I have spent my summer reading and thinking about the unlikely topic of trophoblasts. Trophoblastic cells form the layer of embryonic tissue that attaches the embryo or fetus to the wall of the mother's uterus. Trophoblasts provide protective armor by completely surrounding the embryo, while also carrying nutrients from the mother's blood to that of the developing fetus. The National Institutes of Health define trophoblast as "the extra-embryonic tissue responsible for implantation, developing into the placenta, and controlling the exchange of oxygen and metabolites between mother and embryo."

The word trophoblast means "original feeding tissue," and was so named by the Dutch embryologist Ambrosius Arnold Willem Hubrecht (1853-1915). He discovered it in the course of his study of the placenta of the hedgehog (Erinaceus europaeus). Incidentally, Hubrecht was as fascinated with hedgehogs as I am with trophoblasts. He offered to pay 25 cents for any live hedgehog that was brought to him (Richardson, 1999). At one point, as many as 40 hedgehogs a day were brought in. Out of this ‘slaughter of the innocents' came our first inkling of the existence of the peculiar trophoblastic tissue. But soon trophoblasts were identified in other mammals, including man - or, rather, woman.

Even in scientific circles, trophoblast biology is an arcane subject. I have seen puzzled expressions on the faces of many people with whom I discuss my obsession. Even many physicians look a bit embarrassed, as they desperately rack their brains for some obscure facts about trophoblast left over from medical school days.

Most Western people are vaguely aware of the placenta, or "afterbirth," which is the final act in the life drama of the trophoblast. In some cultures, the placenta is honored. According to one author, the Balinese wash the placenta in perfumed water after birth, wrap it in a cloth, and then bury it on the threshold of the family home in a carefully prepared coconut (Young 2001).

The ancient Egyptians preserved the Pharaoh's placenta in a special jar. The Japanese used to bury placentas in a cedar wood placental pot, and even today the Web site of the Osaka City Bureau of Waste Management offers to dispose of an afterbirth for 1,700 Yen (about $14 USD). Since their Web site shows a picture of an elaborate "afterbirth mound," I assume that their idea of waste management includes a suitable ritualistic burial for this neglected tissue. By contrast, most Western hospitals simply treat afterbirth as a waste product to be disposed of with the other biohazardous trash.

Perhaps our haste to dispose of afterbirth reveals some subliminal fear. In fact, one leading expert on placentas, Dame Anne McLaren of the Gurdon Institute of Cancer, Cambridge, revealed in a scientific account last year that she had "always found trophoblast rather scary." (Sadly, Dame McLaren died in an auto accident on July 10, 2007.)

Trophoblasts are unique in many ways, not least their explosive growth rate. In the mouse, for example, between days 3 and 7 after conception there is a 500-fold increase in tissue volume. This is mainly due to the power of the burgeoning trophoblast. What is more, "trophoblast is able to organize its own program of development within a well-defined time span that is independent of the embryo," according to Y.W. Loke of King's College, Cambridge.

Although the placenta comes between the mother and the developing baby, it is independent of both. It arises before the embryo - the first differentiation of the fertilized egg is into trophoblast - and it has a separate life cycle. Having done its remarkable job, it dies upon delivery of the afterbirth, while the baby (hopefully) goes forward to a long and glorious life. Both scary and autonomous, and growing at an enormous rate, it is rather like the Monster that Ate Pittsburgh.

TO BE CONCLUDED, WITH REFERENCES, NEXT WEEK.

--Ralph W. Moss, Ph.D.

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