Pachytene spermatocyte with synapsed chromosomes
This is the nucleus of a pachytene spermatocyte. We used immunofluorescence to detect two proteins: the histone variant H2AX (red) and SCP1 (green). The autosomes are paired together in this cell, and the SCP1 protein localizes along the entire synapsed chromosomes. The sex chromosomes (XY) lack pairing partners, and associate together. The H2AX protein localizes to unpaired chromosomes in pachytene spermatocytes, identifying the X and Y-chromosomes.