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Gonadotropic factors affecting the onset of reproductive maturity in the female fur seal : Callorhinus ursinus Aman, Allison Craig

Abstract

The investigation was designed to determine, by means of differential adenohypophysial cell counts of gonadotropins and bioassay of adeno-hypophysial gonadotropins, the sequence of gonadotropic events which affect reproductive maturity in the female fur seal, Callorhinus ursinus. Previous investigation (Craig 1963, 1966) has established that ovarian follicular cycles are initiated in March by 2- and 3-year old immature females; the cycle is annovulatory, and maintained until September, when the ovaries become quiescent. Four-year-old females initiate a follicular cycle in March which culminates in ovulation and reproductive maturity in late August. Five cells were identified morphologically and histochemically in the pars anterior. Two were serous cells, with granules composed of simple proteins; by comparison and analogy to cells similarly identified in the adenohypophyses of other mammals, these cells are considered to be the somatotrop and the luteotrop. Three are mucoid cells, with granules composed of mucoproteins; by comparison and analogy to cells similarly identified in other mammalian adenohypophyses, these cells are considered to be the thyrotrop, the folliculotrop, and the interstitiotrop. Differential cell counts were made of folliculotrops and intersti-tiotrops in the pars anteriors of immature, early proestrous, and estrous female seals. The data obtained suggest minimal gonadotropic hormone release among immature females, extensive release of gonadotropic hormones at early proestrus in preparation for estrus, and the further release of ovulatory amounts of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) at estrus. Adenohypophyses from 2- and 3-year-old immature females and estrous females were assayed for quantification of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH), using female fur seal pups 6-8 weeks old as assay animals for FSH, and immature females 2 and 3 years old, as well as pups, as assay animals for LH. The endpoint measured for FSH quantification was ovarian antral follicles 1.0-2.0 mm in diameter. The concentration and content of adenohypophysial FSH among immature females is approximately double that of estrous females; the evidence is commensurate with the results of the differential adenohypophysial cell counts of gonadotrops, and suggests minimal gonadotropin release among immature females, and the release of ovulatory amounts of FSH among estrous females. The bioassays for LH were only partly successful, but demonstrated the ovulatory capabilities of 2- and 3-year-old immature females, and the presence of ovulatory amounts of LH in the adenohypophyses of 2- and 3-year-old immature females. Adenohypophyses from 2- and 3-year-old immature females, early pro-estrous females, late proestrcus females, and estrous females were assayed for concentration.and content of FSH and LH, using intact, immature female rats as assay animals. Adenohypophysial FSH content decreases from a peak 0.8 mg FSH among 2-year-old females to 0.5 mg FSH among 3-year-old females, 0.4 mg FSH among early proestrous females, 0.35 mg FSH among late proestrous females, and 0.18 mg FSH among estrous females; FSH concentration similarly decreases, except among early proestrous females, where a relatively high concentration suggests FSH synthesis at a rate greater than rate of release. Adenohypophysial LH content is highest among 2-year-old immature females, 34 µg, and decreases to 19 µg among 3-year-old immature females, and to 10 µg among early proestrous females; LH content is increased to 28 , µg among late proestrous females, and decreases to 10 µg among estrous females. Adenohypophysial LH concentration follows the same pattern. The evidence suggests minimal release of FSH and LH among 2- and 3-year-old females, sufficient to initiate the annovulatory ovarian follicular cycle in March and support it to its conclusion in September; the evidence of differential adenohypophysial cell counts and assay of adenohy-pophysial FSH in fur seal pups supports this conclusion. Increased release of FSH and LH at early proestrous probably initiates the ovarian ovulatory follicular development and estrogen synthesis characteristic of late pro-estrus and estrus. The apparent storage of LH at late proestrous is probably necessary for the ovulatory release at estrus. Recent investigation has established that in the female rat, reproductive maturity is dependent on the differential maturation of 2 areas of the hypothalamus. The "hypophysiotropic area" differentiates first, and controls, through the synthesis and release of neurohormonal gonadotropic releasing factors, the release of adenohypophysial FSH and LH; the hypophysiotropic area is responsive to negative estrogen feedback. The anterior hypothalamus becomes functional after the hypophysiotropic area, just prior to ovulatory maturity, and is responsive to positive estrogen feedback for the release of ovulatory amounts of FSH and LH. By analogy, a sequence of hypothalamic events similar to that described for the rat is inferred as the determining factor for the onset of reproductive maturity in the female fur seal.

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