A decrease in step 1 spermatids and an arrest of step 8–9 spermatids from the first spermatogenic wave in RARα−/− juvenile mouse testes 13 or 18 days after bromodeoxyuridine (BrdU) injection. A–D: Histological sections of 8- to 9-day-old testis from RARα+/+ (A,C) and RARα−/− mice (B,D) 13 or 18 days after BrdU injection, respectively. ∼50% of step 1 spermatids were found to be labeled in 9-day-old RARα+/+ testes 13 days after BrdU injection, whereas the number in RARα−/− mouse testes was much lower. Arrows in B point to step 1 round spermatids. BrdU-labeled spermatids appeared to be arrested at step 8–9 in RARα−/− mouse testes as shown in D. A–D: Original magnification, ×40. P, pachytene spermatocytes; L, leptotene spermatocytes; PL, preleptotene spermatocytes; MI/MII, meiosis I/II. Arabic numerals, the step of spermatid differentiation. Roman numerals indicate the stage of the seminiferous tubules.