Graafian follicle

Figure 19.7 A diagram of a Graafian follicle. In the rim of cytosol around the nucleus are spherical mitochondria, ribosomes and the Golgi. A mature follicle is relatively large, about 1 cm in diameter.

Menstrual cycling is dependent upon the hormones secreted by the Graafian follicles. Once the primordial follicles are depleted, the secretion of oestrogen and progesterone progressively fails and menstrual cycle activity gradually decreases, a period which is characterised by irregular cycles. The differences in steroid hormone production or plasma levels between pre- and post-meno-pausal women are as follows ... 

In females, the target organs are the ovaries where it increases the number and size (maturation, development and ripening) of Graafian follicles and prepare them for ovulation. During its development, the ovarian follicles secrete its own hormone estrogen. In males, it stimulates spermatogenesis. Under the influence of this hormone, seminiferous tubules produce spermatozoa. 

In the ovary the ovulatory surge of LH increases blood flow [98]. This is accompanied by an increase in vascular permeability in the follicles resulting in oedema which persists through the time of follicle rupture and ovulation. The effect of LH can be mimicked by histamine but not by FSH or serotonin. Leukocytes accumulate in Graafian follicles near the time of ovulation and fibroblasts migrate from the theca layer to the stratum granulosum. 

Careful analysis of the biosynthetic properties of the membrana granulosa, the theca interna, and luteinized cells of the Graafian follicle and corpus luteum have revealed some of the factors determining the secretory products of these structures, but the role of the ovarian interstitial tissue is not completely established. There is much evidence suggesting that the latter tissue may secrete the major portion of ovarian androgens in women and in virilizing syndromes accompanying ovarian disease (Gil, L4, Mia, M5). 

In the nongravid female primary estrogen production is in the ovaries with additional amounts from the stratum granulosum, corpus luteum, and graafian follicles. During pregnancy major estrogen synthesis shifts to the placenta and may increase 100 times over normal near parturition time. The adrenals and male testes also produce small quantities. Estradiol and estrone, which are the main products from ovaries, coexist in 1 2 or even 1 4 equilibrium, a dynamic state of enzyme catalyzed interconversion. 

Metabolism E. are formed in the Graafian follicles of the ovaries and in the corpus luteum, and during pregnancy also in the placenta they also occur in small amounts in the male organism. Estrone is principally biosynthesized from androst-4-ene-3,17-dione, the 17-dehydro product of testosterone, by an aromatase. Estradiol is metabolized in the liver to 2-methoxyestrone via estrone. Estriol is also formed by side reactions. Both metabolites are excreted by the kidneys. Equilenin and equilin are also metabolic excretion products of estrogens. 

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