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Also important is the fact that Sertoli cells transport water from the interstitial space into the lumen, serving as the vehicle for moving sperm from the testis to the epididymis. In addition to basolaterally locatedion channels, recent studies have revealed that aquaporins (water channels) are abundant in the testis, with some being localized to Sertoli cells. Interestingly, various members of the aquaporin gene family contain CRE motifs (CREB binding regions) and are under cAMP regulation, a second messenger that is activated upon FSH-R signaling. In the testis, Jegou et al. first demonstrated a selective action of FSH, proposing that this hormone is involved in fluid absorption/secretion; however, the cell type involved or the mechanism of action was not evaluated. In addition to fluid regulation, Sertoli cells are responsible for the secretion of numerous proteins into the seminiferous tubular lumen, such as the glycoprotein androgen binding protein (ABP). ABP binds androgens with high affinity and transports them to the epididymis. Regulation of ABP by FSH and testosterone has been demonstrated, although whether one or both are required for complete function remains to be resolved. ABP also displays a stage-specific expression pattern within the seminiferous epithelium and its secretion has often been regarded as an index of Sertoli cell function. Recent work with transgenic mice overexpressing rat ABP has shed more light on the role of ABP in spermatogenesis. The predominant phenotypic anomaly observed in these mice is a decrease in fertility, suggesting that altered levels of ABP are in fact associated with impaired fertility.