DNA nicks and increased sensitivity of DNA to fluorescence in situ end labeling during functional spermiogenesis.

Terminal transferase can be used to quantitate DNA strand breaks in situ by labeling free 3'-hydroxyl ends with exogenous nucleotides. Endogenous nicks in DNA temporally appear and disappear during functionally significant structural rearrangements of chromatin. Fluorescence in situ end labeling of mouse and rat testicular cells demonstrated that functional spermiogenesis is associated with abundant DNA nicks that occur in elongating spermatids, most likely as a result of nucleoprotein changes during terminal differentiation. Detectable DNA breaks were not observed in round spermatids and epididymal sperm.