(2 of 8)
The other reason we know so much about what goes on inside the womb is the remarkable progress researchers have made in teasing apart the sequence of chemical signals and switches that drive fetal development. Scientists can now describe at the level of individual genes and molecules many of the steps involved in building a human, from the establishment of a head-to-tail growth axis and the budding of limbs to the sculpting of a four-chambered heart and the weaving together of trillions of neural connections. Scientists are beginning to unroll the genetic blueprint of life and identify the precise molecular tools required for assembly. Human development no longer seems impossibly complex, says Stanford University biologist Matthew Scott. "It just seems marvelous."
How is it, we are invited to wonder, that a fertilized egg--a mere speck of protoplasm and DNA encased in a spherical shell--can generate such complexity? The answers, while elusive and incomplete, are beginning to come into focus.
Only 20 years ago, most developmental biologists thought that different organisms grew according to different sets of rules, so that understanding how a fly or a worm develops--or even a vertebrate like a chicken or a fish--would do little to illuminate the process in humans. Then, in the 1980s, researchers found remarkable similarities in the molecular tool kit used by organisms that span the breadth of the animal kingdom, and those similarities have proved serendipitous beyond imagining. No matter what the species, nature uses virtually the same nails and screws, the same hammers and power tools to put an embryo together.
Among the by-products of the torrent of information pouring out of the laboratory are new prospects for treating a broad range of late-in-life diseases. Just last month, for example, three biologists won the Nobel Prize for Medicine for their work on the nematode Caenorhabditis elegans, which has a few more than 1,000 cells, compared with a human's 50 trillion. The three winners helped establish that a fundamental mechanism that C. elegans embryos employ to get rid of redundant or abnormal cells also exists in humans and may play a role in AIDS, heart disease and cancer. Even more exciting, if considerably more controversial, is the understanding that embryonic cells harbor untapped therapeutic potential. These cells, of course, are stem cells, and they are the progenitors of more specialized cells that make up organs and tissues. By harnessing their generative powers, medical researchers believe, it may one day be possible to repair the damage wrought by injury and disease. (That prospect suffered a political setback last week when a federal advisory committee recommended that embryos be considered the same as human subjects in clinical trials.)