Fig. 5. Abnormal synapsis in Stra8^–/– spermatocytes. Immunostaining of surface-spread nuclei from 4-week-old males using antibodies to SYCP1, SYCP3, H2AX and RAD51, as indicated, is shown. L/Z/P, leptotene/zygotene/pachytene spermatocytes; Z/P, designates the most advanced spermatocytes that were present in the mutant testes – these cells shared characters common to zygonema and pachynema. (A-G,P,Q) Formation of the SC is assessed by immunofluorescence on nuclear spreads. The axial elements and the lateral elements of the SC are stained with anti-SYCP3 antibodies (green). The central elements of the SC are stained with anti-SYCP1 antibodies (red). The SC appears yellow from overlapping of SYCP1 and SYCP3 signals. (A) Early leptonema is indistinguishable between WT (not shown) and Stra8^–/– spermatocytes. (A-C) Bouquet configurations are unique to Stra8^–/– spermatocytes. (D,F) Short fragments of the SC (yellow) have begun to form both in WT and Stra8^–/– spermatocytes at early zygonema. (E) In WT pachytene spermatocytes, 19 fully synapsed bivalents are visible along with the XY pair. (G,P,Q) In the Stra8^–/– Z/P spermatocytes, the chromosomes are either not synapsed (asterisk) or partially synapsed (white arrows), although fully synapsed chromosome pairs are occasionally seen (arrowheads). In addition, lateral elements are frequently heterosynapsed (blue arrows). (H,J) WT and Stra8^–/– spermatocytes express H2AX (red signal) throughout the nucleus during zygonema. (I) In WT spermatocytes during pachynema, H2AX expression becomes restricted to the (almost) unsynapsed XY body. (K) In Stra8^–/– spermatocytes at the Z/P stage, H2AX remains widely distributed throughout the nucleus, except in the synapsed regions (arrowed thick green strands), reflecting the near-absolute lack of homologous pairing in these spermatocytes. (L,M) In WT spermatocytes, the number of RAD51 foci (green signal) sharply decreases during the progression from zygonema to pachynema. (N,O) In the most advanced Stra8^–/– spermatocytes, the density of RAD51 foci is abnormally elevated but, similar to the WT situation, displays an inverse correlation with the extent of synapsis. Scale bar: 10 m (A-O); 4 m (P,Q).