3.4 Oogenesis

• Remark
• Development of the germ cells in the ovary
  
• Structure of the ovary
  
• The follicle stages from primordial follicle to tertiary follicle
  • Primordial follicle
  • Primary follicle
  • Secondary follicle
  • Tertiary follicle
  • Graafian follicle
• Temporal course of the number of germ cells / follicles
  
  • Atresia -- the customary fate of a follicle
• The ovarian cycle
  • The hormonal cycle

The ovarian cycle

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).

The hormonal cycle:

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:

1. the ovarian cycle (follicle maturation) that peaks in the ovulation and the subsequent luteinization of the granulose cells
2. cyclic alterations of the endometrium that prepare the uterine mucosa so fertilized oocytes can "nest" there. In the absence of implantation, the mucosa will be eliminated (menstrual bleeding)

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).

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

• Follicle phase: recruitment of a so-called follicle cohort and, within this, the selection of the mature follicle. This phase ends with ovulation. Estradiol is the steering hormone. Normally, it lasts 14 days, but this can vary considerably!
• Luteal phase: progesteron production by the "yellow body" (= corpus luteum) and lasts 14 days (relatively constant).

Fig. 23
The various follicle stages are distributed over the follicle phase. At its end ovulation occurs. Estradiol is the hormone that is formed in the ovary during the follicle phase and progesterone is synthesized in the luteal phase.