Cyclic hormonal alterations of the endometrium
Over the whole sexually active time span (from puberty to the menopause) the endometrium is subject to cyclic changesthat regulate ovulation. There are three levels of this hormonal regulation: hypothalamus-pituitary-ovary and it takes place via longer and shorter feedback mechanisms.
The menstruation phase
The menstruation phase (1rst to the 4th day) distinguishes the beginning of each menstruation cycle. When an implantation does not occur, the back-formation of the yellow body (corpus luteum) lowers the amounts of circulating estradiol and progesterone hormones, which leads to the expulsion of the functional layer of the endometrium.
The follicular or proliferative phase
During the proliferative or follicular phase (4th to 14th day) the secretion of estrogen through the growing ovarian follicle is responsible for the proliferation of the endometrium (intensive mitosis in the glandular epithelium and in the stroma).
The uterus epithelium clothes the surface again. In this stage a certain number of epithelial cells equipped with cilia can be recognized.
The glands grow longer and the spiral arteries wind themselves lightly into the stroma. At the end of the proliferative phase the estradiol peak (released by the growing follicles) triggers a positive feedback mechanism at the level of the pituitary and the ovulation commences 35 to 44 hours after the initial LH increase .
The luteinizing or secretory phase
During the secretory or luteinizing phase (14th to 28th day) the endometrium differentiates itself due to the influence of progesterone (from the corpus luteum) and attains its full maturity. The glands and arteries begin to entwine. The connective tissue stroma becomes the place of edematous changes.
The time period of the maximal reception ability for the blastocyst lies between the 20th and the 23rd day. This phase of the endometrium lasts 4 days and is usually termed the « ».