Nanog is a newly identified transcriptional factor bearing a homeodomain and 
expressed in pluripotential cells of preimplantation and early postimplantation 
embryos, and embryonic stem (ES) and embryonic germ (EG) cells. Knockout 
experiments indicate that Nanog functions as a key player in maintaining the 
pluripotency of stem cells. Importantly, Nanog expression is highly expressed in 
primordial germ cells (PGCs) of E11.5 and E12.5 mouse embryos. However, its 
temporal and spatial expression pattern and function in germ cells are largely 
unknown. To address these issues, whole embryos and cryosections of embryos were 
immunostained with anti-NANOG and anti-STELLA/PGC7 antibodies. NANOG expression, 
repressed in colonized PGCs of E7.25-E7.5 embryos, became detectable in 
migrating PGCs of E7.75-E8.0 embryos. Both male and female PGCs migrating in 
E9.5 and E10.5 embryos and colonizing the genital ridges of E11.5 and E12.5 
embryos were positive for NANOG immunostaining, while the NANOG expression 
pattern differed between the sexes in the later developmental stage. In female 
gonadal PGCs of E13.5 and E14.5 embryos, NANOG became undetectable in germ cells 
positive for the synaptonemal complex-specific protein SCP3, while in male PGCs 
of E14.5-E16.5 embryos, the number of NANOG-positive germ cells drastically 
decreased during the mitotic arrest. No germ cells positive for NANOG were 
detectable in testes and ovaries of adult mice. Thus, in germ cell development, 
NANOG is expressed in proliferating germ cells, in which nuclear reprogramming 
is progressing.