PLACENTA, FETAL MEMBRANES & TWINNING Langman’s Medical Embryology, 9th ed , Chapter 6 - pp 125-148 Objectives: 1. The components of fetal membrane and their functions 2. Structure and function of placenta 3. Composition of placenta barrier A. Introduction As the embryo grows and develops, its nutrient needs can no longer be supplied by simple diffusion. The placenta (Latin - flat cake) allows the fetus to "plug in" to the mother, accessing nutrients and disposing of waste products. The placenta is unique in its function from several points of view. Its lifetime is short (relative to the fetus) and its function changes during the course of gestation. For the fetus, the placenta serves as lung, GI tract, kidneys, liver and even endocrine organs. The "fetal membranes" are a collective group which include the following: 1) Chorion --gives rise to the placenta, 2) Amnion--surrounds the fetus, 3) Yolk Sac--a portion of which is incorporated into the GI tubes (the remainder of the yolk sac is rudementary in humans), and 4) Allantois -- caudal outlet of bladder remains rudementary in humans. 5) Umbilical cord, B. Chorion Development 1. Chorionic Components - Early in development, the villi cover the entire surface of the chorion. - During the second month, the villi at the embryonic pole expand and proliferate, forming a bushy array of villi called the chorion frondosum. - In contrast, the villi over the aembryonic pole degenerate, forming a smooth surface known as the chorion laeve. 2. Endometrial Contribution -The endometrial cells have accumulated lipids and glycogen, and are now called decidua. -The decidua underlying the chorion frondosum is known as the decidua basalis. A layer of decidual cells tightly adherent to the underlying chorion is known as the decidual plate. - The decidua atop the aembryonic pole is known as the decidua capsularis. -The decidua not in intimate contact with the developing embryo is called the decidua parietalis. - As the embryo develops and the chorion expands, the decidua capsularis degenerates and the chorionic leave fuses with the decidua parietalis obliterating the uterus lumen. 3. Development of Villi (week 2 to week 3) Finger-like projections of proliferating cytotrophoblasts surrounded by syncytiotrophoblasts, form – primary villi. - Secondary villi form as extraembryonic cytotrophoblastic core of the primary villi. mesoderm penetrates the - With the development of blood vessels, the villi are now called tertiary villi. - During this time, cytotrophoblasts proliferate and extend through the syncytium to form a thin, outer cytotrophoblastic that adheres to the endometrial tissues 4.Function of chorion 1) Exchange of metabolite: a portion of placenta, involved in the fetal circulatory system 2) Hormone production: human chorionic gonadotropin (HCG) C. Amnion/Amniotic Fluid - Amniotic fluid is produced by 1) amniotic cells, and 2) infusion of fluid from maternal blood. Additionally, in late fetal life 3) urine output from the fetus and 4) Pulmonary secretions also contribute to the production of amniotic fluid. At the end of pregnancy, urine is daily added to the amniotic fluid. This urine is mostly water, since the placenta is functioning as an exchange for metabolic wastes. - During the latter parts of gestation (fifth month), the fetus swallows and breathes about half the total volume of amniotic fluid per day. Note: The water in the amniotic fluid changes every 3 hours, indicating the enormous exchange between the amniotic cavity and the maternal circulation. - Amniotic fluid functions to: (1) mechanically cushion the fetus, (2) allow fetal movements, (3) prevent fetal adhesions with the amnion, and (4) acts as a temperature sink to maintain constant temperature - An overabundance of amniotic fluid (polyhydramnios) is frequently associated with congenital anomalies, particularly those which prevent the infant from normal swallowing. (either because of esophageal atresia or through lack of nervous control of the swallowing mechanism ( as in anencephaly) - Oligohydramnios, or a decreased amount in amniotic fluid, is associated with poor production of amniotic fluid (i.e. low fetal urine output, such as poor development of kidney, urethra atresia, or inadequate amniotic fluid production). Note: At the end of the 3rd month, the amnion has expanded to such an extent that it comes in contact with the chorion, thereby obliterating the chorion cavity. The yolk sac then usually shrinks and is gradually obliterated. D. Yolk Sac - Early in gestation the yolk sac may provide some limited nutrition. - During the 3rd through 6th weeks of gestation, blood islands form and contribute to the embryo's early circulation. - Also during the 3rd week of gestation, germ cells from the yolk sac migrate into the embryo. - Finally, as the embryo folds and takes shape, a portion of the yolk sac is incorporated into the embryo as the primitive gut. - The remainder of the yolk sac is rudimentary. E. Allantois - Is rudimentary in humans - During fetal life, it fuses to become the ligamentous urachus. - Genes rise to the umbilical vessles F. Umbilical Cord - Cranial and caudal folding of the embryonic disc, accompanied by the lateral folding, produces a purse string closure around the primitive umbilical ring. - The primitive umbilical ring contains: the connecting stalk (with umbilical vessels and allantois), the yolk sac and an intercoelomic canal. - Vessels from the chorionic plate (two arteries and one umbilical vein) intertwine as they enter the connecting stalk. A gelatinous mesoderm called Wharton's Jelly, surrounds umbilical vessels. - The umbilical vein caries O2 and nutrient-rich blood to the fetus, while the umbilical arteries return O2/nutrient depleted blood from the fetus to the placenta - The intercoelomic canal connects the intraembryonic coelom with the extraembryonic coelom (chorionic cavity) and, during early fetal development (~1st trimester), contains the herniated portion of the developing midgut. - Amnion covers the umbilical cord. - The tortuosity of the umbilical cord often gives the impression of knot formation, however no vascular compromise is seen with vessel tortuosity. True knots can form in abnormally long umbilical cords. - In contrast, short umbilical cords can detach the placenta from the uterus before delivery, eliminating adequate oxygenation for the fetus during the delivery process. - Occasionally, a single umbilical artery is found and is often associated with other congenital anomalies. - Abnormal insertions of the umbilical cord into the placenta can cause rupture of the vessels during delivery and exsanguination of the fetus. G. Placental Structure - The placenta proper is composed of the chorion frondosum from the fetus and the decidua basalis from the mother. - The fetal border of the placenta is the chorionic plate (extraembryonic somatic mesoderm and cytotrophoblasts). - The decidual plate of the decidua basalis is the placental border on the maternal side. - The placenta becomes compartmentalized into cotyledons by evaginating decidual septa extending from the decidual plate. The decidual septa do not reach the chorionic plate, thus the compartmentalization is incomplete. - The placenta is usually composed of fifteen to twenty cotyledons. - At term, the placenta is discoid shape and weighs five to six hundred grams. The placenta is expelled from the uterine cavity approximately thirty minutes following the birth of the infant. H. Placental circulation and barrier - Villi extending from the chorionic plate and anchoring to the cytotrophoblastic shell are termed anchoring villi. - The intervillous spaces are lined by syncytiotrophoblasts and bordered by adjacent anchoring villi, the chorionic plate, and the cytotrophoblastic shell. The incorporated maternal blood vessels extend through the cytotrophoblastic shell and bleed into the intervillous space forming lakes of maternal blood that bathe the branching or free villi - Branching or free villi originating from the anchoring villi, extend into the intervillous spaces, and are the chief villi involved in circulatory exchange. - During the fourth month, there is a loss of cytotrophoblasts and connective tissue in the branching villi, approximating the endothelial wall of the capillary and the syncytium. Thus, only two layers separate the fetal and maternal circulations. - The syncytial cells that overlie tightly adherent blood vessels form microvilli to aid in the exchange between the fetal and maternal circulations. - In an effort to enhance functional maternal-fetal exchange, nuclei of syncytiotrophoblasts shift away from underlying blood vessels and clump together to form syncytial knots. - Maternal spiral arteries that enter the intervillous space inject blood deep within the space toward the chorionic plate. The blood then percolates through the branching villi back toward the decidual plate and endometrial veins. I. Placental Function 1. Exchange of Metabolites - The placenta allows exchange of oxygen, carbon dioxide, nutrients, and electrolytes, which maintains the active embryonic and fetal cell metabolism. - Factors affecting exchanges include: Concentration differences, area of exchange, permeability, molecular size, lipid solubility, degree of ionization and molecular configuration, - Additionally, the placenta allows exchange of maternal antibodies, providing passive immunologic protection for the developing fetus. The placenta also functions to remove metabolic wastes from the developing fetus as the kidneys and GI tract are unable to support waste elimination. - Unfortunately, these same features that allow nutrient and supportive transfer between the fetal and maternal circulations also allows drugs and infectious agents to traverse into the fetal circulation. 2.Denfense barrier - The placental barrier, is initially composed of four layers: (a)the endothelial lining of the fetal vessels,(b) the connective tissue in the core of the villus, (c) the cytotrophoblastic layer; and (d) the syncytium. From the fourth month on, the placental barrier becomes much thinner, since the endothelial lining of the vessels comes in intimate contact with the syncytial membrane, thus greatly increasing the rate of exchange. 3. Hormone Production - The placenta (chiefly the syncytiotrophoblasts) produces chorionic gonadotrophin (Beta HCG--Placental LH) which support the corpus luteum in the production of progesterone.( HCG is excreted by the mother in the urine, their presence is used as an indicator of early stages of gestation) - The placenta will eventually (around the 4th month of gestation) produce adequate amounts of estrogen and progesterone that the corpus luteum is no longer needed. - Estrogen and progesterone, enhance and stabilize uterine growth and the development of the mammary gland. - Placental Lactogen (somatomanotrophin) is also produced by the placenta and facilitates glucose extraction from maternal blood. J. Multiple Gestations - Dizygotic or fraternal twins are the most common (70 percent) type of multiple gestations and results from fertilizations of two or more simultaneously shed oocytes. - Because each zygote is different, the sex may differ and the offspring share no more resemblance to each other than other brothers and/or sisters. - Each zygote will implant individually, develop its own placenta, amnion, and chorion. - Monozygotic or identical twins. Occasionally, a zygote will split in the early stages of development. - If the zygote is split at the two-cell stage, two completely separate zygotes will develop with their own placenta, amnion, and chorion. - If separation occurs at the blastocyst stage, the inner cell mass will split into two embryos, with a common placenta and a chorionic cavity, but separate amniotic cavities. * Most common for monozygotic twins. - Rarely does separation occur at the bilaminar germ disc stage, resulting in two embryos with a single placenta, a common chorion, and a common amnion. - Even more rarely, splitting of the zygote may be incomplete and thus, the developing embryos remain partially fused at some point, giving rise to conjoined twins.