Our Initial Conclusions
The ‘take-home’ message is both simple yet significant, and one which is certainly not universally appreciated. This straightforward but effective experiment demonstrated that either pH, bicarbonate (or both acting synergistically) is important for sustaining a high level of sperm motility and velocity in vitro.
Six test derivatives from the same base medium but with differing pH and containing different levels of bicarbonate were tested for influence on sperm motility and curvilinear velocity. Both these variables were shown to correlate with the formulation changes made. These results may have implications for SWB composition for both clinical and laboratory applications, and for diagnostic semen analysis.
A number of authors have utilized CASA to demonstrate the importance of sperm swimming speed for establishing fertilization and pregnancy, and have suggested that simple reporting of progressive motility provides an ‘incomplete picture’ when it comes to assessing a sperm sample’s viability in terms of potential IVF success.2,6,7,8. Therefore, it would seem logical that we should take into account both sperm motility and velocity as viable measures when attempting to improve SWB composition by varying factors (such as pH and bicarbonate levels) that are known to affect these functions in ways that impact on oocyte fertilization. Furthermore, as CASA systems appear to provide a reliable means of quantifying sperm motility and velocity, their consistent use in research into media optimization would allow for better data comparison across studies.
Although many laboratories would not contemplate incubating sperm for extended periods, there may be situations where it may be useful; overnight incubation where insemination has not been possible, for example. Also, perhaps, extended incubation could be useful for comprehensive toxicity testing of laboratory consumables9 or even healthcare products, such as specimen collection condoms or gel lubricants10.
One concern was that excessive bicarbonate could have a detrimental effect on sperm function longer term, leading to ‘burn out’ or over-performance with a subsequent loss of essential nutrients required to maintain a high level of motility. This issue was not observed experimentally, nor was there any evidence of adverse effects on other end-point measures such as the acrosome reaction.
Follow-up experiments are planned which should help to optimize the pH and bicarbonate composition of SWBs that both maximizes motility/velocity yet minimizes the potential of sperm ‘burn out’ due to increased metabolism. Overall, our preliminary work indicates that more needs to be understood about optimal SWB formulation.