Immunolocalization of aromatase and estrogen and androgen receptors in the gonads and mesonephros of Podocnemis expansa during the first year of life

Abstract The endocrine regulation of testicular and ovarian development during early life in chelonians remains poorly understood, despite major morphophysiological changes occurring in this period. This study investigated the immunolocalization of estrogen (ER1 and ER2), androgen (AR), and aromatase (AROM) in the gonads and mesonephros of Podocnemis expansa during the first year after hatching (n = 5; males: 1, 3, 12 months; females: 2, 4 months). After euthanasia, gonads and mesonephros were collected, fixed in 10% buffered formaldehyde, and processed for immunohistochemistry. The intensity of immunoreactions varied according to age and sex, but not with tissue distribution. AROM, AR, ER1, and ER2 were detected in both gonads and mesonephros, confirming the persistence of steroidogenic and steroid-responsive activity after hatching. In males, AROM showed weak to moderate labeling in germ and interstitial cells of seminiferous tubules, whereas ER2 expression predominated in spermatogonia and interstitial endocrine cells. In females, AROM and AR were moderately expressed in the germinal epithelium and oogonia at two months, decreasing at four months, while ER2 persisted in follicles and oogonia. These findings suggest a dynamic endocrine environment influencing posthatch gonadal maturation, with the mesonephros acting as a transient extragonadal source of steroids. Despite the small, sex-unbalanced sample (due to lack of dimorphism), the results provide valuable baseline information and support future quantitative and functional studies on the reproductive endocrinology of P. expansa and other chelonians with temperature-dependent sex determination.