Fertilization

Central Regulation Hub

The brain acts as the primary coordinator making pregnancy possible. The regulatory hierarchy begins in the hypothalamus. This structure synthesizes and secretes gonadotropin-releasing hormone (GnRH, or gonadoliberin).

GnRH travels to its target organ—the pituitary gland. Stimulated by gonadoliberin, the pituitary gland begins to secrete gonadotropic hormones. These include follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These substances are released into the systemic circulation and directly regulate ovarian function, establishing the hormonal foundation for future fertilization.

Ovarian cycle and oocyte maturation

The ovarian cycle is a sequence of events aimed at the maturation of a mature oocyte. Initially, under the influence of pituitary hormones (FSH and Lh), follicles undergo active growth. As they enlarge, the growing follicles actively secrete estrogens.

The culmination of the ovarian cycle is ovulation—the physical release of the oocyte from the dominant follicle. After the follicle ruptures, a new temporary endocrine structure forms in its place—the corpus luteum. Its normal functioning is supported by prolactin (PRL). The corpus luteum acts as the primary source of progesterone. If fertilization does not occur and pregnancy fails to establish, the corpus luteum undergoes physiological atresia (regresses).

Dynamics of blood hormone levels

The synchrony of the reproductive system is reflected in the strict dynamics of circulating hormones throughout the menstrual cycle:

• Estradiol: steadily increases during the first phase of the cycle as the follicle matures. Levels reach their peak immediately before ovulation.
• Luteinizing hormone (LH): characterized by a sharp ovulatory surge (maximal release). This surge serves as the direct trigger for ovulation.
• Follicle-stimulating hormone (FSH): exhibits a moderate peak that precisely coincides in time with the LH surge.
• Progesterone: remains at baseline levels prior to ovulation. In the second phase of the cycle, its concentration increases significantly (forming a broad peak) due to active corpus luteum function. By the end of the cycle, hormone levels drop precipitously.

Uterine cycle (endometrial changes)

Simultaneously with the ovarian cycle, cyclic transformations occur in the uterine mucosa. The primary objective of the uterine cycle is to prepare the endometrium for implantation.

1. First phase (proliferative): occurs under the influence of steadily rising estrogen levels. Active regeneration and thickening of the functional endometrial layer take place, accompanied by the growth of straight glands.
2. Second phase (secretory): begins after ovulation under the influence of progesterone. Endometrial glands become tortuous and dilate, and blood supply significantly increases due to the development of spiral arteries. The endometrium is fully prepared for implantation.
3. Menstruation: a sharp drop in progesterone and estrogen levels at the end of the cycle (due to corpus luteum regression) leads to the shedding of the functional endometrial layer, marking the start of a new cycle.

Process synchronization

The regulatory schema clearly demonstrates the inextricable and synchronous interdependence between brain structures, the ovaries, circulating hormone levels, and the state of the endometrium. Any disruption at the hypothalamic or pituitary level will inevitably affect the ovarian cycle. Conversely, a lack of adequate follicular growth will lead to disordered hormone secretion, preventing the uterine cycle from transitioning into the secretory phase. Each stage strictly depends on the preceding one, forming a unified mechanism that prepares the female body.

Class and exam questions

What else to study on this topic

• The mechanism of gonadotropin-releasing hormone (GnRH) secretion by the hypothalamus and its effect on the pituitary gland.
• The process of corpus luteum atresia and the cellular mechanisms of its regression.
• The role of prolactin (PRL) in maintaining the endocrine function of the corpus luteum.
• The morphology of spiral arteries and their significance for the secretory phase of the endometrium.
• The relationship between the moderate FSH peak and the sharp LH surge prior to ovulation.

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.