Cellular immunolocalization of occludin during embryonic and postnatal development of the mouse testis and epididymis

Cellular junctions in the testis and epididymis play crucial roles for the development and maturation of spermatozoa. In the testis, tight junctions between Sertoli cells form a functional blood testis barrier between 10 and 16 days of age, whereas the tight junctional blood epididymal barrier between adjacent epithelial cells is formed between days 18 and 21. In the present study, occludin, a constituent integral membrane protein of tight junctions, was localized by immunofluorescent confocal microscopy in embryonic (days 13.5-18.5), postnatal (days 5-23) and adult (day 70) mouse testes and epididymides to correlate its expression with the onset of tight junctions and eventual formation of these barriers. At embryonic days 13.5 and 16.5, low diffuse cytoplasmic levels of occludin were observed in cells of the testicular cords. By embryonic day 18.5, the level of occludin was still low but appeared as a filiform-like network streaming toward the center of the cord. At postnatal days 5 and 7 immunostaining became more intense and appeared to outline the periphery of Sertoli cells of seminiferous tubules. Postnatal day 14 marked the appearance of an intense, focal band-like localization of occludin at the base of the tubules, correlating with the appearance of a functional blood-testis barrier. By day 23 and in adults, expression of occludin was noted at the base of the tubule appearing as intense, wavy, discontinuous bands similar in appearance irrespective of the stage of the seminiferous epithelium cycle. In the developing epididymis, intense cytoplasmic immunostaining was present in epithelial cells of many epididymal tubules at embryonic day 13.5. By embryonic day 16.5, intense occludin immunostaining appeared along the lateral plasma membranes of epithelial cells, whereas at embryonic day 18.5, immunostaining was punctate and apically located, suggesting the presence of tight junctions by this age; similar immunostaining was noted at postnatal days 5 and 7. In the adult epididymis, distinct punctate apical staining was observed between adjacent principal cells of all epididymal regions except the proximal initial segment, where occludin was found only in association with narrow cells. These results indicate that in the epididymis, the appearance of occludin at apical sites between adjacent epithelial cells occurs during embryonic development suggesting that tight junctions form earlier than in the testis. While occludin was expressed in a similar pattern between Sertoli cells at all stages of the cycle in the adult testis, its expression in the adult epididymis was cell- and region-specific. Taken together these data suggest that different factors regulate occludin expression in the testis and epididymis.