Bcl-x and bax regulate mouse primordial germ cell survival and apoptosis during embryogenesis

Restricted germ cell loss through apoptosis is initiated in the fetal gonad around embryonic day 13.5 (E13.5) as part of normal germ cell development. The mechanism of this germ cell attrition is unknown. We show that Bcl-x p lays a crucial role in maintaining the survival of mouse germ cells during gonadogenesis. A bcl-x hypomorphic mouse was generated through the introduc tion of a neomycin (neo) gene into the promoter of the bcl-x gene by homolo gous recombination. Mice that contained two copies of the hypomorphic allel e had severe reproductive defects attributed to compromised germ cell devel opment, Males with two mutant alleles lacked spermatogonia and were sterile ; females showed a severely reduced population of primordial and primary fo llicles and exhibited greatly impaired fertility. Primordial germ cells (PG Cs) in bcl-x hypomorph mice migrated to the genital ridge by E12.5 but were depleted by E15.5, a time when Bcl-x and Bar were present, Two additional bcl-x transcripts were identified in fetal germ cells more than 300 bp upst ream of previously reported start sites. Insertion of a neo cassette led to a down-regulation of the bcl-x gene at E12.5 in the hypomorph. Bar was det ected by immunohistochemistry in germ cells from bcl-x hypomorph and contro l testes at E12.5 and E13.5. Bcl-x function was restored, and animals of bo th genders were fertile after removal of the neo selection cassette using O re-mediated recombination. Alternatively, the loss of Bcl-x function in the hypomorph was corrected by the deletion of both copies of the bar gene, re sulting in a restoration of germ cell survival. These findings demonstrate that the balance of Bcl-x and Bar control PGC survival and apoptosis.