Implantation and placentation in mammals| After the ovulation from the ovary is conducted towards the uterus by peristalsis and ciliary action in the fallopian tube of the female| If fertilization occur ,it also occur in the upper part of fallopian tube and ovum is transformed into the zygote | Cleavage of zygote also starts in the fallopian tube| By the time the zygote reaches the uterus ,it has already become a solid ball of cell which look like “little mulberry” and is called morula|outer micromeres of morula form a nutritive and protective layer called tropoblast ,while inner macromeres form an embryonal knob which represent the precursor of the embryo.
The process of attachment of the blastocyst on the endometrium of the uterus is called Implantation|
TYPES OF IMPLANTATION
- CENTRAL IMPLANTATION In this type of implantation,the blastocysts is attached to the surface of uterus and projects into the uterine cavity| It is also called superficial implantation and is found in ungulates|
- ECCENTRIC IMPLANTATION In this type of implantation ,the mucous lining of uterine endometrium gives out folds to surround the embryo which is embedded in a groove of the main uterine cavity| It is found in most in most of rodents.
- INTERSTITIAL IMPLANTATION In this type of implantation ,the embryo is completely buried in the uterine tissue so has no connection with the uterine cavity| it is found in some rodents and some higher primates|
TYPES OF PLACENT
- YOLK SAC PLACENTA It is formed by yolksac and chorion| The yolk sac grows all around the embryo and develops vitelline blood vessels for taking material from maternal blood to the foetus| It is characteristic feature of metatherians such as kangaroo and opossum ,Allantois is poorly developed|
- ALLANTOIC PLACENTA It is formed by allantois and chorion ao is termed as allantochorion ,It is highly vascular and sends small finger like processes, the villi in uterine wall which develops corresponding depressions, the crypts
- CHORIONIC PLACENTA It is formed by chorion alone. The allantois remains small but burrows into body stalk .Allantois does not reach chorion,however its mesoderm and blood vessels grow upto chorion. The choriob develops villi to penetrate the uterine crypts forming chorionic placenta e.g humans, monkeys,and apes.
After the interstitial type implantation ,the surface of the blastocysts gives rise finger like outgrowth called chorionic villi. These villi have foetal blood capillaries and are surrounded by the maternal blood. The chorionic villi are initially fomed all over the chorion but after some time, these degenerates on most part of the chorion. Human placenta is fully formed in about 10 week . on the histologically basis, the human placenta is haemochorial placenata in which all the three maternal tissue of placenta have been digestive by the tropoblast of the chorionic placenta .so the chorionic villi are bathed directly in the maternal blood sinuses.
On the basis of degree of intimacy of foetal abd maternal tissue, human placenta is called deciduate placenta it is so because when the parturition occur, then some maternal tissue, called after death.
FUNCTION OF PLACENTA
Placenta acts a physiological barrier and an ultrafilter between the foetal blood and maternal blood. The following function are as follow.
NUTRITION The primary function of placenta is to provide nutrients like monosaccharides, disaccharides, lipids and amino acids from maternal blood to foetus
EXCRETION The nitrogenous wastes like urea, uric acid,ammonium compound etc are carried away from foetus to maternal blood.
RESPIRATION. Through placenta,O2 is supplied to foetus and Co2 is carried away from foetus to maternal blood .A fully developed foetus takes up about 25ml of O2 per minutes from maternal blood
STORAGE placenta stores materials like glycogens, fats etc till the formation of liver
SIGNIFICANCE OF PRESENCE OF A PLACENTA BARRIOR
- Maternal and foetal bloods may be of different blood groups and incompatible.
- Presence of maternal blood is too high for foetal blood vessels
- Check the entry of sex hormones of mother which may interfere the development of male foetus
- Check the entry of blood protein of mother which may retard the foetal metabolism.