3β-Hydroxysteroid dehydrogenase (3β-HSD) performs essential roles in the regulation of follicular development and the level at which it is expressed may reflect the steroidogenic capacity of follicles and their physiological status. The aim of this study was to investigate the expression of 3β-HSD protein and mRNA in normal follicles categorized by size into small (2 to 4 mm), medium (5 to 7 mm), large (8 to 10 mm) and cystic follicles (> 21 mm). The expression levels of 3β-HSD protein and mRNA were evaluated by western blotting and quantitative real-time PCR, respectively; whereas, the immunolocalization of 3β-HSD was examined in normal and cystic follicles. The results indicated that the expression of 3β-HSD protein and mRNA increased as follicles grew. High levels of 3β-HSD protein and mRNA (P < 0.01) were found in cystic follicles. Immunoreactions of 3β-HSD were localized in the theca cells in the normal and cystic follicles, with reactions indicating 3β-HSD also to be present in the granulosa cells of the cystic follicles, but to be less evident in or absent from the granulosa cells of normal follicles. Our results revealed that the expression and localization of 3β-HSD differed in the cystic follicles and normal follicles and suggest that 3β-HSD in cystic follicles plays an essential role in the formation and persistence of cysts. Our findings provide important additional insights into the pathogenesis of follicular cysts in sows.Key words: 3β-Hydroxysteroid dehydrogenase (3β-HSD), expression, follicular cysts, sows
