BISC 307L 2nd Edition Lecture 23 Current LectureThe PlacentaThe fully formed placenta – the early embryo burrows its way into the endometrium, where theblastocyst tissue(outside of embryo)have fingerlikeprojections calledchorionic villa. (lookat previous notes)As it goes, itdestroys the tissueand the embryonourishes itselffrom remnants ofthat destruction.This destructioncauses bleedinginto 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 embryoto 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. ParturitionProgesterone 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 begin to express receptors for oxytocin athigh levels. Oxytocin is a peptidehormone that is produced in thehypothalamus and stored in theneurosecretory nerve endings inthe posterior pituitary, and itdoes 2 things: 1. Stimulates contraction ofthe smooth muscle. 2. Causes uterine tissue tosynthesizeprostaglandins whichactivate the smoothmuscles, causingcontraction, and theyrelax the muscles andsoften the tissue aroundthe cervix, making it possible to dilate. So the increasing contractions of the uterus push the baby's head against the cervix, which is relaxing due to softening of its tissue. Up until now, it has been tightly closed to prevent infection. Dilation of the cervix is an early sign of labor – it can go from nothing to 10 cm.Another hormone is released by the corpus luteum and the placenta, a peptide hormone called relaxin. Relaxin causes the ligaments at the pubic symphysis in women which are usually very stiff, to relax and stretch. This opening up of the birth canal is very important. Parturition(childbirth) occurs in a positive feedback manner - the growing of the baby and the stretching of everything causes contractions. It smashes against the cervix, and triggers dilation, and activates stretch receptors in the cervix that activate the oxytocin secreting neurons in the brain. So the stretching of cervix stimulates oxytocin release from posterior pituitary, which strengthens the contraction of the myometrium, pushing the baby further, causing the cervix to stretch more, and this repeats ina a positive feedback cycle until the baby comes out. Delivery of the baby is followed by expulsion of the placenta. Fetal Circulation Fetal and adult circulation are similar, but with some significant differences. There are 3 main differences: 1. First of all, the circulatory system of the fetus includes the placenta. And as we know, that’s going to get severed at the moment of birth. But to the fetal circulatory system, the placenta is another organ that has to be supplied with blood. Where does the fetal blood come from that feeds into the placenta? The arterial blood flowing into the placenta are a branch of the illiac arteries, which feed the small intestine. Those arteries are coming off the aorta, coming off the left side of the heart (figure is on next page). This feeds into the capillaries of the fetal half of the placenta, and returns to the fetal bodyin the umbilical vein. 2. As the diagram shows, blue isdeoxygenated blood, and red is oxygenatedblood. The umbilical artery and umbilicalvein run together in the umbilical cord,which is external to the baby’s body. Bloodreturns from the placenta fully oxygenated.That blood has to get to the right side of theheart, where it rejoins the fetal circulation.It gets there by means of a fetus-only veincalled the ductus venosus, a vein thatpasses through the liver and merges withthe hepatic vein into the inferior vena cava.So that’s how the oxygenated blood getsback to the baby’s heart. But the bloodcoming out of the hepatic vein isdeoxygenated, so the merging of these two results in a purplish, partially oxygenated blood thatreturns to the right atrium of the heart. From there, it would normally pass into the right ventricle. But here, we have the second significant way that the fetal and adult circulatory systems are different - the right atrial blood bypasses the fetal lungs because they are collapsed. Because they are