Visualization of X chromosome reactivation in mouse primordial germ cells in vivo.

X chromosome inactivation (XCI), determined during development, remains stable after embryonic cell divisions. However, primordial germ cells (PGCs) are exceptions in that XCI is reprogrammed and inactivated X chromosomes are reactivated. Although interactions between PGCs and somatic cells are thought to be important for PGC development, little is known about them. Here, we performed imaging of X chromosome reactivation (XCR) using the 'Momiji' mouse system, which can monitor the X chromosome's inactive and active states using two color fluorescence reporter genes, and investigated whether interactions would affect XCR in PGCs. Based on their expression levels, we found that XCR of the Pgk1 locus began at embryonic day (E)10.5 and was almost complete by E13.5. During this period, PGCs became distributed uniformly in the genital ridge, proliferated, and formed clusters; XCR progressed accordingly. In addition, XCR of the Pgk1 locus preceded that of the Hprt locus, indicating that the timing of epigenetic memory erasure varied according to the locus of each of these X-linked genes. Our results indicate that XCR proceeds along with the proliferation of PGCs clustered within the genital ridge. This article has an associated First Person interview with the first author of the paper.