Chromatin integrity of ram spermatozoa. Relationships to annual fluctuations of scrotal surface temperature and temperature-humidity index

Abstract

The objective of the present study was to explore the potential relationships of ovine sperm chromatin integrity, quantified using the sperm chromatin structure assay (SCSA), to the heat load of the scrotum and the discomfort felt by the animals because of fluctuations of microclimatic factors at different time periods before ejaculation. Ejaculates were collected once per week from five Chios rams and four East Friesian rams for 12 months and stored in liquid nitrogen. Frozen-thawed semen samples were analyzed using the SCSA, to determine the DNA fragmentation index (DFI) and the percentage of cells outside the main sperm population (%DFI) in each one of the samples. Scrotal surface temperature (SST) of each ram was measured using an infrared thermometer on a daily basis. Ambient air temperature and relative humidity were recorded at hourly intervals throughout the experimental period and temperature-humidity index (THI) was used to assess the discomfort felt by the rams. Mean values of SST (SSTmean) and THI (THImean) were computed for eight different time periods (up to 61 days) preceding each ejaculation day (Day 0). A linear mixed-effect model analysis was performed to describe the relation of SCSA parameters to collection month, SSTmean, and THImean of different time periods before ejaculation. The results of the statistical analysis revealed a relation of %DFI to the SSTmean of the last 12 days preceding ejaculation, namely the period that resembled the phase of epididymal maturation. On the contrary, the variation of DFI was most adequately described by the linear mixed-effect model applied for Days 54 to 48 before ejaculation, which resembled the phase of spermatogonial mitoses. The effect of collection month was significant for DFI and %DFI, with semen samples collected in September and February exhibiting the lowest DFI values; a less profound seasonal pattern was detected for %DFI. The effect of THImean on DFI and %DFI was proven nonsignificant in regard to all time periods. In conclusion, a relation of SCSA parameters to SSTmean of different periods before ejaculation was shown in the present study, implying an effect of scrotal microenvironment on intratesticular and epididymal sperm population. In contrast, we failed to detect any effect of microclimate-induced discomfort felt by the animals on the chromatin integrity of frozen-thawed ram spermatozoa.