Fig. 3

Expression of PIWI proteins and piRNA populations throughout mouse spermatogenesis. Mammalian germ cells express two distinct populations of piRNAs. Pre-pachytene piRNAs are mainly found in neonatal testis and persists at low levels until meiosis. Pre-pachytene piRNAs are associated with MILI and MIWI2 proteins, with MIWI2 being expressed mostly in embryonic gonads and shortly after birth, while MILI is expressed throughout most of the stages of spermatogenesis. Pachytene piRNAs are an extremely abundant population of small RNAs that begin expression during meiosis. They associate with MILI and MIWI proteins, with MIWI protein beginning expression at the same stage as pachytene piRNAs