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Name: Daniel B.
Status: Student
Age: 15
Location: N/A
Country: N/A
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 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 development.

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 organ.

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 genes.

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 layer.

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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