Name: Daniel B.
Date: March 2004
How does a zygote become a complex mulitcellular organism
composed of many types of cells, tissues, and organ systems?
Huge question! First off, to be completely open, most of this remains a
mystery. Nevertheless we have made considerable progress since some of the
first scientists began splitting two cells apart after the first zygotic
division using human hair. We have probably made the most progress by
studying the varioius genes and proteins of those genes that control
development in model organisms like the fruit fly and then look for similar
genes and proteins in humans. It turns out that as the organism develops
through all the stages before and after birth, various sets of genes are
turned on and off to control the differentiation of cells into specialized
cells of the different tissues, organs and organs systems. We do have an
idea of some of the iumportant sets of genes that control some of these
processes but my estimate is that it will be many decades before we can
accurately predict the outcomes of development when certain genes do and do
not function properly...science being measured as the ability to accurately
predict something risky.
Isn't it amazing? This is under genetic control. Genes called homeobox, or HOX genes,
control development. These are actually genes for transcription factors-proteins that
turn genes on and off. They lay out the basic plan of the body by acting as master
switches for whole sets of genes. They determine where your head will go, where you heart
will go, etc. They also control genes for cell signaling such as hormones, that signal
other cells to turn on genes. So its kind of a cascade of one gene turning on another,
which signals another to do something, etc.
Answer: The growth of a zygote into a new complex organism is the most
amazing miracle, and is the reason I find genetics to be such an exciting
and fascinating subject.
Using humans as the example, the zygote is one cell containing 46
chromosomes, or 23 pairs. It contains two entire copies of the genome, and
these work together as the complete "computer program" for the rest of
Some genes make a protein gene product - either a structural protein that
becomes a part of the body structure, or a functional protein such as an
enzyme. Some genes instruct other genes when to switch on and off and so,
when to make or stop making their product. Sometimes the product of a gene
is a receptor molecule situated at the surface of a cell membrane; another
gene situated elsewhere on the chromosomes might code for an enzyme or
hormone that is active when it binds to the receptor. When the receptor is
faulty, the baby might not be able to develop correctly. Some genes instruct
cells to migrate, or physically move about in the developing embryo. Some
genes instruct a cell to grow into an eye cell or muscle cell or nerve cell
once it has migrated to the right position in the embryo, and to cooperate
with other cells in growing into an entire eye or brain or heart or other
How these cells "know" where to go and when to stop their journey and what
to do next is astonishing. The instructions are all present already in the
Some genes are responsible for essential functions needed in every cell of
the body, such as cellular respiration that enables cells to derive energy
from nutrients. These are called housekeeping genes.
Other genes have more specialized functions, and are active only at certain
times or only in certain places in the body. The genes for eye color produce
colored pigment molecules only in the cells of the iris, even though
(almost) all the cells of the body contain these genes. The genes that
instruct a cell to divide are active only at the time of cell division. How
does a cell "know" that it is time for the chromosomes to duplicate to each
become two chromatids, and later to coil up and condense, and then to
proceed with the rest of mitosis?
The genes responsible for these actions have some way of being turned on and
off in response to the presence of chemical or spatial or other stimuli,
caused either by external environmental factors or internal biochemical
circumstances. A cell might respond to the fact that it is completely
surrounded by other cells and is no longer at the surface or in the top
One of the ways scientists have studied embryology has been by studying what
goes wrong sometimes, in infants with birth defects.
The medicine thalidomide was used in the 1960s and was found to be dangerous
for developing fetuses. Thousands of children around the world were born
with missing or malformed arms and legs. By studying the pharmacological
action of thalidomide, we were able to learn something about how limbs
usually do develop correctly. We learned that thalidomide's action relates
somehow to blocking off the blood supply, so now we know that the normal
development of arms and legs depends on having adequate blood circulation
through the arteries and veins.
Although thalidomide was banned for many years, it is now being actively
researched again in carefully controlled situations where research subjects
or patients are for sure not pregnant. It seems it may be useful for helping
improve some of the symptoms in patients with a variety of diseases,
including cancer and AIDS.
The more we know about the development of an embryo and more scientific
explanations we uncover, the more miraculous it seems to me. Enjoy your
studies, and never lose your sense of wonder!
Sarina Kopinsky, MSc, H.Dip.Ed.
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Update: June 2012