FIG. 8.

Undifferentiated A spermatogonia transition to become either A1 or A2 differentiating spermatogonia during the onset of synchronized spermatogenesis. TEM images depict representative cross sections of testes from mice treated with WIN 18,446-only (A) or with WIN 18,446 followed by an RA injection and given 18 h (B, C), 48 h (D), 72 h (E), 96 h (F), 120 h (G), and 144 h (H) of recovery. White arrows denote heterochromatin present along the nuclear envelope within the undifferentiated and differentiating spermatogonia and preleptotene spermatocytes. A model for the progression of spermatogonial differentiation from undifferentiated spermatogonia following the onset of synchronized spermatogenesis indicating that either the population of A1 or A2 differentiating spermatogonia is skipped is diagrammed in I. Bars = 2 μm.