Cell 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.
How 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

sensory 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.
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