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VAN 504 Lecture 09 Structure and types of mammalian placenta Placenta (plah-sen’tah) is a Latin word. It means “a flat cake • The placenta is an organ characteristic of mammals that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. • "True" placentas are a defining characteristic of eutherian or "placental" mammals Placental Development • The morphogenesis of the placenta during early gestation is closely related to those extra embryonic or fetal membranes that are differentiated into the yolk sac, amnion, allantois, and chorion. The fetal membranes participate in the formation of the placenta, either separate or in certain combinations, and give rise to three basic types of placentation which differ in regard to the identity of the fetal membranes involved: chorionic, chorioallantoic & yolk sac placentation. • Among these types the chorioallantoic placentation derived from the fusion of the allantois with the chorion is characteristic of the farm animals. Placenta PLACENTA: • • is a fetomaternal organ. It has two components: – Fetal part – develops from the chorionic sac – Maternal part – derived from the endometrium PLACENTAL SHAPE: defines the proportion of surface area shared between fetal membranes and maternal uterine tissue where exchange occurs PLACENTAL TYPE: defines the structure of cell layers separating fetal blood from maternal blood • The placenta and the umbilical cord are a transport system for substances between the mother and the fetus. Functions of placenta • A site of exchange of gases and metabolites between the maternal and fetal circulation. • A major endocrine organ producing steroid and protein hormones. • Protection. • Nutrition. • Respiration. • Excretion. • Hormone production, (progesterone,estrogen,Gonadotrophins Placental Shapes • Determined by the distribution of villi over the chorionic surface. • Diffuse – noninvasive • Cotyledonary – placentomes are the point of high throughput maternal/fetal contact • Zonary/Discoid – invasive; most direct contact between fetal and maternal blood Diffuse Placenta Uterine Endometrium Fetal Chorion Cotyledonary Placenta Cotyledonary Placenta Umbilicus Fetus Placentome Placentation by Species SPECIES SHAPE Cow Cotyledonary Ewe,goat Cotyledonary Sow Diffuse Mare Diffuse Dog, Cat Zonary Primates, Human Discoid Rodents Discoid TYPE Synepitheliochorial Synepitheliochorial Epitheliochorial Epitheliochorial Endotheliochorial Hemochorial Hemochorial Hormones Produced by the Placenta • Equine Chorionic Gonadotropin: maintains primary CL, responsible for formation and maintenance of accessory CL. • Human Chorionic Gonadotropin: maintain CL • Progesterone: in some species (ewe, mare, woman) the placenta takes over progesterone production later in gestation. ”progesterone block” – inhibits myometrial contractions • Estrogen: peak of E2 signals preparturient period in some species • Placental Lactogen: stimulates growth of fetus and mammary glands • Relaxin: softens connective tissue in the cervix and relaxes pelvic ligaments Fetal Growth Growth: period of development from embryo to fully developed fetus, prior to parturition Example of development relative to time for the bovine Calcification of Bone Matrix Extensive Bone Formation Tooth Formation Hair, eyes, muzzle Hair over entire body 70 days 180 days 110 days 150 days 230 days Types of Placentas • Maternal Blood • Maternal endothelium • Maternal connective tissue • Maternal epithelium • Fetal epithelium Type = degree of invasiveness, based on layers separating maternal blood from the fetal epithelium Least Invasive epitheliochorial • Fetal connective tissue synepitheliochorial • Fetal endothelium endotheliochorial • Fetal Blood Most Invasive hemochorial Histological classification of placentas is based on the degree of removal of the maternal layers KEY CHARACTERISTICS OF MAMMALIAN PLACENTA Foetal capillary (from umbilical artery) Endothelial layer Connective tissue layer (may be minimal) Cellular layer (may be trophoectoderm + maternal epithelium or a syncytium of the two, or solely trophoectoderm) Connective tissue layer (may be minimal) Maternal capillary (in haemochorial placenta of primates the endothelium is degraded) EPITHELIOCHORIAL Maternal endometrial epithelium intact (horse, pig), all layers preserved. SYNEPITHELIOCHORIAL Syncytium of maternal epithelium and Chorion (ruminants), all layers preserved. ENDOTHELIOCHORIAL Removal of maternal endometrial epithelium And connective tissue (dogs, cats) HAEMOCHORIAL Removal of maternal endothelium (human, some rodents) • The primitive type of placenta is a condition where the endothelial walls, blood vessels, and mucosa of both the mother and the fetus remain intact. This is known as an EPITHELIOCHORIAL PLACENTA. • SYNDESMOCHORIAL - when maternal epithelium is broken down (e.g. cows). • ENDOTHELIOCHORIAL - when maternal epithelium and mucosa are both broken down (e.g. dogs). • HEMOCHORIAL - when all of the maternal tissues are broken down to the point that the fetal epithelium is literally bathed directly by the maternal blood cells (e.g. humans, other primates, rodents) • (Essentially, all this has to do with the degree of destruction of the maternal part of the placenta.) • The classification of the placenta by microscopic structure is based upon the maternal and fetal tissues that are actually in contact. • The basic structure comprises on the maternal side blood vessel, connective tissue and epithelium and on the fetal side chorionic epithelium, connective tissue and blood vessel. • While all layers are preserved in the epitheliochorial (horse and pig), and synepitheliochorial placentas (ruminants) • some layers on the maternal side are lost as in endotheliochorial (cat) and hemochorial (human). • In ruminants fetal chorionic binucleate cells migrate to form a syncytum at the junction of the maternal and fetal tissue. Classification based on histological structure 1. Epitheliochorial type: this type is the most superficial placenta and lacks significant invasion of the uterine lining. Pockets of columnar trophoblasts are loosely applied to the maternal endometrial epithelium. No destruction or invasion of the maternal tissues occurs and no layers are removed. • The foetal chorion Is In contact with eplthelium of the uterus hence it is called epithello chorial placenta. In between foetal, maternal parts six layers are present. I. Endothelium of mother blood vessel. ii. Maternal syndesmose connective tissue. lii. Epitheliurn of mother Iv. Chorion of foetus. v. Foetus connective tissue (syndesmose vi.Endothellum of foetal blood vessel The epitheliochorial type is found in horses, pigs and ruminants. Contd 2.Syndeumose chorial placenta The chorion will come in contact with syndesmose of mother’s uterus. Hence it’s called syndesmose chorial. • Ex: Sheep, Cow. 3. Endotheliochorial type: the maternal uterine epithelium and connective tissue disappear after implantation, and the trophoblasts come into direct contact with the maternal endometrial. • The endotheliochorial type occurs in orders from all four major clades of eutherian mammals (Euarchontoglires, Laurasiatheria, Xenarthra and Afrotheria), including carnivores. 4. Hemochorial type: this type is the most invasive placenta. • All maternal tissue layers disappear through erosion, leading to direct connection between the chorion and maternal blood. • There are hemomonochorial (primates), hemodichorial (rabbits), and hemotrichorial (rats and mice) placentas, with one, two and three trophoblast layers, respectively The haemochorial placenta shows the intimate juxtaposition of foetal and maternal blood allowing efficient exchange A The haemochorial placenta Foetal capillaries B Umbilical vein Umbilical arteries Chorio nic villi Maternal Maternal blood arteriole pool Maternal venule Notice the expansions at the’turn around’ to allow slower blood flow and better equilibration with maternal blood 5. Hemo endothelial placenta float In mother’s blood. Hence it called hemo endothelial placenta Hence Rat, Rabbit, Histological Classification Tissues epithelialchorial syndesmochorial endothelialchorial hemohemochorial endothelial Maternal endothelium conn. tissue epithelium + + + + + - + - - - Fetal epithelium conn. tissue endothelium + + + + + + + + + + + + + pig horse ruminant *ruminant dog cat human rat rabbit Species Gross anatomical classification of placentas is based on the pattern of contact between chorion and endometrium & Shape • • • DIFFUSE Uniform distribution of chorionic villi over contact surface (horse, pigs) COTYLEDONARY Villi restricted to defined area (cotyledons) (ruminants) Diffuse Placenta Horse Pig Microcotyledons • ZONARY • Girdle of chorionic villi around middle of chorionic sac (dogs,cats) DISCOIDAL Disc-shaped area on chorionic sac (humans, rodents) • Microscopic Structure of the Placenta • Dogs and cats have an endotheliochorial type of placenta. • In this type of placenta, the endometrial epithelium under the placenta does not survive implantation, and fetal chorionic epithelial cells come to be in contact with maternal endothelial cells. • During implantation, cytotrophoblast cells surrounding the central third of the chorioallantois proliferate to form a syncytium called syncytiotrophoblast. • The syncytiotrophoblast erodes through the endometrial epithelium and flows around maternal capillaries. • Initially, the invading fetal cells are in the form of villi, but villi soon coalesce to form a labyrinthine-type of placenta. Just prior to formation of the placenta, there are a total of six layers of tissue separating maternal and fetal blood. There are three layers of fetal extraembryonic membranes in the chorioallantoic placenta of all mammals, all of which are components of the mature placenta: 1. Endothelium lining allantoic capillaries 2. Connective tissue in the form of chorioallantoic mesoderm 3. Chorionic epithelium, the outermost layer of fetal membranes derived from trophoblast There are also three layers on the maternal side, but the number of these layers which are retained - that is, not destroyed in the process of placentation - varies greatly among species. The three potential maternal layers in a placenta are: 1. Endothelium lining endometrial blood vessels 2. Connective tissue of the endometrium 3. Endometrial epithelial cells • The placenta classification on nature of contact: • There are two types: nondeciduate and Deciduate type. • NONDECIDUATE TYPE PLACENTA: • The chorianic villi are simple projections, they lie in contact with uterus. They have a loose contact. There is no fusion. At the time of birth of embryo uterus is not damaged. This type is characteristic of farm animals. DECIDUATE TYPE PLACENTA: • The allantochorianic villi penetrate into uterine valli. • They are intimately fused. Hence maternal epithelium is lost along with fetal membrane at parturition. Bleeding occurs. • It is restricted to the hemochorial placenta (human). Glucose is the dominant energy yielding substrate for the foetus with little use of fatty acids ENERGY SUBSTRATES Glucose oxidation accounts from 50% oxygen use Most of the rest is due to amino acid and lactate oxidation Rather little is from fatty acid oxidation (Notice that maternal energy metabolism is almost the mirror of this with a shift to fatty acid oxidation and a shift away from amino acids and glucose) Shape or Distribution of Chorionic Villi • • • • Cotelydonary - cow, sheep Diffuse - pig, horse Zonary - dog, cat Discoid - human Cotelydonary Placenta Cow Ewe Placentome Placental Attachment in Ruminant Binucleate Cell Chorion Migrate and fuse with uterine epithelium Uterine Epithelium Syncytium Capillary Stroma Fusion of Binucleate cells and uterine epithelium (multinucleate) Day 23 Migration of Bicnucleate Cells and Formation of Syncytium Binucleate Cell Endometrial Stroma Multinucleated Cells in Uterine Epithelium Cotelydonary Placenta Cow Convex Cotelydon (Chorion ConcaveEwe Caruncle 70 - 120 Endometrium 90 - 100 Binuclear Giant Cells • 20% of fetal placenta • Invade endometrium • Source – Placental lactogen – Pregnancy specific protein B Placental Lactogens (Protein Hormones • Prolactin-like activity:Stimulates Milk Synthesis • Not Present in Pig and Mare • May regulate maternal metabolism to facilitate fetal growth • High levels in the last 1/3 of gestation. • High levels facilitate higher milk production. • Dairy cows have higher blood concentrations than beef cows Microcotelydons Microcotelydon (Fetal) Epithelium Microcotelydon (Maternal) Endometrial Glands Uterine Arteries Increase placental surface area Microcote lydon E n d o m e t ri u Uterine Veins m Zonary Placenta (bitch, queen)