Ovarian and Endometrial Cycles: Hormonal Regulation
Ovarian Cycle
The ovarian cycle is the period of time between the ending of one menstrual period and the beginning of the next.
Follicular or Pre-Ovulatory Phase
This phase lasts from day 1 to 13. During this stage, there is an increased secretion of FSH and combined effects of LH. LH acts on the theca interna, and FSH acts on the granular cells to produce estrogens. The purpose of this phase is to mature primordial follicles in the ovaries. A primordial follicle begins as an oocyte (1-2 km) surrounded by a single layer of epithelial cells (granulosa cells), forming several layers. The follicle develops two layers: the zona pellucida and the theca interna and externa. During this phase, estrogen production increases.
Ovulation
During this phase, which occurs over 1-2 days, an egg matrix is released due to LH. Ovulation determines the preparation of new endocrine tissue (corpus luteum or yellow body), which involves the proliferation, vascularization, and luteinization of the theca and granulosa.
Postovulatory or Luteal Phase
When fertilization does not occur, the corpus luteum progressively increases activity and progesterone concentration for approximately 15-25 days. Around the 25th day, there is a drop in the concentration of ovarian hormones, which triggers menstruation.
Endometrial Cycle
Proliferative Phase
From day 1 to 14, cell mitosis, vascularization, and the formation of glands in the functional layer of the endometrium increase. This is stimulated by estrogen and seeks compensation for the loss of the previous menstruation, preparing the endometrium for the next cycle.
Secretion Phase
From day 15-28, the loss of the endometrial layer ends. Due to progesterone, the formation of blood vessels and glands increases. Menstruation begins with vasoconstriction of the arteries, causing ischemia and necrotic tissue to release necrosis substances. These vasodilator substances break the vessel wall, causing bleeding, cell loss, and the release of a viscous glycogen fluid.
Regulation of Testicular Function
The Leydig cells, under the stimulatory influence of LH, secrete testosterone, the main androgen. Androgens are essential for the control of spermatogenesis, maintenance of secondary sex characteristics, and functionality of secondary sexual organs. These characteristics include facial hair growth, deepening of the larynx, and prostate. The vocal cords and seminal vesicles are responsible for forming the seminal fluid.
The metabolic effects of testicular hormones are clearly anabolic, stimulating cell division and maturation. Their growth effects depend on age. Only testosterone and dihydrotestosterone have significant biological activity. In adolescence, androgens produce linear growth, muscle development, and retention of nitrogen, potassium, and phosphorus. Testosterone also accelerates epiphyseal closure in the long bones.
Hormonal Control of Spermatogenesis
The presence of LH and FSH are necessary for spermatogenesis. These effects are mediated by testosterone, and FSH are the hormones that act directly on Sertoli cells to promote gametogenesis. The following experimental facts show the relationship between gonadotropins, testosterone, and Sertoli cells:
- Sperm production requires a high concentration of testosterone derived from the Leydig cells when stimulated by LH.
- Sertoli cells bind androgens to proteins synthesized by a process dependent on FSH.
- Sertoli cells synthesize follistatin, which inhibits the secretion of FSH by a direct effect on the hypothalamus and anterior pituitary gland.
Pituitary Feedback Control
The rate of synthesis and secretion of testosterone by Leydig cells is stimulated by LH. Free testosterone, in turn, inhibits the secretion of LH at the hypothalamic level. This feedback control system is produced by a dual mechanism:
- Increasing levels of testosterone inhibit the activation of interstitial cells.
- Increased estrogen levels inhibit FSH and ICSH (LH).