CHROMATIN STRUCTURAL-CHANGES IN SPERM AFTER SCROTAL INSULATION OF HOLSTEIN BULLS

The reported effects on semen quality ascribed to testicular heat stre ss generally relate to traits impacting sperm transport and fertilizin g ability but not to the genetic material contained by the sperm. To c haracterize the effects of testicular heat stress on sperm chromatin, susceptibility of DNA in sperm nuclear chromatin to in situ acid denat uration was measured by flow cytometry after staining with acridine or ange using the sperm chromatin structure assay (SCSA). Semen was colle cted from Holstein bulls at 3-day intervals, before and after 48-hour scrotal insulation, until the morphologically abnormal sperm content i n raw semen exceeded 50%. After cryopreservation in egg yolk-citrate e xtender, semen was thawed and sampled during incubation in vitro at 38 .5 degrees C. Overall, SCSA results showed that chromatin susceptibili ty to denaturation was increased for sperm collected post- vs. preinsu lation and was more pronounced for sperm presumably in the testes duri ng insulation than for those sperm presumably in the epididymides. Inc reased susceptibility was detected as early as the first collection po stinsulation; however, chromatin of sperm presumably in the proximal e pididymis during insulation did not appear to have been detrimentally affected. Chromatin susceptibility to denaturation increased with incr eased incubation time in vitro, but the rate of change in susceptibili ty during incubation did not differ among pre- vs. postinsulation spec imens. We conclude that elevated scrotal temperatures adversely affect both epididymal and testicular sperm by reducing sperm chromatin stab ility. The effects of heat stress on the chromatin of epididymal sperm were more subtle than those exhibited by testicular sperm detectable within close proximity to the heat stress event.