Ovary

Gonadotropin Control of Inhibin Secretion and the Relationship to Follicle Type and Number in the hpg Mouse

Abstract
Inhibin is secreted in two distinct heterodimeric forms A & B but the mechanism for the differential control of these two forms is unclear. To evaluate the relationship between secretion of inhibin forms and folliculogenesis, the effects of gonadotropins on inhibin concentrations were studied in parallel with stereological enumeration of ovarian follicle types in gonadotropin-deficient hypogonadal (hpg) female mice treated with rhFSH (10IU/day), hCG (1IU/day) or both for 20 days. Treatment with FSH alone significantly increased blood concentrations of both inhibin A and inhibin B whereas hCG alone had no effect on either inhibin. The combination of FSH and hCG further increased the concentration of inhibin A but had no effect on the concentration of inhibin B beyond that of FSH. The number of primordial follicles per ovary was significantly reduced in FSH treated hpg mice, but was not affected by hCG treatment. Antral follicles were absent in the untreated hpg, present following treatment with FSH and present in only limited numbers following hCG alone. Preovulatory follicles were only seen in the wild type and combined FSH and hCG treatment groups. These results demonstrate that secretion of both inhibins is associated with the presence of antral follicles. Inhibin A secretion is increased by the presence of preovulatory follicles while the concentration of inhibin B is not affected. The observed effects of gonadotropins on inhibin A and B secretion may be explained by corresponding gonadotropin effects on follicle development.

Key words: Ovary • Follicle • Follicle-stimulating hormone • Human chorionic gonadotropin • Inhibin

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