<p> Spermatogenesis is governed by vitamin A, as shown by vitamin A deficient (VAD) testes, which lack advanced germ cells. Vitamin A signaling is mediated by retinoid receptors. There are two families of retinoid receptors, retinoic acid receptors (RARs) and retinoid X receptors (RXRs), each with alpha, beta and gamma subtypes. Retinoic acid receptor alpha (RARA), plays a significant role in the testis such that <i>Rara</i>-null males are infertile because of severe germ cell loss. </p><p> Striking similarities of the testicular phenotypes are detected between <i> Rara</i>-null and VAD mice: severely degenerated testes, lack of germ cells, sloughing of mature spermatids, and infertility. To discern the molecular function of RARA in germ cells, <i>Rara</i> was conditionally deleted using stimulated by retinoic acid 8 (STRA8)-iCRE. With RARA function disabled in germ cells, morphological abnormalities detected in the testes included lack of germ cell organization, lack of lumen, sloughing cells, and vacuolization. Not surprisingly, germ-cell specific <i>Rara</i> conditional knockout mice (cKO) had a dramatic reduction in epididymal sperm number. Further analysis of cKO testes demonstrated decreased spermatogonial proliferation and differentiation, while meiotic defects such as reduced synapsis, synaptonemal fragmentation, and unrepaired double strand breaks were increased. Furthermore, functional spermatogonial transplantation assays pointed to the possibility that RARA regulates spermatogonial stem cell colonization and proliferation, as shown by the reduction of donor-derived spermatogenesis from the cKO donor germ cells. The lack of RARA in the testes clearly shows quantifiable deficiencies during spermatogonial proliferation, differentiation, and meiosis. </p><p> Microarray gene expression studies of mRNAs from the enriched germ cells from wild type and cKO mice provided molecular evidence that RARA regulates spermatogonial differentiation at postnatal day 4 (P4) and meiosis at P8. Cell differentiation, cell adhesion, cell migration, and other pathways related to the early steps of spermatogonial differentiation were found to be functional categories significant in germ cells from P4. These were very distinct from synapsis, synaptonemal complex formation, and crossover formation related to meiosis, which were functional categories significant in germ cells from P8. In conjunction with phenotypic abnormalities, we provide gene expression evidence that RARA mediates retinoic acid function during spermatogonial proliferation, differentiation, and meiosis.</p>