Oogenesis – Embryology

Of the roughly 500'000 follicles that are present in the two ovaries at the beginning of sexual maturity, only around 480 reach the graafian follicle stage and are thus able to release oocytes (ovulation). This number is simply derived by multiplying the number of cycles per year (12) and the number of years in which a woman is fertile (40).

Cyclic changes in the hormone household (hormonal cycle), governed by the hypothalamic-pituitary system, are responsible for the periodicity of the ovulation. In a woman, the rhythmic hormonal influence leads to the following cyclic events:

In the center of this hormonal control is the hypothalamamics-hypophysial (pituitary gland) system with the two hypophysial gonadotropins FSH and LH. The pulsating liberation of GnRH by the hypothalamus is the fundamental precondition for a normal control of the cyclic ovarian function. This cyclic activity releases FSH and LH, both of which stimulate the maturation of the follicles in the ovary and trigger ovulation. During the ovarian cycle, estrogen is produced by the theca interna and follicular cells (in the so-called follicle phase) and progesterone by the corpus luteum (so-called luteal phase).

Definitions

GnRH: Gonadotropin- releasing hormone FSH: Follicle-stimulating hormone LH: Luteinizing hormone

As a rule, the ovarian cycle lasts 28 days. It is subdivided into two phases:

More info

Recruitment of the so-called follicle cohort: In the recruiting a certain number of primordial follicles are stimulated to mature and to go through the following follicle stages. (Details)

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Oogenesis - Embryology