Figure 1.

Zoom in on sex chromosomes during pachytene. Schematic representation of spermatogenesis, showing the different substages of meiotic prophase (leptotene, zygotene, pachytene and diplotene). During meiotic prophase, progression of chromosome pairing and synapsis can be followed by immunofluorescent staining of SYCP3 (red), a component of the axial/lateral elements of the synaptonemal complex. In the enlarged area, the recent findings of Ichijima et al.⁴ and Royo et al.⁵ are summarized: mediator of DNA damage checkpoint 1 (MDC1) mediates spreading of γH2AX (green), phosphorylated by ATR (yellow) from the unsynapsed chromosomal axes (phase I) into the surrounding chromatin (phase II). Spreading of γH2AX and ATR is required to achieve meiotic sex chromosome inactivation (MSCI) and MSCI failure induces apoptosis (MSCI checkpoint) of pachytene spermatocytes, caused by misexpression of one or more sex-linked genes.