Reproductive Figs(7 pages)
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- Lecture number:
- Lecture Note
- University of Southern California
- Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 23 Current Lecture The Placenta The fully formed placenta – the early embryo burrows its way into the endometrium, where the blastocyst tissue (outside of embryo) have fingerlike projections called chorionic villa. (look at previous notes) As it goes, it destroys the tissue and the embryo nourishes itself from remnants of that destruction. This destruction causes bleeding into the space that is created by the chorionic villi. However, the blood does not clot because the chorionic villi produces potent anticoagulants. So you end up with pools of sinus blood in the endometrium, into which the blastocyst tissue of the embryo is invading. Once this forms, you begin to have exchange of nutrients and waste products and gasses between maternal blood and the embryonic tissue. This continues to develop – one of the first parts of the embryo to develop is the circulatory system. And part of the circulatory system is this organ that is external to the fetus’ body, which is half of the placenta. Looking at the bottom right of the figure, you can see that half of the placenta is fetal tissue, and there are loops of blood vessels (arteries/arterioles, and capillaries) and the tufts of fetal capillaries on these tissues – this is the fully formed placenta. But you still see the pools of maternal blood here, on the right that are bleeding into these spaces. Anticoagulants on the surfaces of these chorionic fingers prevent the clotting, so we have exchange of nutrients across the placenta between maternal blood and fetal blood. This is one situation in which the circulatory system is open instead of closed as it normally is. The chorion also secretes some hormones. One is chorionic gonadotropin, an LH type molecule, and the chorionic gonadotropin will prolong the life of the corpus luteum. It will mimic LH in that action, and prolong the life of the corpus luteum for the first few months, because the estrogen and progesterone it pumps out is important. By 2.5-3 months in the placenta itself produces these hormones, and the corpus luteum is no longer as important. Those gonadosteroids are necessary to maintain the uterus. Parturition Progesterone suppresses the activity of the uterine muscle in the myometrium(middle layer). But during the last trimester, the uterine muscle becomes more excitable - it has more voltage- gated channels. It also switches from being a multiunit to a single unit muscle. And the ion channels in this muscle are stretch sensitive ion channels, which open and let in a depolarization when it gets stretched. The fetus is constantly growing and stretching the uterus, and will cause reflex contractions by the myometrium. These are uncoordinated because the gap junction conversion to single unit muscle cells are not completed yet. These uncoordinated contractions are called Braxton-Hicks, and they occur throughout the entire pregnancy but you don’t really feel them until the second half of pregnancy. Near the end of pregnancy, another change also happens in the smooth muscles- they ...
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