Figure 2: Pairing of sex chromosomes at the pachytene stage of meiotic prophase I. Microspread ovarian or testicular cells were immunostained with human autoantibody CREST (red), with specific antibodies against SCP3 (red) and γH2AX (green), and counterstained with DAPI (blue). The merged image (top) is followed by γH2AX signals alone (middle) and red signals of SCP3 and CREST alone (bottom). ( a ) A pachytene oocyte nucleus collected from an XX fetal ovary at 18.5 dpc. 20 sets of homologous chromosomes including the XX pair are fully synapsed. No γH2AX signals are seen. ( b ) A pachytene oocyte nucleus collected from a B6.YTIR fetal ovary at 18.5 dpc. The short Y chromosome is apart from the single X chromosome, the latter of which is covered by a γH2AX domain, a characteristic of unsynapsed chromosome. Nineteen autosomal pairs are fully synapsed without γH2AX signals. ( c ) A pachytene spermatocyte nucleus collected from a B6.YTIR adult testis. The X and Y chromosomes are partially synapsed at their pseudoautosomal regions and covered by a γH2AX domain, named XY or sex body. Nineteen autosomal pairs are fully synapsed.