Placentation in mammals

Placentation in mammals

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  Placentation in Mammals Presented by Sharmila.S AssistantProfessor, Department of Zoology , E.M.G.Yadava Womens Cllege, Madurai
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  Introduction • Placenta isa fetomaternal organ which is the primary site of nutrient and gas exchange between the fetus and the mother. • The term placenta derived from Greek (plakuos = flat cake) . • named on the basis - organs appearance • The placenta a mateno-fetal organ which begins developing at implantation of the blastocyst and is delivered with the fetus at birth. • At most recently have we start to understand the many different functions of this organ carries out in addition to its role in embryonic nutrition.
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  • Generally placentafound in mammals • It is a large organ that develops during pregnancy- internal development • Embryo shows spl kind of tissue called placenta • Formation of placenta – contribution of both embryonic and maternal tissues. • Process of placental development called as placentation.
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  Classification of placenta •based on the type of Foetal Membranes Involved • based on the Distribution of Villi • based on the Nature of Contact • based on the histological intimacy of foetal and maternal tisues
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  Based on thetype of Foetal Membranes Involved • Embryonic tissues which are involved in the formation of placenta, except amnion other three membranes such as chorion , yolk sac and allantois are plays important role in formation of placenta. • Based on this placenta classified into two types 1. Yolk sac placenta 2. Chorio- allantoic placenta
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  1.Chorio-vitelline placenta oryolk-sac palcenta (is formed by yolk sac and chorion ) • In some marsupials, the allantois remains relatively small and never makes contact with the chorionic sac, whereas the yolk sac becomes very large and gets fused with chorion. • The chorionic blood vessels are connected with the vitelline blood vessels of the yolk sac. Hence,the yolk sac contributes the major share in the formation of placenta. Eg. Didelphys, dasyurus
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  2.Chorio-allantoic placenta (isformed on chorion and allntois) • In some marsupials and all eutherian mammals, the yolk sac remains rudimentary and the allantois becomes well developed and vascularised to fuse with chorion. The chorio-allantoic villi constitute the plancenta. In these animals yolk sac placenta is not seen. • This type of placenta is called as chorio-vitelline placenta. The chorio- allantoic villi grow out and get attached with the uterine wall. Eg. Peramoles and eutherian mammals.
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  Based on theNature of contact • chorionic villi may be loosely /intimately connected with the uterine wall • Based on this placenta is classified into two types 1. Indeciduate Placenta & 2. Deciduate Placenta
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  1.Indeciduate Placenta • Herethe implantation is superficial type. • The chorianic villi are simple projections, they lie in contact with uterus. The villi are initially formed by the trophoblast but later on the blood vessels and connective tissues are extended to them. • They have a loose contact. • There is no fusion. • At the time of birth of embryo uterus is not damaged. No bleeding occurs at the time of parturition. • Ex. Ungulate (pig , cow) & Lemurs
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  • Deciduate typePlacenta: • Here the degree of intimacy between the maternal and foetal tissues is great. The allanto-chorianic villi penetrate into uterine villi. • They are intimately fused. Hence at the time of birth, the uterus is damaged. • Bleeding occurs, the uterine wall enters into formation of placenta is calle • It d deciduas. Ex: Primates(monkey , human ) Rodentia (mice,rat) ..etc. • It has three important parts
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  1. Decidua capsularis[reflexa] –( portion of the uterine mucosa immediately overlying the embryo) it covers the blastocyst except embryonic pole and separates it from uterine cavity. 2. Decidua basalis – (beneath the embryo) it is a part of decidua between blastocyst and myometrium. It forms the fetal part of placenta. 3. Decidua parietalis –( the remainder of the uterine lining) it is the rest of the endometrium that lines the rest of the uterine cavity.
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  Based on thedistribution of villi • According to the distribution of villi six kinds of placenta are seen. • 1.Diffused type placenta: Ex Horse, pig, • The villi are uniformly distributed on the surface of blastocyst, except at the extreme ends. • 2.Cotyledonary placenta : Ex Sheep, Cow, Deer. • The villi are arranged in groups. Each group is called cotyledon. Each cotyledon fits into caruncla of uterus.
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  • 3.Intermediate typePlacenta Ex : Giraffe & camel • It is a rare type, it shows free villi on cotyledons. Hence it is called intermediate type placenta • In these three types of placenta during parturition the foetus will not damage uterus. • 4.Zonary placenta: Ex Cat, Dog, Carnivores. • The villi are In the form of transverse zones. in dog a single girdle of vhf will be present. In fox two girdles of villi are present. The villi penetrate into uterine wall. Hence during parturition uterine wall is damaged.
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  • 5.Discoidal Placenta:ex Rat, Bat, Rabbit. • On the entire surface of blastocyst the villi are in the form of discs. When the embryo is growing.It moves away from uterus hence the with look like a disc. These villi are intimately connected with uterus. Hence during parturition much uterine tissue is damaged. • 6.Metadiscoidal placenta: Ex. Man, monkey . • The villi are at first uniformly distributed throughout the entire surface of chorion . • later they are restricted to one / more disc shaped areas on the ventral side . It occurs in primates. • Man has single disc of villi – monodiscoidal placenta • Monkey has two discs of villi – bidiscoidal
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  Basede on thetypes of tisues involved (histology) • number of layers of cells present between foetus and uterus blood supply the placenta is classified into five types. 1.Epithelio chorial placenta : Ex Pig, Horse, (Ungulates Lemmures) • The foetal chorion is in contact with epithelium of the uterus hence it is called epithelio-chorial placenta. In between foetal, maternal parts six layers are present. If all the six layers are present the placenta is called epithelio-chorial placenta.
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  2.Syndeumochorial placenta: ExSheep, Cow. • The allanto-chorianic vith will pierce into the uterus of the mother, the chorion will come in contact with syndesmose of mother’s uterus. Hence it is called syndesmose chorial placenta. 3. Endotheliochorial placenta : Ex Dog, Carnivores. • The chorion of the foetus will come in contact with the endothelim of mother ‘s uterus, hence it is called endothelio-chorial placenta. 4. Hemochorial placenta: Ex: Bat, Man, Primates, Insectivores. • The placental connections are more intimate. The chorion of foetus will float in the blood pools of mother’s uterus. Hence it is called haemochorial placenta. 5.Hemo endothelial placenta Ex: Rat, Rabbit, Hence guinea-pig will float In mother’s blood. • Hence it called hemo endothelial placenta.
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