Morphological and acrosomal changes of canine spermatozoa during epididymal transit

Background: During epididymal transit, functional and structural modifications leading to full maturation enable male gametes to reach, recognize and fertilize the oocytes. In dogs, little is known on the modifications of spermatozoa during the passage in the epididymis. The aim of this study was to describe the motility, morphology and acrosomal patterns of canine spermatozoa retrieved from the epididymis caput, corpus and cauda.Results: After the dilution required for the collection of epididymal content, sperm motility was significantly higher(P<0.0001) in the cauda compared to corpus and caput.Proportions of spermatozoa with normal morphology were significantly higher in corpus (P =0.02) and cauda(P<0.0001) compared to caput. Overall morphological abnormalities of the head and neck/midpiece were similar in the three different epididymal regions. A significantly increased prevalence of tail defects, mainly represented by single bent tails, was observed in the corpus compared to caput (P<0.0001) and cauda (P=0.006).Numbers of immature sperm with cytoplasmic droplets decreased from the proximal to the distal region of the epididymis. Particularly, proximal cytoplasmic droplets were more frequently found in spermatozoa collected from the caput epididymis than in the corpus (P<0.0001) and in the cauda (P<0.0001), whereas the occurrence of distal cytoplasmic droplets was higher in the corpus than in the caput (P=0.0003) and in the cauda (P<0.05).Significantly higher proportions of spermatozoa with intact acrosomes were retrieved from the cauda epididymis than from the caput (P=0.03) and the corpus (P =0.008). This difference was mainly due to a lower proportion of spermatozoa with abnormal acrosomes (mainly swollen acrosomes) rather than with absent acrosomes. Conclusions: Canine spermatozoa undergo several modifications in the epididymis. The acquisition of progressive motility, migration of the cytoplasmic droplet and acrosomal reshaping lead to mature spermatozoa which are then stored in the cauda epididymis. From this site, spermatozoa can be retrieved and used in assisted reproductive techniques as a valuable tool for propagating genetic traits of high value individuals that dies accidentally or undergoes orchiectomy for medical purposes. Further investigations should be also focused on the potential use of spermatozoa recovered from other epididymal regions.