Fig. 3. Spermatogenesis in Stra8^–/– mutants can progress to an early pachytene-like stage. Histological sections through the testes of WT and Stra8^–/– mutants at the age of 8 weeks are shown. (A,B) Overview of seminiferous-tubule sections stained with haematoxylin and eosin. (C) Percentages of the most advanced spermatocytes detected on haematoxylin- and eosin-stained histological sections of 8-week-old WT and Stra8^–/– mutants. PR (preleptotene), L (leptotene), Z (zygotene), P (pachytene), D (diplotene) and M (metaphase I) indicate the most advanced spermatocytes found in a given seminiferous-tubule section, whereas 0 indicates tubule sections containing only Sertoli cells, spermatogonia and eventually abnormal metaphase-like cells (M^*, see the main text for further details). Stra8^–/– mutants display a relative excess in premeiotic (i.e. preleptotene) and early meiotic (i.e. leptotene) spermatocytes, together with a marked deficit in the more advanced zygotene or pachytene spermatocytes and no diplotene spermatocytes. (D-I) Semi-thin histological sections stained with toluidine blue demonstrate the presence, in Stra8^–/– mutants, of spermatocyte nuclei with zygotene (Z) and early-pachytene (eP) nuclear morphologies. The latter represent the most developmentally advanced germ-cell types present in Stra8^–/– testes, because nuclei with an XY body (the condensed chromatin structure containing the sex chromosomes) that are characteristic of mid-pachynema (see XY in H) were never seen in Stra8^–/– testes. S, Sertoli cells. (J-M) Immunofluorescence staining for SYCP3 and TRIM24. (J,K) The SYCP3 immunolabelling pattern (red), which is characteristic of early-pachytene spermatocytes (eP), is shown. The DAPI nuclear counterstain is blue. R, round spermatids. (L,M) TRIM24 (red), which is not detected in WT maturing spermatocytes prior to mid-pachynema, is undetectable in mutant meiocytes. The DAPI nuclear counterstain is blue. SG, spermatogonia. Roman numerals in D,F,H,L designate stages of the seminiferous-epithelium cycle (Russell et al., 1990). Scale bar: 80 m (A,B); 15 m (D-K); 40 m (L,M).