Page 844 - Atlas of Histology with Functional Correlations

forms a connective tissue corpus albicans. The initiation and activation of the

developmental phase of primordial follicular growth in the ovaries is
believed to be dependent on local growth factors and follicle-stimulating
hormone (FSH) and luteinizing hormone (LH). These hormones are
responsible for the later stages of follicular development, maturation,
ovulation, and production of the hormones estrogen and progesterone. The

first half of the menstrual cycle lasts about 14 days and involves the growth
of some primordial ovarian follicles. At this time, FSH is the principal
circulating gonadotropic hormone, and the growing follicles express

receptors for FSH located on the surrounding granulosa cells. FSH controls
the growth and maturation of ovarian follicles and stimulates the
development of theca interna cells around the follicular peripheries. Theca
interna cells exhibit numerous LH receptors and, after LH stimulation,
secrete the estrogen precursors androstenedione, which diffuses into the

follicles, where the granulosa cells, in response to FSH, convert it into
estrogen with the aromatase enzyme. Estrogen then stimulates the granulosa
cells to proliferate and increase the follicular size. As the follicles develop

and mature, the circulating levels of estrogen in the blood rise. Under normal
conditions, only one developing follicle becomes dominant and will reach
maturity to ovulate an oocyte, whereas all the others will degenerate or
become atretic. Increased levels of estrogen inhibit the release of
gonadotropin-releasing hormone from the hypothalamus and decrease the

release of FSH from the pituitary gland. This decreased level of FSH induces
atresia in other follicles that started to develop.

At midcycle, or shortly before ovulation, estrogen levels reach a peak
and produce a positive feedback on the pituitary gland. This peak causes a

sharp surge of LH hormone from the adenohypophysis of the pituitary gland,
with a concomitant smaller release of FSH hormone. Increased blood levels
of both LH and FSH cause the following changes in the ovary:

Completion of the first meiotic division of the oocyte before ovulation

with the release of a secondary oocyte into the uterine tube.
Final maturation of the mature ovarian follicle and ovulation (rupture)
of a secondary oocyte at about the 14th day of the cycle.
Before ovulation, blood flow ceases in the small area of the ovarian

surface over the bulging mature follicle called stigma.
Collapse of the ovulated mature follicle and the luteinization or
modification of the granulosa lutein cells and theca lutein cells.

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