is Chordin dependent and, on the other hand, sizzled suppresses the ventralization by tll1 and bmp1a Sizzled stabilizes Chordin protein, whereas the Chordin-cleavage activity of BMP1a and Tll1 are suppressed Thus Sizzled, positively regulated
by BMP2b and BMP7, negatively regulates BMP1a and Tll1, which in turn, inhibit Chordin, which neg-atively regulates BMP activity If Sizzled diffuses from the ventral side where the mRNA is expressed
to the dorsal side, this mechanism could create the gradient of BMP and Chordin along the dorsoventral axis
Other aspects of BMP signaling, besides dorsoventral patterning, are reported BMP signaling is involved in endodermal patterning, and this is also regulated by the same mechanism as the dorsoventral patterning, the morphogen activity gradient, only that dorsal corres-ponds to anterior, and ventral to posterior endoderm [44] The gradient of BMP2b and Chordin define the regional expression of her5, which is an endodermal marker controlling endodermal cell patterning her5 is negatively regulated by BMP2b, which in turn, is ant-agonized by Chordin Thus similar feedback loops, perhaps by other unidentified proteins might be respon-sible for the regulation
Conclusions
The study of BMPs during embryogenesis has been extensively carried out using Xenopus but, during recent years, zebrafish has also contributed much to gaining new knowledge through the use of genetics However, in spite of extensive and thorough screening that have been done in the zebrafish system, currently, there are no reported bmp4 mutant or mutants of other many factors involved in the BMP pathway This may be explained by assuming that there is a func-tional overlap or redundancy among the different BMP-signaling genes, and that the function could be compensated by other factors or by other members of BMPs; thus, no apparent mutation could be detected
in the screens It may also be explained by the addi-tional whole genome duplication event that occurred
in teleosts
There is still much to be elucidated in the BMP pathway during embryogenesis, Many important ques-tions are unanswered, such as the how the earliest dorsoventral pattern is formed or how specific the
Trang 7threshold of BMP as a morphogen is to induce cell
fate speciation In addition, the biological functions of
some (or many) of the factors involved in modifying
BMP signaling are still undetermined or may be
vari-able depending on the situation of the cell or the
tim-ing durtim-ing embryogenesis
Nevertheless, the availability of mutant strains,
gen-etic crosses, knockout and overexpression systems
make zebrafish a very powerful tool in studying early
development and, taken together with the findings
from Xenopus, would give us a clearer view of the
BMP signaling pathway
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