The War on Cancer

I spent my summer reading and thinking about trophoblasts. Trophoblastic cells form the layer of embryonic tissue that attaches the embryo or fetus to the wall of the mother's uterus. Trophobtasts 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 (NIH) 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), who discovered it in the course of his study of the placenta of the hedgehog (Erinaceus europaeus). Soon trophoblasts were identified in other mammals, including man or, rather, woman. 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 cedarwood placental pot, and even today, the website of the Osaka City Bureau of Waste Management offers to dispose of an afterbirth for 1,700 Yen (about $14 USD). Perhaps our haste to dispose of afterbirth reveals some subliminal fear. One leading expert on placentas, the late Dame Anne McLaren, revealed in a scientific account that she had "always found trophoblast rather scary." Trophoblasts are unique in many ways, not least for 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 tbe 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, the placenta is rather like the Monster that Ate Pittsburgh. In the early part of the twentieth century, scientists began to notice a remarkable similarity between trophoblastic cells and cancer. It was said that if you mixed up microscope slides of both trophoblasts and cancer, you could never again tell them apart. Both tumor tissue and trophoblast are highly proliferative, migratory, and invasive, with an almost limitless ability to perpetuate themselves unless checked. The main difference between cancer and trophoblast is that trophoblast's growth is a naturally self-contained process, limited to the environment of the uterus. In rare instances, however, trophoblast can escape from these natural boundaries, and the result is choriocarcinoma, a highly malignant form of cancer that is deadly, unless treated by chemotherapy. In the vast majority of cases, the cancerlike growth of trophoblast is kept in check by a cascade of hormonal and cytokine signals. Over the past several years, there has been a stream of articles on the similarity between cancer and these trophoblastic cells of pregnancy. Here are excerpts from a few recent examples: