Chapter 3: Figure and Ground
46 - Understanding Eggs (Part One)
There may be no more magical transformation than that of information from microscopic DNA unfolding to become an adult living thing. If there’s any place where nature is telling us something fundamental, it would seem to be there. With that in mind it’s hard to overemphasize the importance of the great debate about how embryos develop—not only for biology and evolution theory, but for any serious effort to draw lessons from nature.
In the classical view, the information needed to create an adult organism was contained entirely within its embryo’s nuclear DNA. This view held that the nucleus developed much as Manaus and its population have traditionally interacted with the Amazon basin—which is to say without much in the way of feedback from its surroundings. Embryo growth and development were thought to be just the mechanical expression of a predetermined plan.
During the past century and more, from the time of August Weismann on, this view had great authority. It gained influence by its correspondence to the genetic “central dogma,” the belief that evolution unfolds from a DNA that is inviolate and unperturbed by feedback from without. In this view, only mutations in an embryo’s nuclear DNA, or the genetic combining of traits from multiple parents, can introduce variations into evolution. In other words, genetic information flows only one way—from the center outward.
But by the end of the twentieth century it had become hard for champions of that view to ignore the mounting evidence that there were other factors involved. New studies show that a growing nucleus is also shaped—and in profound ways—by forces outside an embryo’s nuclear DNA, that it is shaped by exclusively maternal forces that act on it through the cytoplasm of the egg.
Hard-nosed captains of industry as well as admirers of new age goddesses may both prefer not to hear that the maternal egg is nature’s finest example of capitalism at work. An egg is a single cell, swollen immensely by an accumulation of nutrients in its cytoplasm (which fills up the space between an egg’s DNA-packed nucleus and its outer membrane or shell). The nutrients found there are drawn from throughout the mother’s body and are meant to be invested in the growth of an embryo.
But the egg does more than accumulate capital for a new venture. Factors in the cytoplasm also shape a developing embryo from the moment of conception. Far from being just an expression of the plan found within its own nuclear DNA, the eventual form an embryo takes is the product of constant interaction between that DNA and the surrounding character and constraints of the egg.
One effect transmitted this way comes via mitochondria. They were once free-roaming bacteria that had the neat trick of using oxygen and hydrogen to generate a cold chemical “fire” that produces surplus energy. Long ago they took up residence in the cells of larger organisms, and ever since have paid the rent by supplying energy that makes complex life forms possible. Because of this, they are densely packed into the cells of organs where energy requirements are high, such as our eyes, pancreas, liver, nerves, and muscles. Mitochondria make up one-third of the total volume of a typical heart muscle cell.
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