CELL CYCLE PROPERTIES OF DIFFERENTIATING SPERMATOGONIA IN ADULT SPRAGUE‐DAWLEY RATS

The duration of the mitotic cycle and of its components was analysed for each of the six successive generations of differentiating spermatogonia (A 1 , A 2 , A 3 , A4, intermediate and B), using radioautographed whole mounts of seminiferous tubules from testes of adult Sprague‐Dawley rats. Cell cycles were determined from two successive waves of per cent labeled metaphases obtained during the period of 81 hr after a single dose of 3 H‐thymidine. Except for the A 1 spermatogonia, all spermatogonial types (A 2 to B) had similar cell cycle durations of 41‐42.5 hr and comparable pre‐DNA synthesis phases (G 1 ) of 11‐13 hr. Although the combined duration of DNA synthesis (S) and the post‐synthesis phase (G 2 ) remained identical for all the cell types including A 1 , there was a progressive lengthening of the S period at the expense of G 2 during the process of spermatogonial maturation. This change was most marked during the transition from A 1 to A 3 spermatogonia when the S period increased from 14 hr to 21 hr, and the G 2 phase shortened from 13 hr to 7.5 hr. This feature seems to be unique to germ cells and may be associated with an increasing amount of heterochromatin in the nucleus. Excluding the development of type A 1 cells, the entire process of spermatogonial maturation lasted for 208 hr. Combined data on cell cycle times indicated that every 313 hr or 13 days, a new sequence of spermatogonial differentiation was initiated by the A 1 cells. This was equivalent to the duration of one ‘cycle’ of the seminiferous epithelium as measured by other techniques.