SEM_06_Extraembryonic membranes and placentation_PARTE1.docx

Extraembryonic membranes and placentation Learning objectives Understand the evolutionary analogies of the extraembryonic membranes in reptiles, birds and mammals. Know the basic features of the extraembryonic membranes. Understand the different types of implantation. Describe the foetal and m...

Extraembryonic membranes and placentation Learning objectives Understand the evolutionary analogies of the extraembryonic membranes in reptiles, birds and mammals. Know the basic features of the extraembryonic membranes. Understand the different types of implantation. Describe the foetal and maternal contributions to the placenta. Correlate the concepts of placental exchange and placental barrier. Know the different types of placentas. Describe the details of the placenta in domestic animals. An evolutionary approach to the mammalian placenta Initially, aquatic animals (fish and amphibians) only needed the support of the yolk sac for the development of the embryo. In reptiles and birds, as they developed out of the water inside an egg with a shell, in addition to the yolk sac, the development of the amnion, the chorion and the allantois was also necessary. Although mammals do not produce shelled eggs and most of them remain within the mother until birth, the foetal membranes of mammals that make up the placenta remain the same as those that arose in reptiles and birds. Mammals can be classified into three groups based on how they use extra-embryonic membranes to maintain the embryo. This classification reflects the evolutionary adaptations that the extra- embryonic membranes must undergo until they form the placenta of mammals. Monotremes (egg-laying mammals). This group represents the most ancient mammals, which continue to lay shell eggs like their reptilian ancestors. Today there are only four species: three species of spiny anteater (echidna) and the duck-billed platypus. Marsupials (vitelline placenta). Marsupials do not produce shell eggs but the embryo begins its development within the mother's uterus. Marsupial eggs, which contain little yolk, adhere to the wall of the uterus and are able to establish a rudimentary connection with the maternal blood supply, giving rise to a yolk placenta. This placenta is capable of obtaining food, oxygen and other essential elements to start the development of the embryo. However, this connection between the tissues of the uterus and the extra-embryonic membranes is never well elaborated, so the offspring must be born in a very immature state. Newborn embryos, despite their small size, can move through the abdomen to get into a bag where they find nipples that supply them with the necessary milk to complete their development. Placental mammals (allantoic placenta). In placental mammals, the extra-embryonic membranes give rise to the allantoic placenta, which connects the embryo with the mother's uterus in a more elaborate and efficient way. The allantoic placenta is capable of providing the embryo with the nutrition, respiration, excretion and endocrine function that it needs to complete all its embryonic development. Domestic mammals and humans are placental mammals. - Mammals are a group of animals (vertebrates) that have backbones and hair or fur. They are warm-blooded (endothermic), and they have four-chambered hearts. They also feed their young with milk from the mother's body. Mammals can be divided into three more groups based on how develop their offspring is born: monotremes, marsupials, and the largest group, placental mammals. Extraembryonic membranes The extraembryonic membranes are structures that develop adjacent to the embryonic body but result essential for the prenatal survival of the proper embryo. They are formed from the same germ layers as the embryo but do not become part of the individual organism, and after birth, they are eliminated. There are four foetal membranes: Two of them, the chorion and amnion, arise from the outer layer of the conceptus (trophoblast and ectoderm, respectively) and are provided with parietal mesoderm; the other two, the yolk sac and allantois, develop from the inner layer (endoderm) and are provided with visceral mesoderm. These same foetal membranes have been retained, although modified, from the ancient reptiles to the current mammals. Chorion — Chorion, also called serosa in reptiles and birds, is the outermost membrane around the embryo. It develops when the trophoblast, in mammals, or the ectoderm, in reptiles and birds, joint to the adjacent parietal mesoderm to form this bilayed membrane. In insects, the chorion forms the outer shell of the insect egg. In reptiles and birds, it fuses with the allantois forming the chorioallantoic membrane or allantochorion that is arranged in direct contact with the eggshell allowing the allantoic blood vessels to exchange gases with the atmosphere through the porous shell. Besides the chorion, there are other three extra-embryonic membranes that have been already considered in the previous chapter: The yolk sac provides nourishment for the embryo at the earliest stages of development and, among other vital functions, supplies the first blood cells and vessels that transport nutritive yolk products to the developing embryo. The amnion protects the embryo in a sac filled with amniotic fluid. Surrounded by amniotic fluid, the embryo is kept as moist as a fish embryo in a pond. The allantois, besides handling the urinary waste, helps in the exchange of gases and nutrients. In mammals, the allantoic blood circulation gives rise to the placental circulation. From an evolutionary perspective, the chorion, amnion and allantois can be considered adaptations to terrestrial life. The chorion developed as the outermost cellular membrane which together with the amnion, provided protection and a suitable interface for interchanging of gases and nutrients between the embryo and a dry environment. The amnion was also an adaptation to dry land since one of its functions is to prevent the embryo from drying out. The allantois was an evolutionary solution for the disposal of embryonic excretions in terrestrial species when the metabolic waste could not be immediately excreted to aquatic surroundings (like fish and amphibian larvae do); besides, the allantois also made possible the allantoic blood circulation which expanded the respiratory and nutritive capabilities of the primary vitelline circulation that existed in the yolk sac of aquatic animals. Originally, the extra-embryonic membranes were adaptations to the terrestrial life in egg-laying animals, but later over evolution, mammals were capable of reusing these same extra- embryonic structures to form the placenta. - Extraembryonic membranes in birds and mammals. Extraembryonic membranes in birds At the time the egg is laid the embryonic disc is at the bilayered blastula stage. When the egg is incubated, the gastrulation leads to the formation of the three germ layers which expand to give rise to the primitive embryo and the surrounding membranes. The endoderm expands embracing the underlying yolk to form the yolk sac. This membrane also includes the visceral mesoderm that expands attached to the endoderm. The ectoderm expands to form the amniotic foldings which result in the amniotic membrane (inner layer) and the chorion membrane (outer layer) also called serosa. These membranes also include the parietal mesoderm that expands attached to the ectoderm. The last extraembryonic membrane to form is the allantois which expands throughout the extraembryonic coelom. As a result, it becomes intimately attached to the chorion resulting in a tri- laminate membrane named allantochorion. https://sway.office.com/duUyUZFDjuLI1rYD#content=iu6jcDBVhJ3hWb - Extraembryonic membranes in birds. Extraembryonic membranes in mammals In mammals, the extraembryonic membranes develop in a similar manner to those of birds but certain modifications are needed because of their intrauterine mode of development. Such is the case of the yolk sac, amnion and allantois that, although with adaptive modifications, they continue to form despite the fact that practically no yolk accumulates in the mammalian ovum and the embryo develops within a protected environment such as the maternal uterus. Adaptive modifications include changes in the chorion, which, unlike birds where it is smooth, develops multiple finger-like folds called villi. In the first stage, to increase the exchange surface, the trophoblast develops finger-like extensions which protrude into the maternal lining of the uterus (primary villi(). In a second stage, the primary villi become filled by parietal mesoderm (mesenchyme) originated in the gastrulation (secondary villi). In the third stage, the filling mesenchyme inside of the villi forms a network of capillary blood vessels (tertiary villi). The development of the placenta is associated with the development of the vascularised chorion coated with tertiary villi. This chorion covered with tertiary villi is known as chorion villous or chorion frondosum. In certain species in which the placenta is quite efficient, the chorionic villi degenerate at certain locations (non-placental areas). In these places, the chorion loses its villi and becomes smooth chorion where no placental exchange occurs. https://sway.office.com/duUyUZFDjuLI1rYD#content=v1gKiEchg1Rq8n - Development of the chorionic villi. Implantation and maternal recognition of pregnancy Before implantation, the blastocyst is just free-floating in the uterine lumen where it is nourished by uterine glands (womb milk). After floating around for a few days in the uterus, the blastocyst adheres to the wall of the uterus. This is implantation, the moment that is usually considered the beginning of pregnancy. Establishment and maintenance of pregnancy in a number of mammalian species depend upon a tightly regulated interaction between the semiallogeneic conceptus and the maternal uterine endometrium. The term "maternal recognition of pregnancy" encompasses the various ways in which the mother adapts to the presence of a conceptus within her reproductive tract. During implantation, the conceptus tries to gain some measure of control over corpus luteum function, uterine blood supply, the mother's immune system, and other aspects of maternal physiology. Most probably as a result of ongoing genetic conflict between the mother and the conceptus, a bewildering range of placental structures and trophoblast signalling mechanisms are encountered in eutherian mammals despite the fact that the uterus and conceptus share a common interest, which is the successful outcome of the pregnancy. While the basic strategy is to maintain and prolong the cyclical corpus luteum by inhibiting or reducing the secretion of prostaglandins, the factors which control the process show species variation. Successful implantation depends on the blastocyst binding to the endometrium what is called adhesion to the endometrium. There are many molecules that are thought to dictate this interaction, but integrins, a type of cell-adhesion molecule, has been identified as a primary component. Integrins extend from the lining of the uterus and from the surface of the blastula. Integrins have many functions in nearly all tissue types, and they have a role in cell adhesion, conveying information about the extracellular environment to the nucleus, and modulating the local immune response. At this stage, the conceptus is an intruder recognised as foreign by the mother, that likely survives by using strategies analogous to those employed by successful parasites. The attachment of the conceptus to the uterine wall involves a tight intertwining of the chorionic villi on the maternal tissues. After attachment, the blastocyst is no longer easily removed from the uterine wall. In species that carry multiple offspring, the attachment is preceded by a remarkably even spacing of embryos throughout the uterine horns. This process appears to result from uterine contractions, and in some cases, it involves the migration of the embryos from one uterine horn to another (trans-uterine migration). The objective of implantation is always to obtain a very close apposition between embryonic and maternal tissues. There are, however, substantial differences among species in the process of implantation, particularly about "invasiveness," or how much the embryo erodes into maternal tissue. In most domestic species (carnivores are the exception), the embryo implantation only comprises the apposition and attachment but not the embedding in the endometrium. In contrast, in humans, as soon as the adhesion to the endometrium is completed, the cells that lie on the periphery of the blastocyst - the trophoblast - erode the endometrium and induce a set of changes in the uterine wall (decidual reaction). This process leads to the embedding (nidation) of the blastocyst into the endometrium. There are three fundamental patterns of implantation, based on the position of the blastocyst in the lumen of the uterus: Centric implantation. The embryo expands to a large size before implantation and then remains in the centre of the uterus. Examples include all domestic animals: carnivores, ruminants, horses, and pigs. https://sway.office.com/duUyUZFDjuLI1rYD#content=zrDwa8YZN37mYK - Central implantation Eccentric implantation. The blastocyst is small and implants within a uterine crypt on one side of the uterus, usually opposite to the mesometrium. Examples include rats and mice. https://sway.office.com/duUyUZFDjuLI1rYD#content=RRETGTTZdMGKYM - Eccentric implantation Interstitial. The blastocyst erodes through the endometrium into the subepthelial connective tissues, so it essentially creates a wound in the uterine mucosa. This involves the transformation of the uterine stromal and endothelial cells into a tissue called decidua. Such implantation is often called nidation ("nest making"). Examples include primates, including humans, and guinea pigs. This process whereby the embryo leaves the uterine lumen is due to the invasive nature of the trophoblast which divides into an inner layer that retains its original cellular nature (cyto- trophoblast) and an outer layer where cells fuse to form an irregular invasive tissue (syncytio- trophoblast). https://sway.office.com/duUyUZFDjuLI1rYD#content=JlHBjvFk4ZTJpK - Interstitial implantation Usually, attachment and implantation occur within a few days after the blastocyst reaches the uterus: in humans at the end of the first week; in carnivores, small ruminants and pig at the end of the second week. Sometimes implantation takes place a bit later when the organogenesis of the embryo is very much advanced (3-5 weeks in cattle, 3-8 weeks in horses). Extreme cases of late implantation are illustrated in species which presents delayed implantation (embryonic diapause). In such a case, the blastocyst enters in a dormant status, named diapause, which could last for months until stimulatory factors activate it and induce its implantation. This adaptive process ensures optimal survival conditions to the offspring. Some mammals that undergo embryonic diapause include rodents, bears, mustelids (e.g. weasels and badgers), and marsupials, (e.g. kangaroos). It has been described two types: Facultative diapause, also known as lactational delayed implantation due to its regulation through lactation. If a female copulates while she is still nursing a previous litter, the stimulus of sucking can cause the embryos to enter diapause (marsupials, among others). Mandatory diapause or seasonal delayed implantation is a mechanism that allows mammals to program the birth of their young under favorable environmental conditions. This mechanism manifests itself as part of the regular reproductive cycle of some (roe deer and bear, among others). Sources of the embryonic nutrition All the products needed for the embryo nourishment can be collectively named embryotrophe. Before implantation, the maternal uterine glands are the only source of embryotrophe. These gland secretions are known as histotrophe or uterine milk and they provide the embryo with the energy and biochemical building blocks it needs to grow during the first weeks of pregnancy. - Histotrophe But as the embryo grows this arrangement rapidly becomes insufficient. The development of the placenta solves this insufficiency allowing the embryo import directly blood-borne maternal nutrients or haemotrophe. Nevertheless, in some types of placentas, the womb milk will remain as a complement for the nutrition obtained from the maternal blood by the placenta. - Haemotrophe The replacement of the histiotrophe by the hemotrophe is associated with the functional efficiency of the placenta which favours the role of the haemotrophe. Species with a highly efficient placenta (primates including humans) have minimal histotrophic uptake, using only the placental haemotrophic mechanisms throughout most of the gestation. Nevertheless, in most domestic mammals, both sources of nutrients (histotrophe and haemotrophe) persist throughout gestation. For this purpose, many placentas have developed specialised placental regions, called placental areolae, for ingestion of uterine secretions and other cell debris. - Histotrophe and Haemotrophe