Placenta structure and Classification
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1. The document discusses the structure and classification of placentas. It describes the different types of placentas found across mammals based on their gross shape, layers between maternal and fetal blood, and histological structure. 2. The major placenta types discussed are epithelio-chorial (found in marsupials and some ungulates), syndesmo-chorial (ruminant ungulates), endothelio-chorial (carnivores), haemochorial (primates and bats), and haemo-endothelial (rodents). Each type involves a different number of layers separating maternal and fetal blood and tissues. 3. Placentas are also classified
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Placenta structure and Classification
- 1. PLACENTAL STRUCTURE AND CLASSIFICATION Dr. Dinesh C. Sharma Associate Professor & Head-Zoology K. M. Government Girls P.G. College, Badalpur, G.B. Nagar-203207 [email protected]
- 2. The placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply; to fight against internal infection; and to produce hormones which support pregnancy. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development
- 3. The placentas of all eutherian mammals provide common structural and functional features, but there are striking differences among species in gross and microscopic structure of the placenta. Two characteristics are particularly divergent and form bases for classification of placental types: • The gross shape of the placenta and the distribution of contact sites between fetal membranes and endometrium. • The number of layers of tissue between maternal and fetal vascular systems.
- 4. On the basis of mode of Impanation Placenta Superficial Carnivore, Dog, Monkey Eccentric Squirrel, Rat Interstitial Hedgehog, Ape, Human, Bat, Guinea pig
- 5. 1. Superficial Implantation:-Growth of the chorionic sac brings it into contact with the lining of the main uterine cavity. This type of implantation is called central implantation, e.g., ungulates, carnivores, monkey. 2. Eccentric Implantation:-The chorionic sac lies for a time in a fold or pocket which looses off from the main cavity, e.g., beaver, rat, squirrel. 3. Interstitial Implantation:-The chorionic sac penetrates into the substance of the uterine lining, e.g., hedgehog, guinea pig, some bats, ape and man.
- 6. Placenta Yolk sac MetaTherai Allanto-chorionic Eutheria Non-Deciduous Cotyledonary Ruminants. Intermedia te Giraffe Diffuse Horses, pigs Deciduous Zonary dogs and cats Meta Discoidal Human Discoidal Primates, Rodents Contra- Deciduate On the basis of origin of Villi
- 8. 1. Non-Deciduous Placenta or Semiplacenta: In most mammals, the implantation is superficial, i.e, the blastocyst lies in the cavity of the uterus in contact with the uterine wall. The contact may be made more intimate by the surface of the blastocyst by forming finger-like outgrowths which penetrate into depressions in the wall of the uterus. Such outgrowths are initially formed by the trophoblast (i.e., the epithelial layer covering the blastocyst), but later on the connective tissue and blood vessels invade the outgrowths. These outgrowths are called chorionic villi, the blood vessels of chorionic villi are the branches of allantoic blood vessels in case of chorio-allantoic placenta. (In chorio-vitelline placenta, vitelline blood vessels give their branches to chorionic villi).
- 9. At the time of birth, when parturition (the separation of the foetus and its membranes from the mother’s body) occurs, the chorionic villi are simply drawn out from the depressions in the wall of the uterus and, thus, maternal and foetal tissues are separated without further damage to the uterine wall and no bleeding occurs. This type of placenta is called non-deciduate or non- deciduous placenta and is found in pigs, cattle and some other mammals. Further, the chorionic villi of a non-deciduate placenta, because lie in apposition with the endometrium, but, do not fuse with it, so such a placenta is also called semiplacenta.
- 10. 2. Deciduous Placenta or Placenta Vera: In other mammals, however, the degree of intimacy between maternal and foetal tissues becomes further increased. The wall of the uterus becomes eroded to various degrees through the action of the trophoblast and the embryonic tissues penetrate into the uterine wall, establishing a more intimate contact and facilitating the passage of substances from the mother to the foetus and from the foetus to the mother. Here because the chorionic villi fuse with the eroded uterine mucosa, such placenta is called placenta vera (true placenta). At the end of pregnancy the uterine wall is no longer intact and when the foetus with its membranes including the chorion is removed, more or less extensive haemorrhage from the uterine wall ensues (i.e., at birth, when such placenta is discharged, the uterine lining also tears away with some bleeding). Such a type of placenta found in higher eutherian mammals and is called deciduate or deciduous placenta.
- 11. The maternal tissues which are expelled at birth in the case of deciduate placenta are called deciduae. The haemorrhage at parturition is normally stopped by the same mechanism as serves for the expulsion of the newborn, the contraction of the muscular wall of the uterus constricts the blood vessels and, thus, slows down the flow of blood, until clotting of the blood stops the haemorrhage altogether.
- 12. 3. Contra-Deciduate Placenta:- In Perameles and Talpa (mole), somewhat modified type of deciduate placenta occurs, which is called contra- deciduate placenta. In such case, not only there is a loss of maternal tissue but also of the foetal portion of the placenta, both of which absorbed in situ by maternal leucocytes.
- 13. 1. Non-Deciduous Placenta or Semiplacenta: A. Diffuse Placenta:- In some mammals (e.g., ungulates, pig, sow, mare, horse, lemur, etc.) the chorionic villi remain scattered all over the surface of the chorion and their placentae are correspondingly expensive.
- 14. 1. Non-Deciduous Placenta or Semiplacenta: B. Cotyledonary Placenta:- In a cotyledonary placenta, the villi are found in groups or patches, while the rest of the chorion surface is smooth. The rosettes or patches of villi are called cotyledons, and the placenta of this type is found in ruminants (cud-chewing), ungulates such as cattle, sheep and deer.
- 15. 1. Non-Deciduous Placenta or Semiplacenta: C. Intermediate Placenta:- It is a rare type, it shows free villi on cotyledons. Villi are arranged in cotyledons as well as scattered. Hence it is called intermediate type placenta In these three types of placenta during parturition the foetus will not damage uterus. Camel and giraffe
- 16. 2. Deciduous Placenta or Placenta Vera: A-Zonary Placenta:- In a zonary placenta, the villi are developed in the form of a belt or girdle-like band around the middle of their blastocyst or chorionic sac, which is more or less elliptical in shape. Such a placenta occurs in carnivores (e.g., cats, dogs, etc.). Raccoon has incomplete zonary placenta.
- 17. 2. Deciduous Placenta or Placenta Vera: B. Discoidal Placenta:- In insectivores, bats, rodents (mouse, rat, rabbit, etc.) and bear, the villi are restricted to a circular disc or plate on the dorsal surface of blastocyst.
- 18. 2. Deciduous Placenta or Placenta Vera: C. Metadiscoidal Placenta:- In primates also discoidal placenta is found but of special type, i.e., chorionic villi are at first scattered but later on become restricted to one or two discs. a-Monodiscoidal Thus, in man the placenta has a single disc-shaped villous area and is called monodiscoidal placenta. b- Bidiscoidal- In the monkeys, the placenta consists of two disc-shaped villous areas and such a placenta is called bidiscoidal placenta.
- 19. C. Classification of Placenta According to Histology: On histological basis, following types of mammalian placentae have been recognised:
- 20. In placenta formation six tissue or membranes participated : (i) The endothelium of the maternal blood vessels; (ii) Endometrial connective tissue (mesenchyme); (iii) Uterine epithelium; (iv) The ectoderm of the chorion or chorionic epithelium; (v) Chorionic connective tissue (foetal mesenchyme) (vi) The endothelium of foetal blood vessels.
- 21. 1. Epithelio-Chorial Placenta: 2. Syndesmo-Chorial Placenta: 3. Endothelio-Chorial Placenta: 4. Haemo-Chorial Placenta: 5. Haemo-Endothelial Placenta:
- 22. There are three layers of fetal extraembryonic membranes in the chorio-allantoic 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
- 23. Placenta Foetal Layer Uterus layer Example Epithelio- Chorial 3 3 Marsupials, Ungulates (pig, horse, cow, cattle, etc.) and lemurs Syndesmo- Chorial 3 2 Ruminant ungulates (cattle, sheep) Endothelio- Chorial 3 1 Carnivores (dogs, cats, bears, etc.) Haemo- Chorial 3 0 Primates, insectivores (moles, shrews), and chiropterans (bats), Haemo- Endothelial 2 0 Higher rodents (rat, guinea pig, rabbit),
- 26. 1. Epithelio-Chorial Placenta: The epithelio-chorial type placenta is most primitive type and it is found in marsupials, ungulates (pig, horse, cow, cattle, etc.) and lemurs. In this case, placenta is formed of six tissue or membranes: I. The endothelium of the maternal blood vessels; II. Endometrial connective tissue (mesenchyme); III. Uterine epithelium; IV. The ectoderm of the chorion or chorionic epithelium; V. Chorionic connective tissue (foetal mesenchyme) and VI. The endothelium of foetal blood vessels. Because, the immediate contact of the two halves of the placenta involves chorionic epithelium and uterine epithelium, this type of placenta is called epithelio- chorial placenta. The villi of an epithelio-chorial placenta, push in the wall of uterus and lie in pocket-like depressions of the uterine wall.
- 27. 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.
- 28. 2. Syndesmo-Chorial Placenta: In the ruminant ungulates (cattle, sheep), the foetal and maternal components are fused so intimately as to result in a destruction of the uterine epithelium, thus, bringing the chorion into contact with the connective tissue of the uterine mucosa. Only five barriers or tissues, therefore, lie between the two (viz., foetal and uterine) blood streams. This type of placenta is called syndesmo-chorial placenta. 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 iti s called syndesmose chorial.
- 29. 3. Endothelio-Chorial Placenta:- In carnivores (dogs, cats, bears, etc.), the uterine mucosa is also reduced and the chorionic epithelium comes in contact with endoethelial walls of the maternal (uterine) blood vessels. In such a case, therefore, there lies only four barriers between the foetal and maternal blood streams. This type of placenta is called endothelio-chorial placenta.
- 30. The chorion of the foetus will come in contact with the endothelim of mother ‘s uterus, hence it is called endothelio-chorial placenta.
- 31. 4. Haemo-Chorial Placenta:- In the haemo-chorial placenta of primates, insectivores (moles, shrews), and chiropterans (bats), a reduction of the barriers to three occurs, i.e., the endothelial walls of maternal (uterine) blood vessels also disappear and the chorionic epithelium is bathed directly in maternal blood sinuses. Actually, the chorionic villi are surrounded by spaces (sinuses) devoid of endothelial lining, into which maternal blood enters through the uterine arteries flows out through the uterine veins.
- 32. 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
- 33. 5. Haemo-Endothelial Placenta:- In haemo-endothelial placenta of higher rodents (rat, guinea pig, rabbit), the number of barriers between the maternal and foetal blood streams is further reduced to two. In them, the chorionic villi lose their epithelial and connective tissue layers to such a degree that, in most places, the bare endothelial lining of their blood vessels alone separates the foetal blood from the maternal blood sinuses.