Sperm morphometric subpopulations are differentially distributed in rams with different maturity age in cryopreserved ejaculates.

It is widely accepted that sperm morphology is a good indicator of fertility and it has been proposed that sperm quality may be related to subtle changes in sperm head morphology. However, a precise estimation of the morphology of ram sperm would be very useful to improve reproductive success in ovine. Computer-assisted morphometric analysis and clustering analysis have been important tools to study sperm subpopulations in domestic animals. However, to the best of our knowledge, no data exist studing morphometric differences regarding to sperm subpopulations within the ovine ejaculate. The aim of this study was to test the presence and distribution of sperm morphometric subpopulations in cryopreserved ejaculates from yearling and mature rams using an objective method by computer analysis system and to establish the relationship between the distribution of the subpopulations found and sperm quality in each individual ram. Principal component analysis revealed that three principal components for yearlings and four components for mature rams that represented more than 84% of the cumulative variance in both cases. After cluster analysis, three sperm morphometric subpopulations for yearlings (CLY) and four for mature (CLM) rams were identified with defined sperm dimensions and shapes. CLY1 included big, round and short sperm (37%), CLY2 included average size and slightly elliptical and elongated sperm (48%), CLY3 included small, long, elliptical and elongated sperm cells (15%). CLM1 consisted of average size and moderate elliptical and elongated (26%), CLM2 consisted of small, long, elliptical and elongated (31%), CLM3 consisted of small and round (32%) and CLM4 included big, short and round (8%) spermatozoa respectively. There were significant differences in the distribution of the three subpopulations (P < 0.001) as well as in the sperm concentration, total motility (%), sperm viability (%) and the overall (P < 0.05) in the ejaculates among the four yearling rams tested. Same results were found for the four subpopulations and the different sperm quality parameters in the ejaculates among the four mature rams tested. In conclusion, cryopreserved ram semen showed a specific structure with regard to sperm morphometric subpopulations. In addition, the distribution of these subpopulations seems to be related to stud maturity age and the ejaculate quality which would be a very important indicator of sperm function. Thus, analysis of sperm morphometric subpopulation structure together with functional tests could provide valuable information to assess the cryoresistence of ram spermatozoa.