SEM_06_Extraembryonic membranes and placentation_PARTE3.docx

Ruminant placenta Although cows usually produce a single offspring at a time (monotocous), twinning is not infrequent in cattle (up to 5% of births). Nevertheless, it is an undesirable feature, primarily because of the high risk of perinatal mortality but also because of the possibility of freemart...

Ruminant placenta Although cows usually produce a single offspring at a time (monotocous), twinning is not infrequent in cattle (up to 5% of births). Nevertheless, it is an undesirable feature, primarily because of the high risk of perinatal mortality but also because of the possibility of freemartinism. Almost all twins are dizygotic (fraternal); monozygotic twinning is rare in cows. About 95% of female calves born co-twin to a male undergo an abnormal development of the reproductive system. Affected calves are called “freemartins." which is the most common form of intersexuality in cattle. In the majority of bovine twins, the tips of the adjacents embryonic sacs overlap which end up fusing both vascular chorionic membranes. In mixed-sex twins, the fusion of the placental blood vessels not only allows hormones from the male to affect female development but also allows the mixing of haemopoietic precursor cells, leading to the establishment of male and female cells in the blood of each twin (chimeric individual). Sheep and goats, unlike cows, frequently have double (two young) or triple (three young) and freemartin syndrome is rarely present in these species. https://sway.office.com/duUyUZFDjuLI1rYD#content=BH5ulGsuZppTpp - Freemartin syndrome. The phenomenon of ‘freemartin,’ a female born as a twin to a male, is the result of anastomoses between placental circulations of the twin fetuses, resulting in XX/XY chimaeras at the same time that the male hormones interfere with de normal development of the females reproductor system. Implantation is a critical step in the progress of the pregnancy, during which the conceptus acquires a fixed position within the uterine lumen, and leads to the establishment of the placental structures. In ruminants, the implantation process is characterized by three main steps: a long pre-attachment period lasting 2-3 weeks during which the initially spherical conceptus elongates considerably to adopt a tubular shape, an apposition stage when cellular contacts are established between the trophoblast and the uterine epithelium, and an adhesion stage which ends the process and gives rise to the cellular structure of an epithelio-chorial placenta. https://sway.office.com/duUyUZFDjuLI1rYD#content=Ry6cFLcDdqJOlv - Bovine Implantation. The establishment of pregnancy in ruminants occurs during the peri-implantation period. Reciprocal interactions between the elongating conceptus (embryo/fetus and associated extraembryonic membranes) and endometrium culminate in implantation. In ruminant, as in pigs, the amnion keeps fused with the chorion by a persistent chorionamniotic raphe so the calf is usually born without been covered in membranes. The yolk sac is only present transiently, degenerating shortly after implantation has begun. Thus, the choriovitelline placenta is only present for a few days until is quickly replaced by the chorioallantoic placenta. Ruminants basically have an epitheliochorial placenta, but because the uterine epithelium is modified by the invasion and fusion of binucleate cells from the trophoblast, its structure is generally referred to as synepitheliochorial. Allantoic calculi and amniotic plaques, similar to those described in horses, are also found in the ruminant placenta. On the surface of the umbilical cord can also be present many squamous plaques. The umbilical cord contains 4 large blood vessels, two umbilical arteries and two umbilical veins, although only the left umbilical vein enters the embryo as the only source of placental blood (the right umbilical vein converges and fuses with the left umbilical vein just at the entrance of the body). It also contains an allantoic duct (urachus) connecting the bladder with the allantoic sac. https://sway.office.com/duUyUZFDjuLI1rYD#content=kMWqcPJEXNnkac - Embryonic membranes in ruminants In ruminants, the chorioamniotic raphe or mesoamnion persists throughout gestation. In these species, the allantois does not expand dorsally where the amnion remains attached to the chorion by a permanent mesoamnion. Contrary, the allantois expands ventrally to quickly form the chorioallantoic placenta while the yolk sac shrinks towards the umbilicus. The ruminant placenta is also classified as cotyledonary, which means that instead of having a single large area of contact between maternal and foetal tissue, there are numerous smaller areas of the placenta called placentomes. They are the common feature in the ruminant placenta, although their number, size, shape, and other features vary broadly. Sheep, goats and cattle have between 75 and 125 placentomes. Deer also have a cotyledonary placenta, but only 4 to 6 placentomes which are correspondingly larger. Placentomes also differ in shape, being convex in cows, flat in goats but concave in sheep. Common features of the cotyledonary placenta are: Placentomes: the functional units that compose this placenta. Each placentome is made of a cotyledon and a caruncle together. Cotyledons: the foetal part of the placenta. They are made of areas of chorion frondosum that stick out like rounded patches among areas of smooth chorion stretching among them. Caruncles: the maternal side of the placenta. They are oval or round thickening in the uterine mucosa resulting from the proliferation of sub-epithelial connective tissue. Caruncles are readily present in the non-pregnant uterus like small, raised non-glandular areas approximately 0.5 to 1 cm in diameter. Over the course of pregnancy, they grow larger reaching a diameter of up to 10 cm in cows. During parturition, there is a substantial loosening of the cotyledonary villi from the caruncles. After the expulsion of the foetus the capillaries within the placentomes collapse, leading to a decrease in their size. When the uterus contracts and the caruncles shrink, the separation of the cotyledons from the caruncles is enhanced and subsequently, the placenta is cast off. Usually, the placenta, including the amnion which remains attached to the allantochorion by a persistent raphe, is delivered within 12 hours of birth. The placenta is delivered with no significant loss of maternal tissue, and therefore, ruminant placentation is considered non-deciduate. https://sway.office.com/duUyUZFDjuLI1rYD#content=4R8mrHWkL03MLf - Structure of the cotyledonary placenta in ruminants. Ruminants have a cotyledonary placenta. Instead of having a single large area of contact between maternal and fetal vascular systems, these animals have numerous smaller placental areas called placentomes. In each placentome, a patch of chorion frondosum (cotyledon) assembles with the adjacent area of the endometrium (caruncula) Porcine placenta Pigs are polytocous or litter-bearing animals because they give birth to multiple offspring at the same time. Usually, a litter of piglets is between 6 and 12 piglets. Implantation in pigs is characterized by a lengthy pre-attachment period that includes migration of the hatched blastocysts to become evenly spaced throughout both uterine horns. In addition, remodeling of the embryonic vesicle takes place in which the spherical blastocyst elongates to form a filamentous sac of about 10 cm in length with a centrally positioned embryo. https://sway.office.com/duUyUZFDjuLI1rYD#content=sfpkshJQUqBGnL - Implantation in pigs The amnion persists attached to the chorion until the end of gestation so that it is torn when the foetuses go out at parturition; as a result, the piglets are usually born without covering membranes. The yolk sac reaches its maximum at the end of the third week but it regresses rapidly without forming a choriovitelline placenta. The allantois expands ventrally into the extra-embryonic coelom but not dorsally where the amnion remains attached to the chorion by a permanent mesoamnion. Pigs have a non-invasive placenta classified as being epitheliochorial and diffuse with atrophy at the peripheral tips which end up forming the necrotic tips. The allantois vessels do not expand to the tips of the chorionic sac and consequently, these parts remain avascular and shortly afterwards they become necrotic. The umbilical cord contains three blood vessels (two umbilical arteries and one left umbilical vein) and a widely patent allantoic duct. https://sway.office.com/duUyUZFDjuLI1rYD#content=7Usvy8vFtSVL6C - Embryonic membranes in pigs In pigs, the chorioamniotic raphe or mesoamnion persists throughout gestation. In these species, the allantois does not expand dorsally where the amnion remains attached to the chorion by a permanent mesoamnion. Contrary, the allantois expands ventrally to quickly form the chorioallantoic placenta while the yolk sac shrinks towards the umbilicus. As each embryo implants, the single-layered trophoblast interdigitates with a folded endometrial epithelium forming the chorioallantoic placenta. In pigs, the uterine mucosa is provided with transversal folds so that the placental exchange area is maximised by the formation of macroscopic circular folds in chorion called plicae. These transverse folds have microscopic secondary folds referred to as rugae. Another remarkable feature, besides the ridge-like pattern of the chorion, is a conspicuous presence of circular areolas into which uterine glands open. https://sway.office.com/duUyUZFDjuLI1rYD#content=0mrnfcZw31odWC - Structure of the porcine placenta Carnivore placenta Dogs and cats are litter-bearing species (polytocous) and prior to implantation, a number of blastocysts become evenly spaced throughout the uterine horns by efficient trans-uterine migration. Dogs and cats typically have 3 to 8 foetuses. It is not unusual for one or more to die in utero and their remains are destroyed and reabsorbed by an enzymatic breakdown. https://sway.office.com/duUyUZFDjuLI1rYD#content=c6Kvw2iVhf5qoj - Implantation in dogs Since the mesoamnion is transitory in carnivores the amnion separates rapidly from the chorion so that at the moment of parturition, most of the amniotic membrane is expells with the foetus. That is why pups are usually born surrounded by a thin amniotic membrane, which the mother will usually remove as soon as they arrive. However, if she does not do this, she will urgently need some kind of assistance to ensure that the pup doesn’t suffocate. The embryos enter the uterine horns at the end of the first week after fertilisation and migrate inside the uterine horn up to the end of the second week. Around the third week, the wall of the yolk sac develops a choriovitelline placenta with a functional vitelline circulation. About the fourth week, the choriovitelline placenta degenerates and is replaced by the chorioallantoic placenta although rest of the yolk sac may remain in the umbilical cord. The carnivore placenta is classified as zonary because it takes the form of a band that encircles the foetus. In dogs and cats, it is a complete band, while in species like ferrets and racoons, it is incomplete (i.e. two half bands). The umbilical cord is short and has few twists. It has an allantoic duct and four vessels, two umbilical arteries and two umbilical veins but only the left umbilical vein enter the embryo’s body. https://sway.office.com/duUyUZFDjuLI1rYD#content=nlN8qd505pbXq8 - Embryonic membranes in dogs In dogs, the chorioamniotic raphe or mesoamnion is temporary; as a result, the amnion quickly gets 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. Histologically, dogs and cats have an endotheliochorial placenta. In this type of placenta, the endometrial epithelium is eroded by an invading trophoblast so that the foetal chorionic epithelium comes to be in contact with the wall of the endothelial cells of the maternal vessels. Initially, the invading foetal chorion is in the form of straight folds referred to as lamellae. Soon, this initial structure becomes more or less branched and twisted, resulting in the formation of the labyrinthine placenta. Because the carnivore placenta erodes and partially destroys the maternal endometrium, they are considered to have deciduate placentation, although the typical decidual cells, as in primates, are hard to identify. Perhaps, one of the best-known features of the canine placenta is the marginal haematoma. These are bands of maternal haemorrhage at the margins of the zonary placenta. The products of haemoglobin breakdown give them a distinctly green colouration in dogs, whereas in cats they are brownish and usually less obvious. https://sway.office.com/duUyUZFDjuLI1rYD#content=kgeRyZ3DWYZOEZ - Structure of the placenta in dogs