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 and the number of years in which a woman is fertile.
Ovulation represents an exceptional fate of a follicle.
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:
- the ovarian cycle (follicle maturation) that peaks in the ovulation and the subsequent luteinization of the granulose cells
- 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).
- The pulsatile liberation of GnRH, as well as FSH and LH
- The long-loop feedback-effect of estrogen and progesterone on the hypothalamic-hypophysial-system (these two hormones are synthesized in the [ready to rupture] follicle and so originate in the ovary, thus the name "long loop").
As a rule, the ovarian cycle lasts 28 days. It is subdivided into two phases:
- Follicular 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)
During the follicular phase, a cohort of follicles is recruited from which a dominant follicle is selected that will develop into a De Graaf follicle.
The cohort of tertiary follicles that will deliver the de Graaf follicle that will ovulate on day 14 of a cycle has started its maturation about 6 months earlier
- The development of the primary follicle into a secondary follicle takes more than 120 days.
- The development of the secondary follicle into a tertiary (or cavitary or antral) follicle takes about 71 days.
- The terminal development of the tertiary follicle into a Graaf's follicle lasts 2 weeks and starts at D0 of the menstrual cycle and ends at D14 with ovulation. This last phase is dependent on gonadotropins (LH and FSH) and starts as soon as the corpus luteum of the preceding cycle has regressed.