Photoperiod-induced testicular apoptosis in European starlings (Sturnus vulgaris)

To determine the extent to which testicular regression involves apoptotic c ell death, photosensitive adult starlings were photostimulated for up to 9 wk by exposure to long-day (18 h of light) photoperiods. Apoptotic activity in recrudescing and regressing testes was assessed by in situ TUNEL labeli ng. Absolute testis mass in male starlings increased after 2 wk of photosti mulation and subsequently decreased with continued long-day exposure. Semin iferous tubule diameter also increased after 1-3 wk of photostimulation, th en decreased as photorefractoriness developed. Testosterone concentrations increased significantly by Week 2 of photostimulation and declined with fur ther light exposure. TUNEL labeling was significantly elevated in germ cell s with 4 wk of photostimulation, An approximate 7-fold increase in the degr ee of apoptotic cell death was observed over the course of gonadal regressi on, Incidences of TUNEL labeling in somatic Sertoli cells also increased. L ight and electron microscopy examination confirmed that these somatic cells displayed morphological characteristics of apoptotic death. In rodents, Se rtoli cells have not been previously reported to die during gonadal regress ion, These results suggest that seasonal testicular regression in European starlings is mediated by apoptosis.