Cell Differentiation and Embryonic DevelopmentBIO101 - Bora Zivkovic - Lecture 2 - Part 2 There are about 210 types of human cells, e.g., nerve cells, muscle cells, skin cells, blood cel
Trang 1Cell Differentiation and Embryonic Development
BIO101 - Bora Zivkovic - Lecture 2 - Part 2
There are about 210 types of human cells, e.g., nerve cells, muscle cells, skin cells, blood cells, etc Wikipedia has a nice comprehensive listing of all the types of human cells.
What makes one cell type different from the other cell types? After all, each cell in the body has exactly the same genome (the entire DNA sequence) How do different cells grow to look so
different and to perform such different functions? And how do they get to be that way, out of homogenous (single cell type) early embryonic cells that are produced by cell division of the
zygote (the fertilized egg)?
The difference between cell types is in the pattern of gene expression, i.e., which genes are
turned on and which genes are turned off Genes that code for enzymes involved in detoxification are transribed in lver cells, but there is not need for them to be expressed in muscle cells or neurons Genes that code for proteins that are involved in muscle contraction need not be
transcribed in white blood cells The patterns of gene expression are specific to cell types and are directly resposible for the differences between morphologies and functions of different cells.
Trang 2How do different cell types decide which genes to turn on or off? This is the result of processes occuring during embryonic development.
The zygote (fertilized egg) appears to be a sphere It may look homogenous, i.e., with no up and down, left or right However, this is not so The point of entry of the sperm cell into the egg may provide polarity for the cell in some organisms In others, mother may deposit mRNAs or
proteins in one particular part of the egg cell In yet others, the immediate environment of the egg (e.g., the uterine lining, or the surface of the soil) may define polarity of the cell.
When the zygote divides, first into 2, then 4, 8, 16 and more cells, some of those daughter cells are on one pole (e.g., containing maternal chemicals) and the others on the other pole (e.g., not containing maternal chemicals) Presence of chemicals (or other influences) starts altering the decisions as to which genes will be turned on or off.
As some of the genes in some of the cells turn on, they may code for proteins that slowly diffuse through the developing early embryo Low, medium and high concentrations of those chemicals are found in diferent areas of the embryo depending on the distance from the cell that produces that chemical.
Other cells respond to the concentration of that chemical by turning particular genes on or off (in a manner similar to the effects of steroid hormones acting via nuclear receptors, described
last week) Thus the position (location) of a cell in the early embryo largely determines what
cell type it will become in the end of the process of the embryonic development.
The process of altering the pattern of gene expression and thus becoming a cell of a particular
type is called cell differentiation.
Trang 3The zygote is a totipotent cell - its daughter cells can become any cell type As the development proceeds, some of the cells become pluripotent - they can become many, but not all cell types
Later on, the specificity narrows down further and a particular stem cell can turn into only a very limited number of cell types, e.g., a few types of blood cells, but not bone or brain cells or anything else That is why embryonic stem cell research is much more promising than the adult stem cell research.
The mechanism by which diffusible chemicals synthesized by one embryonic cell induces
differentiation of other cells in the embryo is called induction Turning genes on and off allows
the cells to produce proteins that are neccessary for the changes in the way those cells look and function For instance, development of the retina induces the development of the lens and cornea
of the eye The substance secreted by the developing retina can only diffuse a short distance and affect the neighboring cells, which become other parts of the eye.
During embryonic development, some cells migrate For instance, cells of the neural crest
migrate throughout the embryo and, depending on their new "neighborhood" differentiate into pigment cells, cells of the adrenal medula, etc.
Finally, many aspects of the embryo are shaped by programmed cell death - apoptosis For
instance, early on in development our hands look like paddles or flippers But, the cells of our fingers induce the cell death of the cells between the fingers Similarly, we initially develop more brain cells than we need Those brain cells that establish connections with other nerve cells, muscles, or glands, survive Other brain cells die.
Sometimes just parts of cells die off For instance, many more synapses are formed than needed
between neurons and other neurons, muscles and glands Those synapses that are used remain and get stronger, the other synapses detach, and the axons shrivel and die Which brain cells and which of their synapses survive depends on their activity Those that are involved in correct processing of sensory information or in coordinated motor activity are retained Thus, both
Trang 4sensory and motor aspects of the nervous system need to be practiced and tested early on That
is why embryos move, for instance - testing their motor coordination That is why sensory deprivation in the early childhood is detrimental to the proper development of the child.
The details of embryonic development and mechanisms of cell differentiation differ between plants, fungi, protists, and various invertebrate and vertebrate animals We will look at some examples of those, as well as some important developmental genes (e.g., homeotic genes) in future handouts/discussions, and will revisit the human development later in the course.
Nguồn:
http://scienceblogs.com/clock/2006/12/from_two_cells_to_many_cell_di.php