What Scientists Know About the First Nine Months

What scientists have learned about those amazing first nine months--and what it means for mothers

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    Many other genes interact with the HOX system, including the aptly named Hedgehog and Tinman genes, without which fruit flies grow a dense covering of bristles or fail to make a heart. And scientists are learning in exquisite detail what each does at various stages of the developmental process. Thus one of the three Hedgehog genes--Sonic Hedgehog, named in honor of the cartoon and video-game character--has been shown to play a role in making at least half a dozen types of spinal-cord neurons. As it happens, cells in different places in the neural tube are exposed to different levels of the protein encoded by this gene; cells drenched in significant quantities of protein mature into one type of neuron, and those that receive the barest sprinkling mature into another. Indeed, it was by using a particular concentration of Sonic Hedgehog that neurobiologist Jessell and his research team at Columbia recently coaxed stem cells from a mouse embryo to mature into seemingly functional motor neurons.

    At the University of California, San Francisco, a team led by biologist Didier Stainier is working on genes important in cardiovascular formation. Removing one of them, called Miles Apart, from zebra-fish embryos results in a mutant with two nonviable hearts. Why? In all vertebrate embryos, including humans, the heart forms as twin buds. In order to function, these buds must join. The way the Miles Apart gene appears to work, says Stainier, is by detecting a chemical attractant that, like the smell of dinner cooking in the kitchen, entices the pieces to move toward each other.

    The crafting of a human from a single fertilized egg is a vastly complicated affair, and at any step, something can go wrong. When the heart fails to develop properly, a baby can be born with a hole in the heart or even missing valves and chambers. When the neural tube fails to develop properly, a baby can be born with a brain not fully developed (anencephaly) or with an incompletely formed spine (spina bifida). Neural-tube defects, it has been firmly established, are often due to insufficient levels of the water-soluble B vitamin folic acid. Reason: folic acid is essential to a dividing cell's ability to replicate its DNA.

    Vitamin A, which a developing embryo turns into retinoids, is another nutrient that is critical to the nervous system. But watch out, because too much vitamin A can be toxic. In another newly released book, Before Your Pregnancy (Ballantine Books), nutritionist Amy Ogle and obstetrician Dr. Lisa Mazzullo caution would-be mothers to limit foods that are overly rich in vitamin A, especially liver and food products that contain lots of it, like foie gras and cod-liver oil. An excess of vitamin A, they note, can cause damage to the skull, eyes, brain and spinal cord of a developing fetus, probably because retinoids directly interact with DNA, affecting the activity of critical genes.

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