In the rat, a very similar acquisition pattern for the various subpopulations, composed of the same eight developmental stages, was found. As in the mouse, rat testis at p.n. Days 6-7 contains only mitotically dividing spermatogonia cells and somatic cells, some of which are still undergoing mitotic divisions. The developmental stage of p.n. Days 13-14 in the rat correlates to p.n. Day 10 in the mouse in the sense that it is the last postnatal age showing no increase in the proportion of 4d cells, although mitotic activity of the somatic cells either ceased or at least dramatically declined. This suggests that, as in the mouse, the rather constant portion of 4d cells is attributable to primary spermatocytes that have entered the leptotene stage of prophase I. ventolin inhalers

The appearance of zygotene cells in the mouse, at p.n. Day 12, was characterized by significant increase in the proportion of 4d cells, for the first time, and by the appearance of a substantial number of cells in the R1 region in the first window. This very exact pattern was found in rats at p.n. Days 17-18, suggesting the first appearance of zygotene cells at that developmental stage. At p.n. Days 19-20, the 4d cells show up as a discrete R1 group for the first time, and by p.n. Days 22-23, a distinct R2 group begins to appear.