Ovarian structure and folliculogenesis
An ovarian cross-section sequentially reveals all stages of development of the follicular apparatus: from the protective tunica albuginea to mature structures. The process begins with primordial follicles containing an immature oocyte surrounded by epithelium and theca cells. Each cycle, 3 to 30 such elements are recruited into growth.
As granulosa cells proliferate, preantral follicles form, surrounded by the zona pellucida. The accumulation of secreted follicular fluid creates a cavity (antrum), turning them into antral follicles. Concurrently with the development of the winning follicle, the remaining recruited structures undergo atresia.
Preovulatory follicle and ovulation
By the 8th day of the cycle, the largest dominant follicle emerges from the pool, reaching up to 20 mm in size. Its wall consists of richly vascularized granulosa and theca cells, and it encloses an oocyte undergoing meiosis.
The ovulation process itself consists of the rupture of the mature vesicle and the release of the egg into the abdominal cavity. This moment is accompanied by damage to small capillaries and bleeding, after which new vessels rapidly sprout into the empty cavity.
Corpus luteum and its fate
The released granulosa cells undergo luteinization: they accumulate lipids and increase in cytoplasmic volume. This forms the corpus luteum — a temporary hormonally active structure.
The main feature of the corpus luteum lies in its invariable life cycle:
- Its functional lifespan is always exactly 14 days.
- In the absence of conception, the structure undergoes regression.
- In case of fertilization, the corpus luteum continues to develop and reaches its peak of activity.
Hormonal regulation and steroid synthesis
The endocrine function of the ovaries is provided by both granulosa and theca cells. They synthesize sex steroid hormones, among which estrogens, progesterone, and androgens are distinguished.
In the blood, about 90% of these substances circulate in a bound form, while the remaining 10% provide the biological effect. Estrogen fractions differ in their activity:
- Estradiol is the most active hormone.
- Estriol is an intermediate fraction.
- Estrone is the least active form.
The intensity of synthesis directly depends on follicular growth: as it enlarges, the production of all hormones increases, with a predominance of estrogenic activity.
"8-20-14": by the 8th day, a dominant follicle up to 20 mm in diameter forms, and the corpus luteum always lives for exactly 14 days.
Class and exam questions
How many follicles enter growth during a single menstrual cycle?
From 3 to 30 primordial follicles are recruited into growth, but only one reaches the dominant stage, while the rest undergo atresia.
What is the lifespan of the corpus luteum?
The corpus luteum functions strictly for 14 days regardless of the total duration of the menstrual cycle.
Which estrogen fractions are produced in the ovaries and which one is the most active?
Estradiol, estriol, and estrone are distinguished. The most active fraction is estradiol.
In what form do sex hormones circulate in the blood and exert their effect?
About 90% of hormones circulate in a bound state, while 10% are in a free form and exert a direct biological effect.
What else to study on this topic
- Histotopographic structure of the ovary and layers of the follicular wall
- Mechanisms of ovulatory follicular rupture and capillary sprouting
- The process of granulosa cell luteinization
- Comparative characteristics of estrogen reception and binding by blood proteins
- Pathophysiology of follicular atresia
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