It is a common misconception that the ejaculate is presented as a homogenous fluid at ejaculation and offers a stable and good environment for the sperm.
The reality is that the sperm are ejected together with the zinc-rich prostatic secretion in the first ejaculate fraction.
During natural fertilization the sperm most likely enter the cervical mucus before any major contact with the remaining fractions take place.
The remaining fractions consisting mostly of fructose rich seminal vesicle secretions provide an unfavourable environment for the sperm in many ways.
When whole semen collection takes place in the clinic the semen sample, as we call, it is mixed thoroughly and left to liquefy. During liquefaction a number of uncontrolled chemical reactions take place.
It has been shown that the osmolarity of semen samples can increase from 290mOsm at ejaculation to 382mOsm (292-454) in 3 hrs. The wide range is due to the large variation in composition of the semen between individuals.
Maintaining homeostasis is essential to all cells and the sperm cells will accommodate to the changing conditions by pumping water and ions through the channels in the cell membrane. This is done in order to keep their shape constant and motility effective. Most sperm cleaning media, such as density gradient media or swim-up media have an osmolarity of 290mOsm.
When the sperm are introduced to this environment they are exposed to hypoosmotic stress. As a result they will take up water and swell up. Most of the sperm may survive but they will then have to pump water out again to regain their shape and motility. This is an energy depriving process.
The solution to this problem would be to either treat the sperm right after ejaculation when the osmolarity of the media and of the semen are similar.
Another alternative would be to use media that have a higher osmolarity to compensate for the rising osmolarity of the semen sample. PureSperm® 100 + PureSperm® Buffer and PureSperm® 40/80 all have an osmolality of 300-310 to compensate for this. To make sure our media always have the optimal composition, we are currently investigating the effects of a colloid with even higher osmolality.