Alternative to pregnancy: the mechanism of menstruation
If fertilization does not occur, the body triggers the rejection of the prepared endometrium. The involution of the corpus luteum leads to a sharp drop in blood levels of estrogens and progesterone.
The pathogenesis of menstrual bleeding includes the following stages:
- A decrease in the concentration of sex steroids causes spasm of the spiral arteries.
- Ischemia and necrosis of the functional layer of the endometrium develop.
- Against the background of hypoxia, lysosomal proteases are released.
- Secondary vasodilation occurs, and the necrotic tissue is rejected.
An important role is played by prostaglandins (specifically, PG F2α). They enhance ischemia through vasoconstriction and stimulate myometrial contractions, helping the uterus to cleanse itself. Mucosal regeneration begins from basal cells from the very first day of menstruation and is fully completed by the 4th–5th day of the cycle.
Higher levels of regulation (CNS and hypothalamus)
Biphasic changes in the female body are controlled by a complex neuroendocrine network operating on the principle of positive and negative feedback. Hormone receptors are located on cell membranes and then move to the nucleus, where they bind to DNA, altering cellular metabolism.
First level (CNS): Includes the cerebral cortex, limbic system, hippocampus, and amygdala. In response to external stimuli (including psychoemotional factors), the CNS synthesizes neurotransmitters and endogenous opioid peptides (endorphins, enkephalins).
- Stimulate the release of hypothalamic hormones: norepinephrine, acetylcholine, GABA.
- Inhibit the release: dopamine, serotonin.
Second level (Hypothalamus): Peptidergic neurons of the hypothalamus secrete statins (inhibitory) and liberins (stimulatory). The key hormone is gonadotropin-releasing hormone (GnRH). It stimulates the pituitary gland to release both luteinizing (LH) and follicle-stimulating (FSH) hormones. GnRH secretion is genetically programmed and pulsatile.
Pituitary gland and peripheral organs
The activity of the hypothalamus is inextricably linked with underlying structures.
Third level (Adenohypophysis): The anterior pituitary synthesizes FSH, LH, prolactin, ACTH, GH, and TSH.
- FSH and LH act directly on the ovaries.
- Prolactin controls progesterone secretion by the corpus luteum (by activating LH receptors) and stimulates the mammary glands. Outside of pregnancy, its production is tonically inhibited by dopamine. The main stimulator of prolactin is TSH (thyrotropin-releasing hormone) synthesized in the hypothalamus.
Fourth level (Peripheral glands): The main role here is assigned to the ovaries. The adrenal glands and thyroid gland perform their specific tasks while simultaneously supporting the balance of the entire reproductive system.
Fifth level (Target organs): The uterus, fallopian tubes, vagina, and mammary glands. It is in the endometrium that the most pronounced cyclic changes occur, preparing the organ for potential embryo implantation.
Feedback mechanisms
Uninterrupted operation of the system is ensured by feedback loops:
- Long loop: target organs and peripheral glands affect the hypothalamic-pituitary system (e.g., high doses of estrogens block FSH production).
- Short loop: interaction between the pituitary gland and the hypothalamus.
- Ultrachort loop: local regulation within the hypothalamus via neuropeptides and electrical impulses.
For quick memorization of the 5 levels of regulation, use the "Top-Down" rule: Cortex (CNS) → Hypothalamus → Pituitary → Ovaries (periphery) → Uterus (targets).
Class and exam questions
How does severe stress affect the ability to conceive?
The cerebral cortex and extrahypothalamic structures have sex hormone receptors. During stress, the synthesis of cerebral neurotransmitters changes (e.g., serotonin or dopamine levels rise), which inhibits hypothalamic GnRH release and blocks ovulation.
What happens to prolactin secretion when pregnancy occurs?
Under normal conditions, prolactin synthesis is constantly suppressed by dopamine (prolactin-inhibiting factor). During pregnancy and lactation, this tonic inhibition ceases, allowing the mammary glands to grow and secrete milk.
What is the role of prostaglandins in the menstrual cycle?
When sex hormone levels drop, prostaglandin F2α causes a sharp spasm of the endometrial spiral arteries (enhancing ischemia) and stimulates myometrial contractions to expel the necrotic tissue.
What else to study on this topic
- Biochemical mechanism of pulsatile gonadotropin-releasing hormone (GnRH) secretion.
- The effect of endogenous opioid peptides on hypothalamic function.
- Functional diagnostics tests to assess the reproductive system.
- The role of lysosomal proteases in the pathogenesis of endometrial rejection.
- Interaction of hormone-dependent cell receptors with DNA.
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