Chapter 7 EEM and Placentation 2024 PDF

2024 Chapter 7 Chapter 7: Extra-Embryonic Membranes and Placentation Key terms and concepts: Extra-embryonic membranes Histotroph, uterine milk Trophoblast/trophectoderm...

2024 Chapter 7 Chapter 7: Extra-Embryonic Membranes and Placentation Key terms and concepts: Extra-embryonic membranes Histotroph, uterine milk Trophoblast/trophectoderm Diffuse/microcotyledonary placenta Yolk sac Cotyledonary placenta Chorion Zonary placenta Allantois Discoid placenta Amnion Cotyledonary Extra-embryonic coelom Epitheliochorial Mesoderm Endotheliochorial Splanchnic mesoderm Hemochorial Somatopleure Deciduate placenta Splanchnopleure Non-deciduate placenta Implantation Choriovitelline placenta Endoderm Chorioallantoic placenta Learning objectives: By the end of this unit, you should be able to: 1. Explain why vertebrate embryos need extra-embryonic membranes. 2. What is histotroph and what role does it play in nutrition of the embryo? 3. Explain two important differences between somatopleure and splanchnopleure. 4. Understand what anatomic regions and germ layers give rise to each of the extra- embryonic membranes. 5. Describe at least one role of the extra-embryonic tissues PRIOR to implantation. 6. Explain the importance of vascularization of the extra-embryonic membranes. 7. Explain which umbilical vessels carry OXYGENATED blood and which carry DEOXYGENATED blood. Is there anything unusual about the type of vessels that the oxygenated and deoxygenated blood flow through in the umbilicus? 8. Explain the relationship between the maternal and fetal circulation in each placental type (epitheliochorial, endothelial chorial, hemochorial) listed above. I. Introduction to Extra-embryonic Membranes At the same time that the embryo is developing, the extra-embryonic membranes are forming as well. A. The mesodermal layer formed during gastrulation also migrates laterally beyond the embryonic disc and inserts itself between the trophoblast and the hypoblast layers. Although at this stage it appears a bit arbitrary, we can divide the gastrula into embryonic and extra-embryonic regions at the periphery of the embryonic disc. Once the mesoderm has completed its migration, both regions will be composed of three germ layers. This takes a while, however, so there is a 1 2024 Chapter 7 stage in development of the placenta that involves only the endoderm and trophoblast. B. The three corresponding embryonic and extra-embryonic germ layers are continuous at the periphery of the embryonic disc. Discrimination between embryonic and extra-embryonic components becomes increasingly easy as the gastrula loses its spherical shape and the edges of the embryo fold ventrally towards each other and the amnion “lifts up” dorsally. C. The extra-embryonic mesoderm splits into two layers. The space between the layers is the extra-embryonic coelom (space). 1. The mesoderm layer adjacent to the trophoblast is the somatic (extra- embryonic) mesoderm. Together these two layers are the somatopleure. 2. The mesoderm layer adjacent to the hypoblast (endoderm) is the splanchnic (extra-embryonic) mesoderm. Together these two layers are the splanchnopleure. Endoderm stimulates splanchnic mesoderm to form blood vessels and blood cells. D. The major derivatives of the extra-embryonic membranes are: 1. Yolk Sac - from splanchnopleure [vitelline = yolk sac]; continuous with the gut, contains little or no yolk in mammals; is very vascular 2. Allantois - from splanchnopleure; develops as an outpouch of the hind- gut; stores urinary wastes, very vascular 3. Amnion - from somatopleure; creates an aqueous environment for the embryo/fetus; is avascular 4. Chorion - from somatopleure; since it is the outermost layer, it is an important component of the placenta; is avascular E. Formation of the amnion and chorion 2 2024 Chapter 7 1. The somatopleure folds over the top of the developing embryo, and pinches off from the rest of the extra-embryonic membranes, forming the amnion, which surrounds the amniotic cavity. This moves ventrally to surround the embryo except where it wraps around the umbilical cord. 2. The remaining somatopleure, which now surrounds the entire embryo, is known as the chorion. The outer layer is still the trophoblast. F. Formation of the allantois: In most species that we commonly deal with, the yolk sac does not remain the major source of splanchnopleure. A second out pocketing of the hind gut, the allantois (also endoderm) acquires a layer of splanchnic mesoderm. The yolk sac atrophies, and the allantois expands into the extra-embryonic coelom until in some species, it surrounds the embryo. This results in the formation of two fluid-filled cavities that surround the embryos of domestic species. In humans, the amnion functionally fuses with the chorion resulting in a single fluid-filled cavity. G. Vascularization of the extra-embryonic Membranes: Vascularization of the extra- embryonic membranes, which is necessary for distribution of absorbed nutrients, is dependent on splanchnopleure. Although both the amnion and chorion have a mesodermal component, these layers are relatively avascular since they contain only somatic mesoderm. H. Fusion of the chorion with the underlying splanchnopleure can form two kinds of placentas, distinguished by the source of endoderm. 1. Choriovitelline placentas: composed of the chorion and yolk sac. The yolk sac is a transitory structure for most mammals, lasting longer in the horse and carnivores. 2. Chorioallantoic placentas: composed of the chorion and allantois. 3 2024 Chapter 7 At the time of hatching or birth, each individual amniote (including you and me) has suffered natural death and amputation of portions of itself that were vital to its life up to that time. These are the fetal membranes--chorion, amnion, and allantois. Harland Mossman, Vertebrate Fetal Membranes II. Looking Ahead to Implantation A. In order to form a placenta, where maternal and embryonic blood vessels are very close together, the embryo must first burrow into the lining of the uterus. This process of implantation begins with the embryo just touching the uterine lining, and then proceeds to some degree of interdigitation of tissues, depending on the species. B. In species which give birth to multiple young, embryos migrate throughout the uterine horns, distributing themselves at regular intervals. Even spacing is essential for their survival and normal intrauterine growth, since each needs a sufficient blood supply. C. The extra-embryonic structures are of two major types from a functional point of view. One type, such as the avascular amnion, surrounds the embryo in a protective, fluid filled sac. The other type is the highly vascular placenta, whose purpose is to pick up oxygen and needed metabolites from the mother's blood stream, while dumping carbon dioxide and waste products. There are several good ways to build a placenta, and there is quite a bit of species variation in placentation. D. The implanting embryo must be protected from the maternal immune system in order to survive in the uterus. E. The embryo must also signal to the mother that she is pregnant; this latter process varies markedly among species and will be discussed extensively in Reproductive Physiology later in the year! F. Implantation is a process that takes a varying amount of time depending on the species. It usually begins at about the time that gastrulation has been completed. This is not true, however, for the horse, which proceeds with its embryonic development as it floats around the uterine lumen, then implants and forms a placenta later in gestation. Variation and error in reports of timing of developmental events is common and frustrating. There are several reasons for these inconsistencies. Some authors time developmental events from ovulation or fertilization (neither of which can be observed), others from time of copulation or insemination (which may precede fertilization by several days), or by signs of estrus. Therefore, the dates given in this class are approximate, and are meant only to give an idea of the relative times when events occur in major domestic species. 4 2024 Chapter 7 III. Comparative Placentation: Different Solutions to Common Problems Why bother with a placenta? A. What major problems does the mammalian embryo/fetus face? B. How does the placenta help solve these problems? IV. Placentation A. The uterus is highly specialized for carrying the developing offspring. It is under hormonal control. 1. It is lined with cuboidal to columnar epithelium, and includes glands which develop under the influence of the hormones of pregnancy, secreting histotroph or uterine milk for nourishment of the embryo. This is particularly important before formation of the placenta, but continues to be important to supplement placental exchange in many species. 2. The ability of the uterus to respond to the fetal extra-embryonic membranes by allowing implantation is also regulated by hormones. Hormonal control of the uterus and implantation will be extensively discussed next semester in Reproductive Physiology. B. In all species, interaction of fetal extra-embryonic membranes and uterus induces mitosis and differentiation of both tissues to optimally carry out functions of the placenta, i.e., absorption (epithelium), distribution (blood supply), hormone synthesis. 1. In most species, the junction between fetal and maternal tissue is characterized by interdigitating villi, which maximize surface area for exchange. Blood flow is also high volume, slow, low pressure, and organized in such a way to maximize exchange of material between the fetal and maternal circulation. 5 2024 Chapter 7 C. In species where implantation occurs later, including most farm animals, extra-embryonic membranes expand very rapidly early in development to provide increased surface area for nutrient uptake. D. Development of the placental blood supply 1. Connection to fetal cardiovascular system: The amnion is attached to the body of the embryo where the body wall opens ventrally (umbilicus), and forms the outer wall of this structure. The umbilicus contains the yolk stalk and allantoic stalk, and their associated vessels. The vitelline vessels are those associated with the yolk sac, and atrophy along with the yolk sac as gestation progresses. The umbilical vessels comprise the allantoic circulation, and are responsible for the metabolic exchange with the mother. The umbilical arteries branch from the caudal aorta and carry poorly oxygenated blood from the fetus to the placenta, and one or two umbilical veins, depending on the species, carry blood high in oxygen and nutrients back to the fetus. 2. Maternal circulation to the placenta is part of the systemic (left heart) circulation. 3. Since the placenta must function both as lungs and GI tract for the developing embryo, the placenta carries a very large blood volume from both mother and fetus, and blood pressure is low to maximize time for diffusion. 6 2024 Chapter 7 V. Species differences A. Time of implantation. This can range from very early, as about the 6th day in primates and rodents, when the embryo is only a blastocyst, to longer than one month. B. Extent of uterine/ trophoblast involvement (gross appearance) 1. Diffuse/ microcotyledonary (pigs, horses) 2. Cotyledonary (ruminants). Uterine caruncles induce formation of cotyledons in overlying trophoblast, and together they form a placentome. In ruminants, caruncles, are specialized to form future areas of specialized contact with the fetus. In other species, the entire uterus is capable of responding to the fetus. 3. Zonary (carnivores) 4. Discoid (rodents, humans) C. Number of layers separating fetal and maternal blood supplies 7 2024 Chapter 7 1. Potentially six layers form the placental barrier: maternal endothelium, maternal connective tissue, maternal uterine epithelium, fetal chorionic epithelium (trophoblast), fetal connective tissue, and fetal capillary endothelium. At the initiation of implantation, all these layers are intact in all species. However, in some species, the fetal membranes invade into the uterus, decreasing the number of uterine layers. Species are classified by the number of layers separating maternal and fetal blood supplies. Maternal and fetal blood never mix during pregnancy. D. Amount of uterine tissue lost at parturition: deciduate vs. non-deciduate E. Relationship among these classification systems: 1. Late implanting species tend to have non-deciduate, epitheliochorial placentas which are either diffuse (such as the pig and horse) or cotyledonary (ruminants). The surface area of the membranes is generally very large, so that uptake of histotroph before implantation is optimal. 2. The placentas of early implanting species are more aggressive (deciduate) with fewer layers separating maternal and fetal blood supplies (carnivores, zonary, endotheliochorial; primates, rodents, discoid, hemochorial). 3. The amount of uterine tissue lost at parturition depends on whether or not the placenta is deciduate, and is a major determinant of the time necessary for the uterus to regenerate, and fertility to return. THOUGHT QUESTIONS: What are the major problems to be overcome to allow the prolonged development of the fetus within the uterus that occurs in mammals? What features of the placenta address these problems? How are all three layers (endoderm, mesoderm, and ectoderm) utilized for the fetal component of the placenta? 8