Placenta is an organic connection between the foetus and uterine wall, for physiological exchange between foetus and mother’s blood.
The placenta develops at the point of implantation. At first, the trophoblast cells absorb food and oxygen from the increasingly vascularised uterine lining. The allantois grows out from the embryo and fuses with the chorion to form the allanto – chorion which will develop into the placenta. The allantois gives rise to the umbilical cord which contains blood vessels connecting foetus and placenta.
1. Classification of Placenta: Three main factors involved in the classification of mammalian placenta are – 1. Nature of extraembryonic membranes involved. 2. Distribution of vill (finger like projection, formed from the outer surface of chorion) and shape of placenta. Connections between foetus and mother via the placenta The villi provide a large surface area for interchange of materials between the embryo and the mother. 3. Histological types.
2. Types of Placenta:
1. Epitheliochorial: Placenta with all the six barriers between foetal and maternal blood. E.g. horse, ass, marsupials. 2.
Syndesmochorial: Uterine epithelium breaks down only five barriers left. E.g.
cow, buffalo, sheep, goat, camel. 3. Endotheliochorial: Uterine epithelium and connective tissue eroded only four barriers left. E.
g. tiger, lion, cat, dog. 4. Haemochorial: Placenta with only three barriers, the maternal part of placenta eroded.
E.g. human, ape, lemurs. 5. Haemoendothelial: All barriers except endothelium of foetal part of placenta get eroded.
E.g. rat, rabbit.
1. Non-deciduous: No part of uterine portion of placenta is broken off.
E.g. horse, ass. 2. Deciduous: A portion of uterine tissue called decidua is detached and passed out at birth, E.g. most of the mammals. 3.
Contradeciduous: Even the foetal part of placenta is retained and gets absorbed to provide nourishment. E.g. talpa, parameles.
On the basis of distribution of villi on the surface
1. Diffuse placenta: Villi distributed uniformly all over the surface. E.g. horse, pig. 2.
Cotyledonary: The villi form tufts which fit into corresponding areas, the caruncles in uterine part of placenta. E.g. cow, buffalo, sheep. 3. Intermediate: Villi occur singly as well as in tufts, E.g. camel, giraffe 4.
Zonary: Villi arranged in two transverse bands. E.g. tiger, lion, cat, dog, elephant. 5.
Discoidal: When the villi are confined to a disc-like area. E.g.
rat, rabbit, bat. 6. Metadiscoidal: The placenta in which the villi are initially distributed uniformly all over the surface but later on get confined to a disc-like area fitting into a corresponding depression on the uterine wall. That is, the placenta is diffuse at first but later on becomes discoidal. E.
g. human beings and apes. 3. Function of Placenta:
All the nutritive elements including glucose, fatty acids, amino acids, nucleotides, vitamins and minerals from the maternal blood pass into the foetus.
Oxygen passes from the maternal blood to the foetal blood through the placenta and CO2 passes in reverse direction.
Excretory products diffuse into maternal blood and are again excreted by the mother.
Stores glycogen, fat etc.
Allows only essential materials to pass into the foetal blood.
Secretes hormones such as estrogen, progesterone and human chorionic gonadotropin (HCG).