26-05-2012, 10:51 AM
PLACENTA IN MAMMALS
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MAMMALIAN EGG
Mammalian eggs - Alecithal egg, because it is having no yolk (or) little yolk.
As they are lacking yolk in their egg, the energy needed to lead the development is met by the steady supply of nutrients and oxygen to the developing embryo from the mother.
This embryonic nutritional supplement process is attended by the special structure called, Placenta.
Implantation of the Embryo
Morula has some cells superficially and other cells are laid inside [ inner cell mass….. Embryonic part ].
The superficial layer forms the extraembryonic part.
This serves to attach the embryo with the uterine wall of the mother. This superficial layer takes care of supplying the nutrients to the embryo from the mother.
This layer is referred as TROPHOBLAST.
The trophoblast later lifted off from the inner cell mass, and remains attached on one side of the embryo ...
BLASTOCYST.
At this stage, the embryo is attached with the uterine wall through the trophoblast………..IMPLANTATION.
PLACENTATION
The blastocyst attaches with the uterine wall either superficially (or) penetrates into the uterine wall.
This connection between the embryo and the uterus of the mother is referred as PLACENTATION.
By further activity of the trophoblast cells, the uterine wall epithelial cells are destroyed to develop into an organ called PLACENTA.
Placenta facilitates the diffusion of the nutrients and oxygen from the maternal blood vessels to the embryo.
PLACENTA ‘S COMPOSITION
The participating layers of tissue in the placenta formation are provided
BY both mother and foetus.
MATERNAL PART:
1] The blood of the mother,
2] The endothelial wall of the maternal blood vessels,
3] The connective tissue around the maternal blood vessels,
4] The uterine epithelium.
FOETAL PART:
1] The epithelium of chorion,
2] The connective tissue of the chorion,
3] The endothelial wall of the blood vessels of the chorion,
4] The blood of the foetus.
TYPES OF PLACENTA
(i) CHORIO-VITELLINE PLACENTA:
The connection is formed between the uterine wall and part of the chorion lined inside by the yolksac with its net work of vitelline blood vessels.
Ex: Marsupials --- Didelphys and Macropus.
This type is otherwise called as YOLK SAC PLACENTA.
(ii) CHORIO-ALLANTOIC PLACENTA:
The connection is formed between the uterine wall and part of the chorion lined inside by the allantois with its net work of allantoic blood vessels. Here the yolk sac is rudimentary (or) non-functional.
Ex: All eutherians.
CLASSIFICATION OF PLACENTA
The mammalian placentae are classified into many types based on the following criteria;
(i) Nature of implantation of the placenta in the uterine wall.
(ii) Degree of association of placenta with the uterine wall.
(iii) Distribution of villi in the chorion.
(iv) Participatory tissues of the placenta.
FUNCTIONS OF THE PLACENTA
Supplies the nutrients to the developing embryo.
Aids the process of respiration in the foetus.
Removes the nitrogenous wastes from the foetus by way of diffusion.
Immunizes the developing foetus by injecting the antibodies into the foetus.
Transports drugs to the foetus.
Stores the fats, glycogen, iron, etc.
Carries out the protein metabolism.
Secretes the hormones like progesterone, estrogen, gonadotrophin, and relaxin.
Protects the foetus from the infections of the mother.