Embryo implantation

thumb|right|200px|Blastocysta

Implantation of the embryo is possible in case of fertilization of the follicle in the ovary. Under normal conditions, the blastocyst implants in the endometrium in the upper third of the body of the uterus on its back or front surface - between the mouths of the glands. The Endometrium (mucous membrane lining the uterine cavity) is maintained in the secretory phase and also participates in the formation of the Placenta.

thumb|right|200px|Embryo

The Endometrium is in its peak secretory phase at the beginning of implantation. The tissue is soaked, the glands and vessels have a tortuous course. can be distinguished In this phase there are three phases of the Endometrium:
 * surface layer compact,
 * middle layer spongiosa,
 * a thin deep basal layer (bordering the myometrium).

Subsequent progress
If the oocyte is not fertilized menstruation occurs. Contraction of endometrial arterioles causes ischemia in  the zona (stratum) functionalis  with subsequent degenerative changes in their glands and vessels. After a few hours, the contraction of the arterioles relaxes, a sudden congestion occurs, the walls of the vessels burst and there are blood spurts in the mucosa at the border of the pars basalis and pars functionalis. The Blood, that escapes washes away parts of the disintegrated pars functionalis. In the course of 3-4 days, the entire pars functionalis is completely removed from the endometrium, and only a weak pars basalis remains from the endometrium - it is supplied by the basal arteries forming a network, which is the source for the regeneration of the endometrium during the proliferative phase of the next cycle.