Fig. 4. Stra8^–/– preleptotene spermatocytes can achieve a premeiotic S phase and express the meiotic cohesins Rec8 and Smc1b. Histological sections through the testes of WT and Stra8^–/– mutants at the age of 8 weeks are shown. (A-G) Photomicrographs illustrating that Stra8 transcripts (purple and red signals) are expressed (1) in all preleptotene spermatocytes and (2) in differentiating A spermatogonia that were identified as A1-A3 on the basis of their presence at precise stages of the seminiferous-epithelium cycle (Chiarini-Garcia and Russell, 2001). (H,J) Immunohistochemical detection of BrdU (red signal) in WT and Stra8^–/– preleptotene spermatocytes at 2 hours after its administration. (I,K) Show the same seminiferous tubules as in H,J, but stained with haematoxylin and eosin to allow identification of the cell types on the basis of their nuclear morphology. (L-Q) Immunohistochemical detection of BrdU in seminiferous tubules 8 days after its administration: BrdU that was initially incorporated in preleptotene spermatocytes has been transferred to spermatocytes with pachytene nuclear morphologies both in WT and Stra8^–/– males. Note that M,P correspond to superimpositions of fluorescent signals (brown false colour), and of an haematoxylin and eosin stain of the tubule sections displayed in L,O. Also note that N,Q are high-magnification views of the areas delineated by boxes in L,M,O,P. (R-Y) Detection of Rec8 and Smc1b transcripts in the seminiferous epithelium. A1/A2/B, A1/A2/B spermatogonia; P, pachytene spermatocytes; PR, preleptotene spermatocytes; R, round spermatids; S, Sertoli cells; St10/St15/St16, step 10/15/16 spermatids. Roman numerals designate stages of the seminiferous-epithelium cycle. Note that, in C,E,G,S,U,W,Y, the in situ hybridisation signals were converted to a red false colour, and the DAPI nuclear stain was converted to a bright-field image and then to a blue false colour using Photoshop. Scale bar: 80 m (A,H-P); 30 m (B-G,N,Q); 40 m (R-Y).