Placenta II PDF - Lecture Notes

Placenta II Prof. Dr. Abdelghany Hassan Abdelghany Professor of Anatomy and Embryology Faculty of Medicine, Alexandria University Structure of the placenta 1. The placenta has two components : fetal part (chorion frondosum) and maternal part (decidua basalis). 2. As pregnancy advances, th...

Placenta II Prof. Dr. Abdelghany Hassan Abdelghany Professor of Anatomy and Embryology Faculty of Medicine, Alexandria University Structure of the placenta 1. The placenta has two components : fetal part (chorion frondosum) and maternal part (decidua basalis). 2. As pregnancy advances, the placenta increases in size by formation of new villi, and in thickness due to the extensive branching of the villi. Structure of the placenta  Structure of the placenta 3. Spaces between the villi are called intervillous spaces. They are filled with maternal blood. The chorionic villi erode the spiral arteries and blood leaks into the intervillous spaces. 4. A number of incomplete septa arise from the decidual plate and project into the intervillous spaces, but do not reach the chorionic plate. - Thus, the maternal surface of the placenta is divided into compartments called cotyledons (spaces between the septa) but the intervillous spaces are communicating with each other. Placental membrane (placental barrier) - It separates the fetal blood in the vessels of the chorionic villi and the maternal blood in the intervillous spaces. - A number of substances (beneficial or harmful) freely cross the placental barrier while others do not cross. - Throughout pregnancy, the structure of the placental barrier changes. Early in pregnancy - Till week 20 of gestation, it is formed of four layers: a. endothelium of the fetal vessels. b. connective tissue (primary mesoderm) of the villus. c. cytotrophoblast. d. syncytiotrophoblast.  After 20 weeks of gestation - It is formed of two layers: a. endothelium of the fetal vessels. b. syncytiotrophoblast. - The connective tissue and cyotrophoblast degenerate. - The rate of exchange between the maternal and fetal blood increases. Towards the end of pregnancy - a fibrinoid material is deposited on the surface of the villi so that the barrier is formed of three layers; fibrinoid material, syncytiotrophoblast and the endothelium of the fetal blood vessels. Thus, the permeability and rate of exchange decrease. Placental circulation It is important to know that the placental barrier prevents mixing of the maternal and fetal blood but exchange of gases, nutrients, and waste products occur through the placental barrier. 1.Maternal circulation : - oxygenated blood in the spiral arteries of the endometrium is poured into the intervillous spaces to bath the chorionic villi. - then, deoxygenated blood in the intervillous spaces is drained back into the maternal blood through the endometrial veins. Placental circulation 2. Fetal circulation : - oxygenated blood laden with nutrients leaves the intervillous spaces to the embryo through the umbilical vein. - fetal deoxygenated blood leaves the fetus through two umbilical arteries to the intervillous spaces. Functions of the placenta 1. Respiration (Exchange of gases) : by simple diffusion of O2 and CO2. 2. Nutrition (Exchange of nutrients) : as water, electrolytes, as amino acids, fatty acids, carbohydrates and vitamins. 3. Passive immunity : by transmission of maternal antibodies to the foetus. 4. Excretion : fetal waste products as urea and uric acid pass to maternal blood. 5. Selective barrier (protection) against the transmission of diseases from the mother to the fetus. However, maternal infectious agents as viruses of rubella, measles, cytomegalovirus, and toxoplasma can cross the placenta causing congenital malformations or fetal death. Functions of the placenta 6. Hormone production : By the end of the 4th month, the placenta secretes the following hormones by the syncytiotrophoblasts: a. progesterone (it replaces the corpus luteum and prevents menses during pregnancy). b. estrogen. c. gonadotrophins : as human chorionic gonadotrophins (hCG), somato mammotropin, human chorionic thyrotropin, and human chorionic corticotropin. d. relaxin hormone: to soften the ligaments of the pelvis in preparation for birth of the fetus. Abnormalities of the placenta A. abnormalities of the shape : 1- bilobed or trilobed or horseshoe placenta. 2. Placenta membranacea : a thin layer of placenta attached to a large area of the uterus. B. abnormalities of the position : - Placenta previa which is diagnosed by ultrasound and delivery is by caesarian section to avoid antepartum hemorrhage.  Abnormalities of the placenta C. abnormalities of the number : 1. Accessory placenta : it is formed of a large and a small parts with one umbilical cord. The small part may be retained after shedding of the large part during labor with severe hemorrhage after labor. 2. Twin placenta : two placentae, each has its own umbilical cord. This occurs in presence of twins.  Abnormalities of the placenta D. abnormal attachment of the umbilical cord : 1. Battledore placenta (Placenta marginalis) : The cord is attached to the margin of placenta. 2. Velamentous placenta : The cord is not attached to the placenta, and the umbilical blood vessels reach the placenta by passing in the amniotic membrane. Abnormalities of the placenta E. abnormal penetration of the uterine wall : 1. Placenta accreta : due to excessive penetration of the endometrium. 2. Placenta percreta : the chorionic villi penetrate the myometrium all the way to the perimetrium (uterine peritoneal covering). - Here, the placenta fails to separate from the uterus after birth of the fetus and may cause severe postpartum hemorrhage.  Test your self - True or false : 1. In batteldore placenta, the umbilical cord is attached to the placental margin. 2- placenta percreta means that the umbilical cord is not attached to the placenta. 3- placental barrier is thin at the beginning of pregnancy. 4- cotyledons are the spaces between the villi.  Test your self - Which structure represents the maternal part of the placenta? A. Decidua marginalis. B. Decidua parietalis. C. Decidua capsularis. D. Decidua basalis.