Summary. Significant uterine growth occurred in normal and hypogonadal ( hpg ) mice between Days 7 and 21 but thereafter no further growth was observed in hpg mice. The ovaries of hpg mice were significantly smaller than those of normals at all ages, but there was no significant difference between the number of non-growing follicles in the ovaries of mutants and their normal littermates at any age studied, and normal and hpg mice showed a marked reduction in the number of non-growing follicles during the first month of life. The size and composition of the growing follicle population in hpg mice, however, differed markedly from those in normal animals and by 21 days of age the number of growing follicles in mutants was significantly reduced. There was no significant difference in the number of Type 3b follicles before 60 days of age, but the number of all other follicle types was significantly less in hpg mice at all ages studied. Follicles in which the antrum is fully developed (Type 7 and 8) were never seen in the ovaries of mutants and corpora lutea were never observed. Interstitial tissue development was also very poor in hpg ovaries.

The hypothalamic GnRH content in normal mice remained low until Day 20, before rising sharply to adult levels (∼800 pg) between Days 20 and 30. The pituitary FSH content increased over the first 10 days of life to reach a peak of about 5000 ng, before declining to the adult value of about 2000 ng by Day 30, whilst the plasma FSH concentration was high in the first 10 days, but fell to adult levels over the next 20 days. Pituitary LH content increased significantly between Days 5 and 10 to reach the adult level of about 600 ng.

Hypothalamic GnRH was undetectable at all ages in hypogonadal mice, but the pituitary content of FSH and LH had risen to the attenuated mutant adult value by Day 15, and unlike normals, plasma FSH concentrations were not elevated during the neonatal period.

These results suggest that minimal gonadotrophic stimulation of the ovary from birth has no effect on the total number of follicles but reduces the number of growing follicles and prevents follicle growth beyond the early antral stage. Gonadotrophins therefore appear to have a role in the initiation and continuance of follicle growth in the adult mouse.