Figure 1. Developmental expression of piRNAs and PIWI proteins during mouse spermatogenesis. Pre-pachytene piRNAs are present prenatally in pro-spermatogonia and postnatally in spermatogonia. Pachytene piRNAs are present postnatally in pachytene spermatocytes and round spermatids. MIWI is expressed in pachytene spermatocytes and round spermatids. MIWI2 is expressed in pro-spermatogonia. MILI is expressed from pro-spermatogonia to round spermatids. Miwi-null mutants exhibit spermiogenic arrest. Mili- and Miwi2-null mutants exhibit meiotic arrest. The spermatogenic arrest points are indicated by Xs in red. This figure is modified from Figure 1 in the publication from Zheng and Wang.⁶²