The placenta is a embryonic Annexe characteristic of the Mammifères Placentaire S also called euthériens, but which also exists in other forms at others animal and Végétaux. The role of the placenta is to bring to the Embryon and the Fœtus the Nutriment S, the Dioxygène and to evacuate their Déchet S like the metabolic Carbon dioxide and other waste like the Urée.
The placenta, in the mammals, is consisted the embryo and the uterine Muqueuse of the mother. It contains at the same time Sang fetal and maternal, brought by blood-vessels of the two individuals, but both in contact, are never separated by a hémato-placental barrier. The exchanges of substances are done through this barrier. The toxic substances (alcohol, Drug, Drug S, Toxin S microbial, Virus) can cross the barrier and cause malformations at the embryo (delay of development, backwardness, anomalies of formation of the bodies).
However the placenta constitutes an effective barrier against a certain number of pathogenic. For example, Mycobacterium tuberculosis (bacillus of Koch, agent of the Tuberculosis), practically does not pass the placental barrier.
Its form, like its organization, can vary significantly according to the species.
At the human being, the placenta is organized in cotyledons, or functional units of the placenta located on the uterine face of this last. They are thus not physiologically in contact with the pocket of water. They are generally individualized and form on the external face a quite identifiable wafer. These cotyledons are fragile, and are often injured at the time of the delivery of the placenta.
One occasionally finds cotyledons aberrant, which can make fear with the Accouchement important bleedings because of their position and their atypical form.
The placenta also ensures of the endocrine functions in particular by producing hormones, among which one can announce:
- Hormones steroids: progesterone and the estrogens (estriol, oestradiol and oestrone).
- the hCG (human chorionic gonadotrophin, chorionic hormone gonadotrope)
- the hormone placental lactogene (HPL) or PL,
- the leptine,
- the placental growth hormone (PGH).
In addition, the placenta is also a particularly interesting structure from the immunological point of view, since this fabric implements strategies enabling him to avoid the attacks of the immune system of the mother. one can in particular quote the absence of traditional HLA, the presence of a not very polymorphic particular HLA, the HLA-G, but also the presence on the syncitiotrophoblaste of Fas-ligant, or the local Déplétion in Tryptophane (an amino-acid).
Sites ectopic of the placenta
References
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