Embryology - The Placenta PDF

# Embryology - The Placenta ## Vision Colleges ### Saeed Vohra MD., PhD [email protected] ## Objectives At the end of this lecture, the students will be able to: - Describe the mature placenta and mention its functions and anomalies. - Describe the fate of the placenta. ## The Placenta - T...

# Embryology - The Placenta ## Vision Colleges ### Saeed Vohra MD., PhD [email protected] ## Objectives At the end of this lecture, the students will be able to: - Describe the mature placenta and mention its functions and anomalies. - Describe the fate of the placenta. ## The Placenta - The placenta is the primary site of nutrient and gas exchange between the mother and fetus. - It has two components: 1. A fetal part that develops from the chorionic sac, the outermost fetal membrane. 2. A maternal part that is derived from the endometrium, the mucous membrane comprising the inner layer of the uterine wall. - It has 2 surfaces: 1. Fetal surface (smooth) - Covered by amnion - Provides attachment to the umbilical cord. 2. Maternal surface (rough & irregular) - Presents 15-30 cotyledons which are limited by fissures. ## Measurement (at full term) - Diameter - 15 to 25 cm. - Thickness – 3 cm. at the center - Weight - 500 gm - Surface area – 14 square meters. ## Normal site of attachment - Near the fundus. ## Fundus of uterus - The uterus, chorionic sac, and placenta enlarge as the fetus grows. Growth in the size and thickness of the placenta continues rapidly until the fetus is approximately 18 weeks old. - The fully developed placenta covers 15% to 30% of the decidua and weighs approximately one sixth that of the fetus. ## Functions of Placenta - The placenta and umbilical cord form a transport system for substances passing between the mother and fetus (connect the placental circulation with the fetal circulation). - Nutrients and oxygen pass from the maternal blood through the placenta to the fetal blood. - Waste materials and carbon dioxide pass from the fetal blood through the placenta to the maternal blood. - The placenta and fetal membranes perform the following functions and activities: protection, nutrition, respiration, excretion, and hormone production. - Shortly after birth, the placenta and fetal membranes are expelled from the uterus. - Functions of placenta: - Protection - Nutrition - Respiration - Excretion - Hormone production - Nutrients and O₂ Pass From maternal blood to fetal blood through the placenta. - Waste and CO₂ From fetal blood to maternal blood through the placenta. ## Functions of Placenta - Exchange of metabolic & gaseous products between maternal & fetal blood streams. - Exchange of gases: O₂ is supplied (25ml/per minute) & CO₂ is removed from fetal blood. - Exchange of nutrients (carbohydrates, lipids, polypetides, amino acids & vitamins) & electrolytes. - Transmission of maternal antibodies (IgG, immunoglobulins). - Production of hormones: Progesterone, Estrogen (mainly estriol), relaxin, placental lactogen and HCG. - Removal of waste products (urea, CO₂ & other waste products). - Protective barrier: Placental membrane protects the fetus as maternal and fetal blood don't mix (prevent antigenic reactions). - These comprehensive activities are essential for maintaining pregnancy and promoting normal fetal development. ## Placental Development - Early placental development is characterized by the rapid proliferation of the trophoblast and development of the chorionic sac and chorionic villi. - By the end of the third week, the anatomic arrangements necessary for physiologic exchanges between the mother and embryo are established. - A complex vascular network is established in the placenta by the end of the fourth week, which facilitates maternal-embryonic exchanges of gases, nutrients, and metabolic waste products. - Chorionic villi cover the entire chorionic sac until the beginning of the eighth week. - As this chorionic sac grows, the villi associated with the decidua capsularis become compressed, reducing the blood supply to them. These villi soon degenerate, producing a relatively avascular bare area, the smooth chorion (chorion laeve). - As the villi disappear, those associated with the decidua basalis rapidly increase in number, branch profusely, and enlarge. This bushy area of the chorionic sac is the villous chorion (chorion frondosum) villous. - The fetal part is formed by the villous chorion. The chorionic villi that arise from the chorion project into the intervillous space containing maternal blood. - The maternal part of the placenta is formed by the decidua basalis, the part of the decidua related to the fetal component of the placenta. - By the end of the fourth month, the decidua basalis is almost entirely replaced by the fetal part of the placenta. - The fetal part of the placenta (chorionic villi) is attached to the maternal part of the placenta (decidua basalis) by the cytotrophoblastic shell. - Endometrial arteries and veins pass freely through gaps in the cytotrophoblastic shell and open into the intervillous space. - The shape of the placenta is determined by the persistent area of chorionic villi. Usually this is a circular area, giving the placenta a discoid shape. As the chorionic villi invade the decidua basalis, decidual tissue is eroded to enlarge the intervillous space. - This erosion produces several wedge-shaped areas of decidua, the placental septa, that project toward the chorionic plate. - The placental septa divide the fetal part of the placenta into irregular convex areas-cotyledons. - Each cotyledon consists of two or more stem villi and their many branch villi. - By the end of the fourth month, the decidua basalis is almost entirely replaced by the cotyledons - The decidua capsularis contacts and fuses with the decidua parietalis on the opposite wall, thereby slowly obliterating the uterine cavity. ## Placental Circulation - The intervillous spaces of a mature placenta contain approximately 150 ml of blood coming from 80-100 spiral arteries, which is exchanged about 3 or 4 times per minute. - Fetal blood arrives to the placenta by 2 umbilical arteries that branch to form capillaries in the villi where gaseous exchange occur in with maternal blood in intervillous space. Finally oxygenated blood returns to fetus via umbilical vein. - The intervillous space of the placenta, which contains maternal blood, results from lacunar networks that developed in the syncytiotrophoblast during the second week of development. - The intervillous space of the placenta is divided into compartments by the placental septa; however, there is free communication between the compartments because the septa do not reach the chorionic plate. - Maternal blood enters the intervillous space from the spiral endometrial arteries in the decidua basalis. The spiral arteries pass through gaps in the cytotrophoblastic shell and discharge blood into the intervillous space. This large space is drained by endometrial veins that also penetrate the cytotrophoblastic shell. Endometrial veins are found over the entire surface of the decidua basalis. The numerous branch villi-arising from stem villi-are continuously showered with maternal blood that circulates through the intervillous space. The blood in this space carries oxygen and nutritional materials that are necessary for fetal growth and development. - The maternal blood also contains fetal waste products such as carbon dioxide, salts, and products of protein metabolism. - The amniotic sac enlarges faster than the chorionic sac. As a result, the amnion and smooth chorion soon fuse to form the amniochorionic membrane. - This composite membrane fuses with the decidua capsularis and, after disappearance of the latter, adheres to the decidua parietalis. - It is the amniochorionic membrane that ruptures during labor. - Preterm rupture of this membrane is the most common event leading to premature labor. When the membrane ruptures, amniotic fluid escapes through the cervix and vagina to the exterior. ## Umbilical Cord - Consists of the following: - Two Umbilical arteries (medial umbilical ligament). - One Umbilical vein (ligamentum teres). - Wharton's jelly surrounding them. - Distal part of the allantoic diverticulum (Urachus). - Vitello-intestinal duct (Meckel's diverticulum). - The attachment of the umbilical cord to the placenta is usually near the center of it's fetal surface, but it may attach at any point. ## Cord Abnormalities - Long cords have a tendency to prolapse and/or to coil around the fetus. The cord may be compressed between the presenting body part of the fetus and the mother's bony pelvis, causing fetal hypoxia or anoxia. If the deficiency of oxygen persists for more than 5 minutes, the baby's brain may be damaged. dead. - A very short cord may cause premature separation of the placenta from the wall of the uterus during delivery. - The umbilical vessels are longer than the cord, twisting and bending of the vessels are common. They frequently form loops, producing false knots that are of no significance; however, in approximately 1% of pregnancies, true knots form in the cord, which may tighten and cause fetal death resulting from fetal anoxia. ## Anomalies of Placenta - **Placenta Praevia** (name of condition?): The blastocyst implants close to or overlying the internal OS of the uterus (Centralis, marginalis, & lateralis). - **Placenta accrete**: Abnormal adherence of chorionic villi to the myometrium(normal labor, placenta fails to separate after birth). - **Placenta percreta**: Chorionic villi penetrate the full thickness of the myometrium(bleeding third trimester). - **Batteldore placenta**: Cord is inserted at periphery of the placenta. The attachment of the cord to the fetal membranes is termed a velamentous insertion of the cord (not in the center & placenta so it's abnormal). ## Amniotic Fluid - Amniotic fluid plays a major role in fetal growth and development. Initially, some amniotic fluid is secreted by cells of the amnion. The embryo, suspended in amniotic fluid by the umbilical cord, floats freely. - Amniotic fluid has critical functions in the normal development of the fetus: - Permits symmetric external growth of the embryo and fetus. - Acts as a barrier to infection. - Permits normal fetal lung development. - Prevents adherence of the amnion to the embryo and fetus. - Cushions the embryo and fetus against injuries by distributing impacts the mother receives. - Helps control the embryo's body temperature by maintaining a relatively constant temperature. - Enables the fetus to move freely, aiding muscular development by movements of the limbs. - Assists in maintaining homeostasis of fluid and electrolyte. - Most fluid is derived from maternal tissue and interstitial fluid by diffusion across the amniochorionic membrane from the decidua parietalis. There is also diffusion of fluid from blood in the intervillous space of the placenta. - Fluid is also secreted by the fetal respiratory and gastrointestinal tracts and enters the amniotic cavity. The daily rate of contribution of fluid to the amniotic cavity from the respiratory tract is 300 to 400 ml. - Beginning in the 11th week, the fetus contributes to the amniotic fluid by excreting urine into the amniotic cavity. By late pregnancy, approximately 500 ml of urine is added daily. The volume of amniotic fluid normally increases slowly, reaching approximately 30 ml at 10 weeks, 350 ml at 20 weeks, and 700 to 1000 ml by 37 weeks. - The water content of amniotic fluid changes every 3 hours. Large amounts of water pass through the amniochorionic membrane into the maternal tissue fluid and enter the uterine capillaries. Amniotic fluid is swallowed by the fetus and absorbed by the fetus's respiratory and digestive tracts. - It has been estimated that during the final stages of pregnancy, the fetus swallows up to 400 ml of amniotic fluid per day. The fluid passes into the fetal bloodstream and the waste products in it cross the placental membrane and enter the maternal blood in the intervillous space. - Excess water in the fetal blood is excreted by the fetal kidneys and returned to the amniotic sac. ## Twin Placenta - Twins are either dizygotic (DZ) or monozygotic (MZ) twins (identical) twins. - Approximately two thirds of twins are DZ and the rate of DZ twinning increases with maternal age and has hereditary influence. - Dizygotic Twins result from fertilization of two oocytes, DZ twins develop from two zygotes and may be of the same sex or different sexes. - For the same reason, they are no more alike genetically than brothers or sisters born at different times. The only thing they have in common is that they were in their mother's uterus at the same time. - DZ twins always have two amnions and two chorions, but the chorions and placentas may be fused. - Monozygotic Twins result from the fertilization of one oocyte and develop from one zygote, MZ twins are of the same sex, genetically identical, and very similar in physical appearance.. - MZ twinning usually begins in the blastocyst stage, approximately at the end of the first week, and results from division of the embryoblast into two embryonic primordia. Subsequently, two embryos, each in its own amniotic sac, develop within the same chorionic sac and share a common placenta-a monochorionic-diamniotic twin placenta. - Uncommonly, early separation of embryonic blastomeres (e.g., during the two-to eight-cell stages) results in MZ twins with two amnions, two chorions, and two placentas that may or may not be fused. In such cases, it is impossible to determine from the membranes alone whether the twins are MZ or DZ. ## Bottom Line - Fetal membranes are the extraembryonic structures that develop from zygote but do not form any part of the embryo proper. - F.Ms. Contains: Chorion, amnion, yolk sac allantois and umbilical cord etc. - Function: Protection, nutrition, excretion of the waste products & respiration. - Fate of the F.M: After the delivery of the baby, the F.M are separated from wall of the uterus along with placenta & UC they are expelled form uterus with 30 min after birth. - **Placenta**: Is a temporary endocrine gland that consents the fetus to the uterus via umbilical cord. - **Sources of placental development**: Fatal & Maternal Chorion and Decidua Basalis. - **Functions**: Hormones, Gasses exchange, Transmission of antibody “passive immunity", act as Protective barrier for any infection & temporary storage. - **Placenta cannot be considered as fetal membranes**.