"We are going to talk now about an organ that my next guest believes does not receive the respect it deserves. It's the placenta: that mysterious connection between mother and fetus that is both indispensable and until recently considered disposable. Dr. Susan Fisher is a professor of obstetrics and gynecology and reproductive sciences at the University of California San Francisco. We learned about her story in The New York Times, in which her descriptions of the placenta just jumped off the page. So we had to call her up."
Lewis: You were recently featured in a New York Times article that really spoke of the placenta with awe. How well have experts explored the placenta?
Dr. Fisher: I'm glad you picked up the "awe," because I think it is an awe-inspiring organ. And I think "awe" is a good way to describe what we know presently. We know that it's important. We know that the placenta is required to establish pregnancy. And then for pregnancy to go normally. But we really know very little about this organ compared to the permanent organs of our body. For example, the liver, the heart, the stomach.
Lewis: Why has there been so little interest in the placenta?
Dr. Fisher: Yeah, I think that's an interesting question. One reason is that it has a 9-month lifespan. So, we are all invested in the biology of the heart, because without the heart we wouldn't be alive. But I think we all forget that without this organ, the placenta, we wouldn't be alive either. Just because it lives for nine months of pregnancy and then we leave it behind, doesn't mean that it isn't equally important as the heart.
Lewis: I guess if you could it the afterbirth, you've already kind of dismissed it. Haven't you?
Dr. Fisher: You have already relegated it to a very unimportant function. And I also think the reason is because the structure is quiet complex.
Lewis: What function does the placenta serve for a growing fetus?
Dr. Fisher: So the specialized cells of the placenta are called trophoblasts. They are generated within about four to five days of the embryo being developed. Those trophoblasts have many types of functions. Their first function is to mediate adhesion to the uterus, thus implantation. So following implantation, their job is to attach the developing embryo to the uterine wall. And this is a very invasive process in humans, in which the embryo burrows very far into the uterine wall and taps a blood supply much like a cancer cell would. There's a mechanical roll that a trophoblast and placenta plays in anchoring the embryo and fetus to the uterine wall and tapping a blood supply. There's another extremely important set of functions that the placenta plays, and those include acting as pseudo adult organs for the embryo fetus in the development. So, it's the placenta that brings nutrients and oxygen and all kinds of valuable substances to the embryo and fetus so that it's able to grow. And like the kidneys, it also exchanges waste material back into maternal blood. There's another super important function, and that somehow by means that are really very poorly understood, the placenta is able to transplant the baby to the mother for the duration of pregnancy. And what I mean by that is the baby has half the genes of the father. So, by all the laws that we know of the organs, such as kidneys, the baby should be rejected by the mother's immune system. And this important function is also attributed to the placenta.
Lewis: So, do you think by studying the placenta, and really understanding how the placenta works like a parasite or a cancer that it's the embryo cells that create the placenta, that you might be able to understand more about cancer or more about immune rejection?
Dr. Fisher: Yes, very much. And, in fact, this is one of the reasons why we are so interested in the placenta. Obviously it has its own very interesting functions and we study it primarily because of our desire to make pregnancy a better process for women and children and families. But in doing so, we've also found some very interesting principles that cancer researchers now have found that apply in that field. For example, the placenta is able to target blood vessels for invasion very much like cancer cells and to tap a maternal supply of blood. In doing so, the placental cells mimic cells of blood vessels. So, this mimicry, which we found happens during trophoblast invasion of the uterus, has now been found to occur in some kinds of cancers; for examples, melanomas.
Lewis: What mysteries of this organ fascinate you the most? It sounds like everything fascinates you, but you have to pick a few for more.
Dr. Fisher: I think one of the most interesting mysteries of the placenta, and what perennially fascinates graduate students who come to our lab, is the question of why the organ isn't rejected by the mother. If you transplanted a kidney from a baby to a mother, that kidney, without a great deal of pharmacological intervention, would be rejected. But when the placenta, with essentially the same genes, is transplanted to the mother during pregnancy is not rejected. So, you can imagine, this is one of the most fascinating biological questions that remain unanswered today. And we are extremely interested in explaining this. There would be enormous explanations for improving pregnancy outcomes for families, and for also using the strategies that the placenta has so successfully developed to transplant itself to transplant other organs. So, we envision that this replicating this natural process in the setting of transplanting other organs could lead to transplantations without dampening the immune system without using pharmacological means.