Fluorescence Microscopy and Flow-Cytometry Assessment of Substructures in European Red Deer Epididymal Spermatozoa after Cryopreservation

SIMPLE SUMMARY: Flow cytometry (FC) is the recommended technique for assessing sperm quality. In comparison with fluorescence microscopy (FM), FC supports analyses of much larger sperm populations and generates more reliable results. However, FC is not always accessible, and sperm populations are often evaluated with the use of FM. In the present study, FC and FM were used to assess the functionality of various organelles in European red deer epididymal spermatozoa stored in liquid nitrogen. Spermatozoa were collected from the epididymides of hunter-harvested European red deer stags. The epididymides were stored in a refrigerator (2–4 °C) for 12 h before analysis. The study demonstrated that the refrigerated storage of the epididymides for 12 h had no significant effect on the sperm quality before cryopreservation, but it significantly influenced the percentage of early necrotic sperm after thawing. The results of FM and FC assays differed significantly, excluding in the assessment of the plasma membrane integrity. However, the results of both assays revealed significant correlations between the examined variables, except for mitochondrial activity. The study demonstrated that the spermatozoa from epididymides chill-stored for 12 h can be used for cryopreservation. Fluorescence microscopy and FC are equally reliable techniques, but FM was more useful for evaluating mitochondrial activity. ABSTRACT: Thawed spermatozoa, sampled post mortem from the fresh epididymides of European red deer and epididymides stored for up to 12 h at 2–4 °C, were evaluated by fluorescence microscopy (FM) and flow cytometry (FC). The sperm samples were extended and cryopreserved. The sperm motility (CASA), sperm viability (SYBR(+)/PI(-)), acrosome integrity, mitochondrial activity, apoptotic changes, and chromatin stability were assessed. Sperm were analyzed by FM before cryopreservation, and by FM and FC after thawing. Epididymal storage time (for 12 h) had no significant effect (p > 0.05) on the examined variables before cryopreservation. After thawing, the storage variants differed (p ˂ 0.05) in the percentage of apoptotic sperm (FM and FC) and DNA integrity (FC). The results of FM and FC differed (p ˂ 0.05) in all the analyzed parameters, excluding SYBR(+)/PI. Significant correlations (p ˂ 0.01) were observed between the sperm viability, acrosome integrity, and the percentage of non-apoptotic spermatozoa, regardless of the applied technique. In FM, the above parameters were also significantly correlated with mitochondrial activity. The study demonstrated that European red deer spermatozoa stored in the epididymides at 2–4 °C for 12 h can be used for cryopreservation. Both techniques were equally reliable, but FM was better suited for evaluating mitochondrial activity whereas FC was more useful in the evaluation of DNA fragmentation.