In the laboratory mouse, formation of spermatozoa from type A spermatogonia requires 34·5 days (Oakberg, 1956b; Clermont & Trott, 1969). Early spermatids appear in the mouse testis at the age of 22 to 24 days (Nebel, Amarose & Hackett, 1961) and 2 weeks later, the testicular histology has been reported to reach the adult state (Molne & Brabrand, 1968). Fertility is usually attained before the age of 60 days, depending mainly on the genetic constitution and diet.
The yield of spermatogenesis was studied quantitatively during the period of rapid growth of the testes before the animal becomes fertile.
Groups of CD-1 random-bred male mice (Charles River Breeding Laboratories) were killed at the age of 27, 32, 39 and 44 days. Two 4-month-old males were included for comparison. Germinal cells were counted on 5-μ cross-sections of the testes stained with haematoxylin and PAS, using ten tubules in stage VII of spermatogenesis (Clermont & Morgentaler, 1955; Oakberg, 1965a). The crude counts of type A spermatogonia (A), preleptotene spermatocytes (PL), pachytene spermatocytes (P) and step 7 spermatids (ST) were converted into true counts using the Abercrombie formula (Abercrombie, 1946) and corrected for the differences in the number of Sertoli cells using the `Sertoli cell factor' (Clermont & Morgentaler, 1955). The values were expressed per 10·1 Sertoli cells, i.e. the number observed in adult males. The differences between the ratios of different cell types were compared by Χ2 and the mean