SEM_06_Extraembryonic membranes and placentation_PARTE2.docx

Placenta The placenta (plural placentas or placentae) is an apposition of foetal tissues (chorioallantois) to maternal tissue (endometrium) which has two main features: to maximise the embryo’s nutrition and excretion (exchange capabilities), but also to minimise immunological rejection by the mate...

Placenta The placenta (plural placentas or placentae) is an apposition of foetal tissues (chorioallantois) to maternal tissue (endometrium) which has two main features: to maximise the embryo’s nutrition and excretion (exchange capabilities), but also to minimise immunological rejection by the maternal immune system (invasiveness). These competing drives have resulted in a range of different types of placenta across species. The primary function of the placenta in all species is to promote selective transport of nutrients and waste products between the mother and foetus. Such transport is facilitated by the close approximation of maternal and foetal vascular systems within the placenta. It is important to recognise that normally there is no mixing of foetal and maternal blood within the placenta because some placental tissue always interposes between the foetal and maternal circulation, what constitutes the placenta barrier. The degree of trophoblast invasion is very variable between species, and thereby, the number of layers of the placental barrier. Therefore, the placenta presents a selective permeability since the placental barrier will only allow the passage of certain structures at the same time that it will prevent the passage of others. The list of substances that can cross the placental barrier will depend on the type of placenta and the characteristics of the substance that intends to cross it; as a general rule, the size of the molecules greatly influences, so to cite just two opposite examples, cells do not normally cross the placenta while gases do so easily. On the other hand, the placenta should not only be conceived as a simple permeable membrane. Transfer across the placenta occurs by passive diffusion but there is also active transport such as receptor-mediated endocytosis. The following discussion reflects some general principles about the main functions of the placental. Transport of nutrients (nutritive function). Glucose is the major energy substrate provided by the placenta to the foetus. It is transported across the placenta by facilitated diffusion (through specific carriers). Although the foetus receives large amounts of intact glucose, a large amount is oxidised within the placenta to lactate, which is used for foetal energy production. Amino acid concentrations in foetal blood are higher than in maternal blood. Amino acids are therefore actively transported to the foetus. There is substantial metabolism of some amino acids as they cross the placenta. There is much more variability among species in placental permeability to fatty acids than to glucose or amino acids. In some animals, there is little transport of fatty acids from mother to foetus, while in others a significant amount of transport takes place. Transport of gases (breathing function). Gases like oxygen and carbon dioxide diffuse through and across tissues in response to differences in partial pressure. In pregnancy, the partial pressure of oxygen (P02) in maternal blood is considerably higher than in foetal blood. Therefore, oxygen readily diffuses across the placenta from maternal to foetal blood. Despite its low PO2, foetal blood is able to transport essentially the same quantity of oxygen to foetal tissues as maternal blood. This is because the haemoglobin concentration in foetal blood is about 50% higher than in maternal blood, and the majority of haemoglobin in the foetus is foetal haemoglobin, which has higher oxygen- carrying capacity than adult haemoglobin. Carbon dioxide is produced abundantly in the foetus, and the PCO2 of foetal blood is higher than maternal blood. Carbon dioxide, therefore, diffuses from foetal blood, through the placenta, into the maternal circulation, and is disposed of by expiration from the mother's lungs. Transport of other molecules (excretory function). Placental exchange processes are also involved in the removal of waste products from the foetal metabolism. They cross over into the maternal blood to be excreted by the mother (urea, creatinine, uric acid, bilirubin). Transport of antibodies, infectious agents and drugs (protective function). There are marked differences between species in whether immunoglobulins are transported across the placenta. In primates and rodents, there is a substantial transfer of immunoglobulin G from maternal to foetal circulations prior to birth. This process requires immunoglobulin-binding proteins in the placenta. In contrast, there is no trans-placental transfer of immunoglobulins in domestic animals like cattle, sheep, horses and pigs. In those species, the neonate is essentially devoid of circulating antibodies until it absorbs them from the first milk (colostrum). Regarding infectious agents, the placenta forms in principle a "protective barrier" but the ability of a real infectious agent to cross the placental barrier will depend greatly on the type of placenta and the size of the harmful agent. In addition, the placenta also presents an incomplete barrier against certain toxins or drugs; Depending on the size of their molecules, certain antibiotics and corticosteroids tend to cross the placental barrier. Transport of hormones (endocrine function). The placenta is also a hugely important endocrine organ, producing many hormones which affect the status of pregnancy and the maternal physiology. They include steroid hormones, such as oestrogens and progesterone, and protein hormones, the most important of which are the chorionic gonadotrophins. - The placental barrier. The placenta is the connecting link between the mother and developing embryo allowing the relationship between maternal and fetal circulation. The placental barrier describes the layer of tissue making up the placenta, which is semi-permeable and serves as a selective membrane between mother and embryo circulations. The function of the placental barrier is to screen what substances can pass from maternal blood to fetal blood and which can not. Types of placentas based on the placental membranes Choriovitelline placenta. In some mammals, the yolk sac becomes very large and fuses broadly with the chorion to form a choriovitelline membrane coated with villi attached to the uterine endometrium. This attachement constitutes the placenta viteline o chorioviteline provided with vitelline blood vessels in close apposition with the vascular uterine endometrium. Nutrients absorbed by the villi are conveyed by the vitelline circulation toward the embryo. Most marsupials only develop a choriovitelline placenta. Some domestic mammals, such as horses and dogs, develop a temporary vitelline placenta at the beginning of pregnancy but this placenta always plays a subsidiary role until the chorioallantoic placenta is established. - Choriovitelline placenta. It is supported by the development of a choriovitelline membrane provided with vitelline blood vessels. Chorioallantoic placenta. Chorioallantoic placenta is the term used to describe the placenta when the allantois fuses with the chorion. The corionic villi of this membrane constitute the fetal tissues which develop into the final placenta in eutherian mammals. In these species, soon after implantation, the yolk sac becomes rudimentary while the allantois attaches to the chorion and the allantoic blood vessels get into close contact with the maternal circulation. - Chorioallantoic placenta. It is supported by the development of the chorioallantoic membrane provided with allantoic vessels. Types of placentas based on the endometrial erosion Non-deciduate placenta: The chorionic villi are simple and minute. The chorion apposes the uterine wall without fusing with the endometrium. At the time of birth, the chorionic villi are simply drawn out from the maternal tissue without any damage to the uterine wall, hence no bleeding occurs. This type of placenta is found in most domestic animals, such as in horses, pigs, cattle and other ruminants. Deciduate placenta: In species with an invasive corion (primates, rodents and partially in carnivores), the degree of intimacy between maternal and foetal tissues increases. The chorionic villi become complex and penetrate deeper into the uterine tissue. Chorionic villi erode and fuse with the uterine mucosa to various degrees so that the passage of substances from the mother to the foetus and vice versa is facilitated. Decidua is the term for this modified layer of the uterine lining during pregnancy, which forms the maternal part of the placenta. When the placenta is cast off after the time of birth there is lost, not only of the embryonic membranes but also of the intertwined maternal tissue with extensive haemorrhage. - Deciduate and non-deciduate placenta. In mammals with apposed placentation, there is no fusion of maternal and foetal tissue. Therefore separation is easily achieved at parturition without damage to maternal tissue. In contrast, in species with conjoined placentation, an intimate connection is formed between maternal and embryonic tissue, so that at birth some maternal tissue (decidua) is lost when the foetal membranes are expelled from the uterus. Types of placentas based on the number of histological layers (histological classification) Just before the formation of the placenta, there are a total of six layers of tissue separating maternal and foetal blood. Three layers are on the foetal side: Endothelium lining the allantoic capillaries. Connective tissue in the form of chorioallantoic mesoderm. Chorionic epithelium, the outermost layer derived from the trophoblast. Three layers on the maternal side: Endometrial epithelium. Connective tissue of the endometrium. Endothelium lining the endometrial blood vessels. A classification scheme for the placentas is based on naming the tissues that remain in contact once the process of formation of the placenta has finished. According to this criterion, the following types of placenta are considered: Epitheliochorial placenta. The chorion villi attach to the maternal uterine epithelium but there is no maternal invasion as such. It is found in horses, pigs and camels. - Epitheliochorial placenta. The chorionic and the uterine epithelia are only apposed and there is no loss of maternal tissue. One modification of this type is the synepitheliochorial placenta where some specialised trophoblastic cells detach and migrate through to the maternal epithelium and fuse with it. It is seen in ruminant placentas. - Synepitheliochorial placenta. Some specialised trophoblast cells detach and migrate through to the maternal epithelium and fuse with it. Endotheliochorial placenta. The chorion erodes the uterine epithelium and invades the maternal connective tissue stopping short at the maternal blood vessels. It is found in carnivores. - Endotheliochorial placenta. Endometrial epithelium and connective tissue are lost during placentation leaving the chorion in direct contact with the maternal vessels. Haemochorial placenta. It is a more invasive form of the placenta where the trophoblast erodes the endometrium and destroys the walls of the maternal capillaries so that trophoblast is in direct contact with maternal blood. It is found in humans, rodents, bats, among other species. - Haemochorial placenta. The reduction of the maternal tissues is complete leaving the chorion as if bathed in maternal blood. Classification based on the shape of the placenta (morphological classification) Examination of the placentas from different species reveals striking differences in their shape and the area of contact between foetal and maternal tissues: Diffuse: Almost the entire surface of the allantochorion is involved in the formation of the placenta. Seen in horses and pigs. - Diffuse placenta in horses. The chorion frondosum is diffusely distributed over the entire surface of the foetal sac. - Diffuse placenta in pigs. The chorion frondosum is diffusely distributed over the surface of the fetal sac, like in horses, but the allantochorion degenerates at the extremities forming the necrotic tips. Cotyledonary or multiple placentation: Chorionic villi are restricted to multiple, discrete areas of attachment where the chorioallantois interacts with defined structures in the maternal endometrium. The foetal portions of this type of placenta are called cotyledons, the maternal contact sites are referred to as caruncles, and each of the cotyledon-caruncle complexes forms a functional unit of placenta called placentome. This type of placentation is observed in ruminants. - Cotyledonary placenta. In ruminants, the chorionic villi assembled into large macroscopical clusters named cotyledons are separated by areas of chorion laeve. Zonary placenta. The zonary placenta takes the form of a band that encircles the fetus. In dogs and cats, it is a complete girdle-like band, while in species like ferrets it is incomplete. - Zonary placenta: The placenta takes the form of a complete or incomplete band of tissue surrounding the foetus. Seen in carnivores, seals, bears, and elephants. Discoid placenta: A single area of placenta is formed and is discoid in shape. Seen in primates and rodents. - Discoid placenta. In discoidal placentation, which occurs in humans, monkeys and rodents, the chorion frondosum is arranged in a circular plate. Equine placenta Equids are monotocous species, that is, they usually produce only one egg / young at a time in each reproduction cycle. However, twinning is relatively common in horses (1-2%), but they do not usually come to a successful conclusion. Due to its poor prognosis, in most cases, one of the twins is eliminated by "crushing" at the beginning of gestation (around day 30); Otherwise, the spontaneous abortion of a twin is the most likely outcome; it reduces the reproductive capacity of the mother or may even poses a serious risk to her health. Early embryonic development in horses is characterized by a very late implantation. To do this, the blastocyst as soon as it reaches the uterus, is surrounded by an acellular "capsule" formed by glycoproteins that are deposited between the trophoblast and the overlying zona pellucida. This capsule persists even after hatching and allows the blastocyst to keep its spherical form and prevents the attachment of the embryo to the endometrium during a period of intra-uterine migration before implantation takes place at the base of the uterine horn. This preimplantation movements and contact between the embryo and endometrium are thought to be a significant part of the maternal recognition of pregnancy in horses. After this mobility phase, the capsule is lost and implantation takes place around day 21 when the embryo becomes fixed in one spot within one uterine horn. This process, appropriately enough, is called fixation and marks the beginning of implantation. At this stage, the initially spherical conceptus enlarges and acquires a triangular appearance. Finally, the embryonic vesicle expands and acquieres a crescent shape that fills the whole uterine horn. https://sway.office.com/duUyUZFDjuLI1rYD#content=PLy8DXhs97XH1O - Implantation in horses. Before implantation, the horses´ blastocyst migrates throughout the uterine tube surrounded by a temporary capsule. Like in carnivores placenta, the equine mesomanion does not persist for a long time so the amnion becomes rapidly free inside of the embryonic sac. As a result, when parturition comes, the amnion is usually delivered intact and it is only broken by fetal movements once the foal is midway through the birth canal. If this does not happen, the amnion should be manually ruptured and removed from the area of the nose and mouth to allow the foal to breathe. For the first weeks of gestation (between the second and eighth week), the yolk sac of the equine embryo is quite large compared with other animals. The wall of the yolk sac fuses with the chorion to form a functional, although temporary, choriovitelline placenta that provides the embryo with nutrients until it is replaced by the development of the chorioallantoic placenta. From the fourth week, the allantois expands rapidly and soon fuses with the chorion allowing the development of the chorioallantoic placenta which is fully developed by the 60th day. Although the yolk sac recedes as the allantois expands throughout the sac, the vitelline and allantoic placentas co- exist for approximately four weeks. Subsequently, the functional role of the choriovitelline placenta ceases. The umbilical cord has two segments: a somewhat longer portion bordered by the amniotic sac and a shorter segment surrounded by the allantoic sac. There is considerable variability in the length of the cord, with measurements from 36 to 84 cm. There is often considerable twisting of the umbilical cord that could occasionally lead to excessive torsion and strangulation of the placental circulation. The umbilical cord contains two umbilical arteries and one left umbilical vein. Altough the two major venous branches expand over the allantochorion (right and left umbilical veins), they fuse to become one single vein, the left umbilical vein, which is the only one to enter the amniotic portion of the cord. In addition, the umbilical cord contains a thin-walled allantoic duct (urachus) and some remnants of the vitelline duct. - Development of the embryonic membranes in horses In horses, the chorioamniotic raphe or mesoamnion is temporary; as a result, the amnion quickly becomes free inside of the embryonic sac. In these placentas, the allantois expands dorsally interposing itself between the amnios and the chorion while the yolk sac forms a temporary choriovitelline placenta. Eventually, the allantois also expands ventrally, to form the definitive chorioallantoic placenta. A unique feature of the equine placentation is the development and ultimate degeneration of the endometrial cups. These structures are derivatives of the embryonic trophoblast which secretes equine chorionic gonadotropin hormone (eCG). This hormone, like the human chorionic gonadotropin hormone (hCG)s, promotes the maintenance of the corpus luteum during early pregnancy, causing it to secrete the hormone progesterone. The endometrial cups are formed several weeks into gestation and are immunologically destroyed 2 to 3 months later. These hormone- producing structures derive from a narrow band of thickened trophoblast, called chorionic girdle, arranged circumferentially around the embryonic sac at a point where the membranes of the allantois and yolk sac meet. Trophoblastic epithelial cells from the chorionic girdle penetrate and destroy the endometrial epithelium. After they migrate through the endometrial connective tissue, they develop into ulcer-like structures named endometrial cups. Gradually, cells of the maternal immune system accumulate in the periphery of the cups and eventually invade and destroy the cups. https://sway.office.com/duUyUZFDjuLI1rYD#content=w7t8uZ9dbGV0I6 - Chorionic girdle and endometrial cups. The chorionic girdle can first be detected as a narrow band of thickened trophoblast that develops circumferentially at the junction between the expanding allantois and the yolk sac. Trophoblastic epithelial cells of the girdle invade the endometrium to form ulcer-like structures named endometrial cups which have been shown to be the principal source of equine chorionic gonadotrophin (eCG). The equine placenta is classified as diffuse and epitheliochorial. On casual examination, the mature chorioallantois appears uniform in structure (diffuse placenta), but a closer look reveals the chorionic surface to be composed of thousands of polygonal structures 1-2 mm in size called microcotyledons. Microcotyledons consist of a cluster of highly vascularised chorionic villi which extend into elaborate invaginations of the endometrium called crypts. The epitheliochorial nature of this interface remains intact throughout gestation, but there is a progressive thinning and flattening of the connective tissue and epithelial layers which brings the foetal and maternal blood supplies into closer apposition. In addition to the placenta, uterine glands, present in the maternal stroma between adjacent microcotyledons, empty their secretions into the space between the maternal and fetus tissues. The copious glandular secretions are absorbed by the areolas (also spelt areolae) which are inter-microcotyledonary areas of the chorion oppose to the uterine glands. Another feature in the equine placenta is the presence of allantoic calculi (also called hippomanes) which are commonly found floating or attached to the wall of the allantoic cavity. They are brownish masses of cellular debris surrounded by mucoproteins and minerals probably originating from the foetal hindgut. Likewise, the amniotic plaques are also quite common in the amnion. They are epithelial elevations on the amniotic wall that can be also present near the umbilical cord. There is no known functional role for these epithelial structures. In this placenta, the amniotic raphe quickly disappears, and the amnion rapidly becomes free and completely detached from the chorion. That is why foals are usually born surrounded by the amnion. The new-born has to break the sac on its own, but if this fails, the foal can suffocate without human intervention. After birth, there is no significant loss of maternal tissue, and therefore, horses’ placentation is considered non-deciduate. The average time needed for placental expulsion is about one hour and should not take more than two hours. Most clinicians consider the foetal membranes to be retained if they are not passed within three hours of birth. Retention of the foetal membranes is one of the most common peripartum problems in the mare, with an incidence in the range of 2% to 10%. https://sway.office.com/duUyUZFDjuLI1rYD#content=xZS1XycsuaFLI7 - Structure of the placenta in horses. Initially, the villi attach to the endometrium in the form of a simple diffuse apposition of foetal and maternal tissues. Later the chorion frondosum develops diffusely distributed tufts of chorioallantoic villi named microcotyledons which fits into endometrial indentations called crypts. The ducts of the uterine glands open on to the surface of inter-microcotyledonary areas comparable to areolae in pigs.