Quantification of cell types throughout the cycle of the human seminiferous epithelium and their DNA content. A new approach to the spermatogonial stem cell in man.

The numbers of each different cell type in the human seminiferous epithelium were determined throughout the 6 stages of the cycle in both semithin and ultrathin sections obtained from 15 young adult men with normal testicular histology. Up to 4 types of A spermatogonia (Ad, Ap, Al and Ac) were distinguished. In addition, the DNA nuclear content of seminiferous epithelium cells was determined on Feulgen-stained sections. Both Ad and Ap spermatogonia showed a 2c DNA content and were present in the 6 stages of the cycle, though their numbers decreased in stages III-V. Both Al and Ac spermatogonia showed a DNA content varying from 2c to 4c. Al spermatogonia were observed in stages III-V; their numbers plus those of Ad spermatogonia in these stages were similar to the numbers of Ad spermatogonia in the other stages lacking in Al spermatogonia. Ac spermatogonia appeared in stages III-VI and their numbers plus those of Ap spermatogonia in stages III-V were similar to the numbers of Ap spermatogonia in the other stages lacking in Ac spermatogonia. The results suggest that Ad spermatogonia are the stem cells. Some of them replicate their DNA; during this replication they appeared as Al spermatogonia. Al spermatogonia divide, giving rise to both Ad and Ap spermatogonia. Some Ap spermatogonia replicate their DNA; during this process they are transformed into Ac spermatogonia which divide, giving rise to B spermatogonia.