Figure 1. Progression of mouse spermatogenesis that is controlled by key regulators.

Fetal male germ cells remain quiescent at G₀ of mitotic cycle until birth. Thereafter, spermatogenesis resumes with undifferentiated type A spermatogonium cells that have the capacity to self-renew. Later, type A spermatogonia differentiate and expand via mitosis to become differentiating type B spermatogonia. Coinciding with puberty, mitotically dividing type B spermatogonia enter meiosis I in response to retinoic acid (RA). Prophase I is the longest step of meiosis I during which cells undergo complex events, such as programmed DNA double-strand breaks, synapsing and recombination of homologous chromosomes. During anaphase I of meiosis I, homologous chromosomes separate, and cells undergo second meiotic division that ultimately generates haploid round spermatids. Following meiosis, haploid round spermatids differentiate into mature sperms. The prolonged prophase I is accompanied by drastic transcriptome changes controlled by key regulators. Such regulators are expressed spatiotemporally and control the timely expression of genes during entry into meiosis, meiotic progression and spermiogenesis.