Retinoic acid regulation of spermatogenesis in the mouse

Fertility in male mammals is dependent on spermatogenesis, the process by which spermatogonial stem cells (SSCs) differentiate into sperm. Tight control of this process is required to maintain constant fertility as occurs in mice and men. The interaction of cyclical spermatogonial differentiation, termed the spermatogenic cycle, and spatially organized spermatogonial differentiation, termed the spermatogenic wave, gives rise to a constant production of mature sperm but the triggers initiating and maintaining these processes have not been well defined. Previous work has implicated retinoic acid (RA) as a potential regulator of the spermatogenic cycle; however its role in the spermatogenic wave has remained elusive. Utilizing a transgenic mouse model in which active RA signaling can be observed on an individual cell basis, the role of RA in spermatogonial differentiation was explored and definitively demonstrated, confirming RA regulation of the spermatogenic cycle. The distribution of both RA and an RA-responsive marker of germ cell differentiation was found to be uneven throughout the neonatal testis, possibly heralding the onset of the spermatogenic wave. Additionally, RA distribution was observed to be periodic along the length of the seminiferous tubule, implicating RA as an initiator of the spermatogenic wave. Neonatal exposure to RA resulted in abolishment of the spermatogenic wave, confirming a causative role for RA in its initiation. Microarray analysis of normal and vitamin A deficient adult testis coupled to transcriptional profiles of isolated testicular cell types demonstrated that RA response was associated predominantly with germ cells and occurred in a stage-dependent manner, correlating well with the observed stage-dependent variation in RA concentration and expression of RA metabolism genes. These observations, coupled to the RA-responsiveness of many RA metabolism genes, imply that RA itself may be capable of perpetuating a cycle of high and low RA concentration in a stage-dependent manner. This, in theory, could maintain the spermatogenic wave in the adult. Taken together, the results discussed in this work demonstrate a fundamental role for RA in multiple aspects of male spermatogenesis and highlight the need to better understand the targets and production of this key regulator.