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The placenta may be defined as a structure produced by the fusion of the extraembryonic membranes of the foetus with the uterine endometrium of the mother for the purpose of physiological exchange. Endometrium is the mucous membrane lining the uterus.
TYPES OF PLACENTA-
On the degree of contact between chorionic villi and the endometrium.
(a) Non-deciduate placenta: •The chorionic villi are simple and minute.
• They lie on the crypts of uterine wall apposing endometrium and do not fuse with it.
•At the time of birth, when parturition (separation of the foetus and its membranes from the uterine wall) occurs, the chorionic villi are simply drawn out from the depression without any damage to the uterine wall, hence no bleeding occurs.
• Examples:- This type of placenta found in pig, cattle, horse and other ruminants is known as non-deciduae or semi-placenta.
(b) Deciduate placenta: •The degree of intimacy between maternal and foetal tissues increased.
• The allanto-choric villi become complex and penetrate deeper into the uterine tissue.
• Chorionic villi fuse with eroded uterine mucosa to various degrees, so that the chorionic epithelium comes to lie in the connective tissue or into the maternal blood, facilitating the passage of substances from the mother to the fetus and vice versa.
• Such a placenta is cast off at the time of birth; there is a loss, not only of embryonic membranes but also of encapsulating maternal tissue with extensive hemorrhage. Such a placenta is called deciduate placenta, and the uterine wall participating in the formation of such a placenta is the decidua.
Examples:- In higher eutherian mammals including dog, rabbit and man
c) Contra deciduate placenta: • No part of the placenta is thrown out.
• The foetal portion of the placenta remains in the uterus, where it is broken up or absorbed in situ by maternal leukocytes; such a condition is called contra deciduate.
Examples:- In Таlpa (mole) and Parameles (bandicoot)
Type of placenta according to the distribution pattern of chorionic villi:
(a) Diffuse placenta: •The chorionic villi remain scattered all over the surface of the chorion and their placentae are correspondingly extensive.
Examples:- Among primates (lemurs) and some ungulates (pig, horse), t
(b) Cotyledonary placenta: •The true villi are grouped in well-spaced prominent rosettes or patches known as cotyledons.
• The rest of the chorionic surface is smooth.
Examples:- In ruminants (cud-chewing) ungulates such as cattle, sheep and deer
c) Zonary placenta: • The villi have developed in the form of a belt or a girdle around the middle of the chorionic sac.
• Raccoon has incomplete zonary placenta
Examples:- In carnivores (cat, dog)
(d) Discoidal placenta:
The villi are restricted to a small disc shaped area of the blastocyst. The functional placenta has therefore has the shape of a disc and is known as discoidal placenta.
In the monkeys , the placenta consists of two discs and is therefore called a bidiscoidal placenta.
In humans and anthropoid apes, the early blastocyst is all covered with villi but later they disappear from its free surface persisting on a small disc shaped area attached with the uterine wall. Such a secondarily discoidal placenta is called metadiscoidal placenta.
Epithelio-chorial placenta: •This type is the least modified type of placental condition illustrated by all marsupials, some ungulates and lemurs.
• The allantois and chorion unite and become jointly vascularized by allantoic blood vessels.
• This composite membrane forms simple chorionic villi fitting into corresponding pockets or pits in the mucosal lining of the uterus.
• In this type of placenta all the six tissues (three foetal and three maternal) lie as barriers between the foetal and maternal blood streams.
• Therefore, the molecules of nutrients and oxygen during their passage from the mother to foetus pass through
1.the endothelium of maternal blood vessels,
2.endometrial connective tissue 3.uterine epithelium,
4.the epithelium of chorion,
5.allanto-chorionic connective tissue (foetal mesenchyme), and
6.the endothelium of foetal blood vessels.
• Over the same path gaseous wastes from the embryo travel in reverse direction.
•In this case the relationship is purely one of the apposition, because the chorionic epithelium is merely applying itself against the uterine epithelium.
• At the time of parturition, the villi can be pulled out of the pocket in which they are embedded, leaving the surface of uterine wall intact.
• There is no bleeding and damage to any tissue.
(b) Syndesmochorial placenta: •The epitheliochorial placenta is modified slightly by an advance in the sub group of ruminant ungulates.
• In these mammals, the prominent villi penetrate deeper in the uterine pits so as to erode the uterine epithelium, which allows the chorionic ectoderm to come into direct contact with the vascular maternal connective tissue.
• Only five barriers in between the foetal and uterine bloodstreams are present.
• At the end of gestation, however, the chorionic villi are merely withdrawn, and the maternal mucosa is not torn away.
• Therefore, no bleeding occurs at the time of parturition.
• E.g. cow, sheep, goat, buffalo, camel, etc
(c) Endotheliochorial placenta: •In carnivores the foetal-maternal union shows a marked advance in intimacy.
• There is the destruction of the uterine epithelium and the underlying connective tissue and so the chorionic epithelium comes into direct contact with the endothelial wall of the maternal blood vessels
In this case the number of barriers between the foetal and maternal blood vessels is reduced to four.
• At birth, there is a destructive separation of the placenta through which the foetal layers with enclosed maternal vessels split off from a deeper regenerative zone of maternal tissue.
• E.g. dogs, cats
(d) Haemochorial placenta: •All the three maternal tissues are eroded but the foetal tissues remain intact.
• The destruction of the endothelial wall of maternal blood vessels causes the chorionic epithelium to bath directly in maternal blood, which circulates in the lacunae.
• This facilitates rapid gaseous exchange and diffusion of nutrients from the maternal blood into the blood vessels of chorionic villi.
• The villi may be ramified by dendritic structures and form a complicated network.
• The fusions between chorionic and uterine tissues are so intimate that at birth the placenta tears away as a unit.
• It is found in lower rodents, insectivores, bats, and man
(e) Haemendothelial placenta: •In higher rodents and rabbit, the number of barriers between the maternal and foetal blood streams is still reduced.
• The chorionic villi lose their epithelium and connective tissue layers to such a degree that in most places endothelial lining of their vessels alone separate the foetal blood from the maternal sinuses.
• E.g. Rabbit and some rodents
The placenta performs the following functions:
Food materials pass from the mother’s blood into the foetal blood through the placenta.
The trophoblast ol the placenta digest protein before passing them into foetal blood.
Through the placenta oxygen passes from the maternal blood to the foetal blood, and carbon dioxide passes from foetal blood to maternal blood.
Nitrogenous wastes such as urea pass from foetal blood into maternal blood through placenta and are filtered out by the kidneys of the mother.
The placenta stores glycogen, fat etc. for the foetus before liver is formed.
Placenta functions as an efficient barrier (defensive wall) and allows useful: materials to pass into the foetal blood. Harmful substances such as nicotine from cigarette and addictive drugs such as heroin can pass through placenta. Therefore, pregnant women should avoid cigarette and drugs. Viruses and bacteria can pass through placenta.
7. Endocrine function:
Placenta functions as an endocrine gland it secretes hormones such as oestrogen, progesterone and human chorionic gonadotropin (HCG).
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