Motility Subpopulations with Distinct Motility Characteristics Using Swim-Up-Selected Sperm Cells from Norwegian Red Bulls: Effects of Freezing–Thawing and Between-Bull Variation

SIMPLE SUMMARY: For the breeding of cattle using artificial insemination, the cryopreservation of sperm cells is an essential tool. To predict the male fertility potential, it is of utmost importance to control the sperm quality after freezing–thawing, before sending the semen samples to be used at farms for inseminations. Sperm motility is a crucial parameter to keep track of and can be analyzed using the Computer-Assisted Sperm Analyzer, which can accurately and objectively address the question of sperm function with a user-friendly method at the AI station. The sperm cells can be divided into different subpopulations according to swimming pattern and velocity, like rapid progressive, rapid non-progressive and slow-motility spermatozoa. In this study, Norwegian Red bull sperm motility was studied and compared between fresh and frozen–thawed sperm cells after more in vivo-like conditions, using sperm selection via a swim-up method and incubation at body temperature over time. The difference between the bulls was mainly due to the subpopulation with a rapid progressive swimming pattern, which was also the only population significantly correlating with fresh and frozen–thawed spermatozoa from the same bull. These results indicate that rapid, progressive sperm cells are the possible indicators of bull spermatozoa functionality and freezability. ABSTRACT: Discrete subpopulations of motile sperm cells have been found for several species and are implicated to be important for sperm functionality. The aim of this present study was to examine the motile subpopulations in swim-up-selected bull spermatozoa and the relationship between subpopulations in fresh and frozen–thawed sperm cells. In experiment 1, swim-up (SWUP)-selected and non-selected (control) sperm cells were analyzed using a Computer-Assisted Sperm Analyzer (CASA). In experiment 2, the semen from nine bulls was cryopreserved and analyzed using CASA both before and after freezing and after incubation at physiological temperatures. The SWUP population had a higher proportion of total motility, progressivity, and velocity compared to the control (p < 0.05). Likewise, both incubation over time and cryopreservation affected motility and motility parameters (p < 0.05). The population of rapid progressive (RapidP) sperm cells dominated the SWUP fraction and was higher than in the control samples (p < 0.05). Furthermore, RapidP was also the main part of fresh semen, but decreased significantly over time during incubation and due to cryopreservation. In conclusion, RapidP was the main population in SWUP-selected spermatozoa and seems to be an important subpopulation contributing to the differences between treatments and in response to the freezing of sperm cells.