BISC 307L 1st Edition Lecture 22 Current Lecture The Placenta o o Early embryo burrows its way into the blastocyst tissue which has finger like projections called chorionic villae and as it goes it destroys the tissue and nourishes this causes bleeding which doesn t clot because the chorionic villae produces anticoagulants pools of blood that the blastocyst tissue is invading and this causes the exchanging of blood between the embryo and the mother develops the circulatory system which develops partly into placenta o Maternal blood is bleeding into these spaces exchange of nutrients across the placenta o Exception to the rule that vertebrates have a closed circulatory system o Chorion secretes chorionic gonadotropin which prolongs the life of the corpus luteum mimics LH o Essential in the first 2 months o BY the 3rd month the placenta itself starts secreting progesterone and estrogen but the first two months rely on the corpus luteum Fetal Circulation o o While the fetal circulatory system is developing All the parts are there but different in some ways 1 In utero has the placenta included arterial blood of fetus flows through the iliac arteries and goes to placenta and returns through umbilical vein fully oxygenated That blood gets to the right side of the heart by means of the 1 DUCTUS VENOSUS fetus only passes through the liver and merges with the hepatic vein deoxygenated and this merging is partially oxygenated blood to the right atrium 2 The right atrial blood bypasses the fetal lungs since developing and collapsed and blood vessels compressed most goes directly to the left atrium because there is the hole called the foramen ovale or can go into right ventricle to aorta but this is high resistance and not as likely over course of fetal life the right side of the heart gets stronger because there is more pressure from left atrium goes to left ventricle and then aorta and out For blood that makes it into the right ventricle there is an extra artery called the ductus arteriosus which pushes the blood to the aorta as well 3 Fetal hemoglobin has a different subunit composition than adult human and has higher affinity for oxygen than adult hemoglobin this way it can get as much oxygen from maternal blood as possible when going through placenta o When baby is born this whole circulation has to switch in the matter of minutes what happens The lungs will expand with the first few breaths 1 Inflation causes decrease in vascular resistance in the lungs and blood will start flowing in the lungs 2 Blood will be oxygenated in the lungs so the blood will return with more oxygen than ever experienced there will be a drop in the pressure on the right side so the higher pressure on the right atrium is no longer there and this now makes it so the left side pressure goes up causing an opposite pressure gradient one way valve that allows blood flow from right to left but not from left to right which quickly grows and becomes the septum from the foramen ovale The ductus arteriosus is highly sensitive to oxygen and when blood is fully oxygenated it causes strong constriction of this artery causing it to seal off and eventually disappear The ductus venosus also constricts causing less blood returning from the placenta don t cut immediately and then the umbilical cord can be severed All these changes happen within seconds or a minute or two o Problems Failure of the foramen ovale PDO hard to fix and ductus arteriosus easier to fix to close PDO associated with increase in blood clot formation Parturition really should be slide 8 o o high Progesterone secreted by placenta suppresses the activity of the uterine muscle o during the last trimester the uterine muscle becomes more excitable multi unit to single unit muscle o Stretch sensitive ion channels in uterine muscle o So as this happens the fetus is growing and stretching the uterus causing reflex uncoordinated contractions uncoordinated because not smooth transition of muscle types o Smooth muscle cells start expressing receptors for oxytocin produced in hypothalamus which causes stimulation of contraction in smooth muscle and it causes the uterine tissue to synthesize prostaglandins which activate smooth muscle and soften the tissues around the cervix making it possible to dilate o Increasing contractions of uterus are pushing the baby s head against the cervix which is relaxing and this combination causes dilation of cervix early sign of labor o Relaxin released also peptide hormone which causes the ligaments at the pubic symphysis to soften making it possible to open the birth canal o Parturition is everything we have talked about acting in a positive feedback way Stretch receptors in the cervix that feed into the brain and activate the oxytocin secreting cells to secrete oxytocin from posterior pituitary which prolongs strength of the contraction o Delivery of baby is followed by the expulsion of placenta Lactation o o Two important structures 1 Milk ducts care milk to opening 2 Alveoli bottom made up of milk secreting epithelium and surrounded by smooth muscle cells milk synthesized and stored o Before puberty there isn t much of this o During puberty Rise in estradiol stimulates growth of milk ducts but little growth of the alveoli Alveoli needs Progesterone in order to grow second half of ovulation you have high progesterone so women s breasts swell a little bit Estrogen that stimulates growth of glandular tissue and the progesterone remain high in pregnant women so the breasts grow Two hormones necessary to stimulate the final growth and milk secretion 1 Prolactin from Ant pit high at end of pregnancy 2 Fetus secretes placental lactogen similar to prolactin Breasts do not produce milk in pregnancy because inhibited by high levels of estradiol and progesterone At birth No more placenta no more secretion of estrogen and progesterone levels fall Inhibition of milk production relieved and milk is produced Positive feedback cycle Touch receptors on nipple that connect to the hypothalamus and the post pit In hypothalamus this increases the secretion of PRH and decrease dopamine prolactin release inhibiting hormone increases prolactin secretion in ant pit stimulates milk synthesis in alveoli breasts swell In post pit there will be oxytocin secretion causing contraction of the myoepithelial cells causing milk ejection AS long as breast feeding continues then this whole process happens for left hand side it will still be stimulated Stops when the