Pituitary Regulation: Three Key Hormones
The anterior pituitary controls the ovaries via three hormones, each with its own target cells and morphogenetic effects:
- FSH (follicle-stimulating hormone) acts on follicular cells. It initiates the growth of the primordial and secondary follicle pool, supports their development, and stimulates the enzyme aromatase.
- LH (luteinizing hormone) acts on thecal (interstitial) cells. It serves as the trigger for ovulation, ensures the transformation of the follicle into the corpus luteum, and stimulates the production of androgens, which are indirectly required for estrogen synthesis.
- LTH (lactotropic hormone / prolactin) acts on the luteal cells of the corpus luteum, stimulating progesterone production and exerting a supportive effect on ovarian structures.
«Two-Cell Theory» of Estrogen Synthesis
The key concept of steroid biosynthesis in the growing follicle lies in the cooperation of two cell types.
- First stage (in thecal cells): Under the influence of LH, cells capture cholesterol and convert it first to progesterone and then to testosterone.
- Second stage (in follicular cells): Testosterone diffuses across the basement membrane from the theca into the follicular layer. Here, under the influence of FSH, the enzyme aromatase (with the involvement of $H_2O$) converts androgens into estradiol.
The synthesized estrogens are partially accumulated in the follicular fluid, creating an environment for the oocyte, and partially returned via the basement membrane into the bloodstream.
Histological Analogy with the Male Reproductive System
A comparison of the structures of the ovary and testis clearly demonstrates the homology of cell functions in different sexes:
- Leydig cells (testis) and thecal cells (ovary) are interstitial and responsible for testosterone synthesis.
- Sertoli cells (testis) and follicular cells (ovary) are capable of converting testosterone to estradiol.
Peptide Regulation and Follicular Atresia
Follicular cells synthesize not only steroids but also the protein hormone gonadocrinin. It accumulates in the follicular fluid and causes the death (atresia) of neighboring follicles, ensuring the selection of the dominant one, or triggers the atresia of the follicle itself in case of ovulation disorders.
The process of atresia is accompanied by a shift in the type of hormonal secretion:
- At the early stage, while the follicular cells are alive, the atretic follicle serves as the primary source of estrogen synthesis.
- At the late stage, follicular cells die, and atretic bodies are formed from the derivatives of the theca. Secretion shifts toward androgens and progesterone.
Note: At the onset of the menopausal transition, the mass maturation of primordial follicles (formation of theca interna) and their subsequent mass atresia cause a sharp surge in estrogens and androgens. Over time, a drop in estrogen production against the background of preserved androgen synthesis by the ovaries and adrenal cortex can lead to masculinization.
Biosynthesis in the Corpus Luteum and Feedback
When a follicle transforms into a corpus luteum, a genetic switch occurs. In the follicular cells, which become luteal cells, the genes for aromatase enzymes are turned off, and the genes for the initial stage of biosynthesis are turned on. As a result, luteal cells (upon stimulation by LTH) halt at the stage of converting cholesterol to progesterone.
Estrogens and progesterone secreted by the ovaries regulate their own production through a negative feedback loop. They act on neurons of the limbic system, which transmit a signal to the hypothalamus to reduce the production of luliberin.
- Estrogens inhibit FSH secretion, which is why it remains suppressed for almost the entire cycle.
- Progesterone inhibits LH secretion, suppressing it during the second half of the cycle.
This ensures the suppression of the development of new follicles in the current cycle.
Briefly: answers to topical questions
What stages of development does the corpus luteum undergo?
The life cycle of the corpus luteum includes four sequential stages of development.
- Proliferative and vascularization stage — collapse of the follicular cavity, ingrowth of connective tissue with blood vessels, and colonization by cells.
- Stage of glandular metamorphosis — differentiation of follicular and thecal cells into granulosa lutein and theca lutein cells.
- Stage of bloom (acme) — active secretion of progesterone.
- Stage of regression (involution) — degeneration and atrophy of luteal cells, replaced by connective tissue with the formation of the corpus albicans (corpus albicans).
What are the histological layers of the wall of a tertiary follicle (Graafian follicle)?
The wall of a mature tertiary follicle (Graafian follicle) is formed by several histological layers.
- Granulosa layer (granulosa) — stratified follicular epithelium.
- Basement membrane — a structure that externally bounds the follicular epithelium.
- Inner thecal layer (theca interna) — the inner part of the connective tissue capsule.
- Outer thecal layer (theca externa) — the outer part of the follicular connective tissue capsule.
Which specific enzymes, besides aromatase, are involved in steroidogenesis in ovarian cells?
In addition to aromatase, the sources explicitly name two enzymes/enzyme systems associated with disorders of ovarian steroidogenesis.
- 17α-hydroxylase — when defective due to a mutation in the CYP17A1 gene, primary amenorrhea, absent secondary sexual characteristics, and arterial hypertension are described.
- 17,20-lyase — when deficient, the clinical picture is similar to a 17α-hydroxylase defect; arterial hypertension is atypical, and frequent complaints of lower abdominal pain due to multiple ovarian cysts are possible.
Mnemonic analogy: Theca cells act like a raw material factory (producing testosterone under LH influence), while follicular cells act like a processing plant (converting raw materials into estradiol under FSH influence).
Class and exam questions
Why is FSH necessary for estrogen production?
FSH does not synthesize the substrate itself, but it activates the enzyme aromatase in follicular cells, which is necessary for the conversion of testosterone (derived from the theca) into estradiol.
What interaction between estrogens and progesterone is observed in the myometrium?
In the myometrium, they act as antagonists: progesterone reduces sensitivity to oxytocin to maintain pregnancy, whereas estrogens at the end of pregnancy increase this sensitivity, triggering labor.
How is the hormonal background maintained in the premenstrual period?
When mature developing follicles are absent in the ovaries, the hormonal background is maintained by late-stage atresia structures, which secrete predominantly androgens, as well as by the corpus luteum.
What else to study on this topic
- Genetic regulation of enzyme systems during follicular luteinization
- The mechanism of neurohumoral signal transmission via the limbic system
- Specific effects of estrogens and progesterone on target organs (synergism and antagonism)
- Morphogenetic effects of gonadotropins at different stages of folliculogenesis
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