NATURE VS. NURTURE (on a very small scale)

Often we stumble into the middle of an argument that pits the nature of our genes against the nurture of our environment. Which is more important to the development of an individual? Which holds sway at what period of any life? Some people will claim that the nurture part is the most important. This implies that the environmental and social conditions around you determine what you become. Others claim that our nature will hold forth. This means that you are what you are and all the meddling in the world will not alter your true self. With the abundance of variations available to us for review, it is very hard to pin down one or the other of these positions as true or false. Indeed they may both be true or false or some combination of true and false.

Modern science has more or less settled on the side of nature. This in itself is to be expected, because the basis of modern science is the study of nature in the form of the universe that surrounds us. Advances in genetics, understanding the roles of genes in the molecular cascade that produces all of our biochemistry have made it obvious that a very large part of who and what we are is determined by our genes. Are we completely determined by our genes? Are we 90% determined by our genes? Are we 50% determined by our genes? No one quite yet has the answer. The complexity of the workings of our brains will require much more study and experimentation before we can say with certainty, "I am only everything that was given to me at conception." Or "I am as I was conceived and a lot more."

Our physical characteristics have been accepted as the workings of our genes. At one time in our social past, a parent may have been able to say with pride that his tall strong son was that way because he was well fed as an infant. In part this was true. If the boy were malnourished, he may not grow as big and strong as he did. But we also know that if the child did not carry the needed genes for stature, he would not grow tall. Today a proud parent might say, "I gave my child the genes to make him tall and strong, or a boy, or a girl, or to have brown eyes, etc."

Without taking sides one way or the other, let's investigate the boundary between nature and nurture. Recent studies on developing embryos have revealed some very subtle changes, as well as some drastic changes, in development that occur in response to very small environmental influences.

The life cycle for any mammal begins at conception. The male and female gametes fuse to form a fertilized egg. Under normal conditions, the egg than begins cell division, and grows until ultimately an infant mammal emerges from its mother. Although is easy to describe this process in one rather long sentence, it is a very complex series of events. Each cell division doubles the number of cells found within the embryo. Each cell division is a series of many biochemical steps. The developing embryo is washed by fluid from the mother and through her the environment. These environmental influences alter the way the embryo develops.

You may not smoke cigarettes or drink whiskey but you are perhaps familiar with the warning labels on such products. These labels make it very clear that smoking during pregnancy may damage that developing infant. Likewise, the labels on alcohol products caution us not to drink alcohol during pregnancy. The reason for these warnings is simply that the chemicals found in cigarette smoke and alcoholic beverages (primarily ethanol) are harmful to the developing embryo. Very small amounts of some of these compounds can cause irreparable damage to an otherwise normally developing embryo. Beyond the mutagenic effect of very small exposures during early embryo development there are damaging effects from chronic (long-term) exposures to many compounds. Although ethanol exposure to a very young zygote may induce birth defects, an unfortunately common outcome of a mother's drinking (alcoholic beverages) during pregnancy is a disorder called fetal alcohol syndrome. Although the developing fetus does not exhibit any major birth defects, the long-term exposure to alcohol causes the child's nervous system to malfunction. We see in the example of ethanol that the timing and dosage of the exposure can alter the outcome for an otherwise normal embryo. Likewise, there are many compounds that can and do interfere with normal embryo development.

Usually, when we speak of nature vs. nurture, we are implying that the genetic makeup of an individual is applying for expression while social and psychological pressures diminish or alter that natural genetic expression. However, in a very simple sense "nurture" is anything other than normal genetic expression. Nurture is not only the proper foods at the proper time but also the lack of proper foods. Nurture is also the presence of some toxin, cigarette smoke or alcohol that may affect the developing embryo. While the word nurture usually implies some positive outcome, we should remember that nurturing might also have negative effects.

A scientist at the Oak Ridge National Laboratory in Oak Ridge, TN was studying mutagenesis. He wanted to test the effects of altering the amount of genetic material contributed by the mother to the developing embryo. He designed an experiment that he thought would cause more or less of the mother's chromosomes to become deactivated. He was then going to observe the offspring and try to determine the effect of maternal genetic material on the offspring. Working with mice, he exposed newly fertilized eggs to a compound called ethylene oxide. Ethylene oxide is a mutagen. Mutagens are compounds that are known to cause genetic mutations. To study the effects of ethylene oxide on newly developing embryos he simply exposed female mice just after conception to small amount of ethylene oxide gas in their breathing air. He was hoping to alter the amount of genetic material contributed by the female mice to their offspring.

But that's not what happened at all. He stumbled upon some amazing results from these experiments. The developing mice embryos grew into mice that had very human-like defects. His experiment had somehow revealed a parallel between human birth defects and similar defects in mice. Since the initial exposure to ethylene oxide was very short and very small, he knew that the defects were all caused by this very brief exposure. Before his discovery it was assumed that birth defects of this nature were induced much later in a pregnancy. Now it became obvious that major birth defects were actually formulated almost at the instant of conception. And that they were the consequences of exposures to environmental mutagens.

One of the reasons that scientists use mice as experimental animals is because they are very fertile. Another reason scientists use mice, especially genetic scientists, is that mice and humans share 90% of their genomes. The reproductive cycle of a mouse is very rapid. Even a modest size laboratory can produce thousands of offspring mice in a matter of weeks. During this work with ethylene oxide exposure many embryos died early. Many defective mice were born.

The most fascinating aspect of this work however was that there were no genetic mutations even among the deformed mice. Their chromosomes where intact. The ethylene oxide, a known mutagen, had not caused genetic mutations. Yet, it had caused great changes in the developing embryos. Careful studies revealed that the damage was done to the developing zygote very early after conception. Once this damage was done, the health of the mother had very little effect on the developing embryo. An embryo that was damaged by exposure to ethylene oxide could not repair itself back to a normal structure, even if the mother was kept in the best of health.

As zygotes begin to develop, they follow a very carefully genetically orchestrated plan. The genes within their cells map each step of the zygote's development. You may think of a developing embryo as an orchestra of many different instruments. Not all instruments play the same note at the same time. Not all genes within the developing zygotes are expressed at the same time. While the zygotes are just a single cell, only certain genes need to be expressed to guide its development. Once several cells have emerged the developing embryo needs other genes to continue. If we think of an orchestra playing a symphony we can imagine groups of instruments sounding and then remaining silent while other instruments play. Each musician follows the score of the symphony. Somehow the exposure to ethylene oxide disrupted the ability of the cellular musicians to read their sheet music correctly. From that point on the music of the developing cells was just a lot of noise. Some cells tried to continue as if nothing were wrong. Cell lines destined to become a liver continued to form liver structures. The problem with this was that these liver cells might have developed outside the growing body rather than inside. This is just one example of many developmental defects that were seen during this series of experiments.

In the beginning stages of embryo development each cell has the potential to become any other cell or groups of cells. This means that a single cell in the developing embryo has the potential to become a heart, a leg, or a liver. Slight disturbances in the early stages of development can cause things to go terribly wrong. A small whiff of a gas called ethylene oxide may cause life-threatening deformities.

While we can safely say that the genetic composition of a zygote maps its adult form, we must concede that the effect of the environment can be equally important. The introduction of a chemical wrench into the genetic gears of a developing cell may kill the embryo. If it doesn't kill the embryo, it most certainly causes the embryo's development to go terribly wrong.