Retinoic Acid Signaling in Male Reproductive Biology: From Germ Cell Regulation to Contraceptive Innovation Within a...

Spermatogenesis is a highly coordinated biological process in which diploid spermatogonia undergo mitotic expansion, meiotic division, and terminal differentiation into haploid spermatozoa. This process is tightly regulated by intrinsic germ cell programs and extrinsic signals from Sertoli cells within the seminiferous epithelium. Among the signaling pathways governing male germ cell development, all-trans retinoic acid (RA), a bioactive metabolite of vitamin A, has emerged as a master regulator of meiotic initiation and spermatogonial differentiation in mammals. RA functions through nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which regulate transcriptional networks essential for germ cell progression, including the activation of Stimulated by Retinoic Acid 8 (STRA8), a key determinant of meiotic entry. Intratesticular RA homeostasis is maintained by a balance between synthesis via aldehyde dehydrogenase (ALDH) enzymes and degradation by cytochrome P450 family 26 (CYP26) enzymes, ensuring precise temporal and spatial control of germ cell development. While rodent models have defined core mechanisms of RA signaling, the canine testis provides a valuable comparative and translational system due to its physiological similarity to human spermatogenesis and relevance to reproductive management. Recent studies highlight conserved RA signaling pathways in dogs, including receptor-mediated transcriptional regulation, feedback control of RA metabolism, and post-transcriptional modulation via microRNAs. Importantly, pharmacological manipulation of RA signaling can reversibly disrupt spermatogenesis, supporting its potential applications in non-hormonal male contraception. This review integrates molecular, developmental, pharmacological, and comparative evidence and presents RA signaling as a central regulatory axis of spermatogenesis with important translational applications.