FIG. 1.

The RA pulse hypothesis. Various in situ hybridization and immunohistochemistry experiments have led us and others to propose a model whereby retinoid storage, synthesis, and/or degradation is regulated such that RA levels are lowest during Stages II–VI and peak at Stage VIII of the cycle of the seminiferous epithelium during testis development. Processes known to be regulated by RA, including spermatogonial differentiation, spermiation, and blood-testis barrier reorganization, all take place during Stage VIII, and there is currently no known role for RA during Stages II–VI.