Comparison of DNA fragmentation of frozen-thawed epididymal sperm of dogs using Sperm Chromatin Structure Analysis and Sperm Chromatin Dispersion test.

The aim of this study was to compare sperm DNA fragmentation of frozen-thawed epididymal sperm of dogs using the SCSA (Sperm Chromatin Structure Assay) and SCDt (Sperm Chromatin Dispersion test). For this purpose, epididymis from neutered dogs were minced and incubated in a Tris-based extender. The recovered sperm were frozen in a two-step cooling protocol with Tris-based, egg yolk extender and increasing glycerol concentrations, and stored in liquid nitrogen. After thawing, each replica was incubated at 38°C for 24h. Sperm DNA fragmentation index (sDFi) was assessed by SCSA and SCDt at 0, 3, 6 and 24h of incubation and compared within treatments. The relationship and agreement between techniques were evaluated by Pearson's coefficient and Intraclass Correlation Coefficient (ICC). The results were expressed as mean±standard error of the mean (SEM). Both techniques indicated there was a significant increase of DNA fragmentation after 24h of incubation. Moderate correlation (r=0.65; P<0.01) but lack of agreement (ICC=0.451; P>0.05) was found between SCSA and SCDt. The lack of agreement indicates that SCSA and SCDt measure different aspects of DNA fragmentation. Four halo morphologies were detected after 24h of incubation using the SCDt: un-fragmented DNA with a small halo, fragmented DNA with large halo and two new halo presentations never described before for dog sperm: receding sperm with a disappearing halo and "bald" sperm without chromatin dispersion halo around the core. Sperm without a halo of chromatin dispersion are not described by the manufacturer and are similar to un-fragmented sperm, which could lead to erroneous results when using the SCDt. Further studies with different incubation periods and including the new morphologies described in this study should be performed. In conclusion, although SCSA and SCDt can evaluate the changes in the sperm DNA fragmentation dynamics of frozen-thawed epididymal dog sperm, these provided different findings on sperm DNA fragmentation.