Appearance of Semen and Sperm Concentration

Gross Appearance of the Ejaculate

It is important to note the appearance of any semen sample, particularly to identify abnormal conditions. The opacity of semen provides a rough indication of concentration. As shown below, a typical bull ejaculate is much more opaque than a typical dog ejaculate, reflecting substantial differences in sperm concentration.

It is normal for the color of an ejaculate to vary among individuals. Some bulls, for instance, ejaculate distinctly yellowish semen while most have light, cream colored semen. More dramatic alterations in color are seen when semen is contaminated with blood, in which case it is important to identify the source of the bleeding.

Semen from most species is homogeneous in consistency, and the presence of flakes or small clumps usually indicates an inflammatory process in the reproductive tract and the male. Such semen should not be used for insemination and the male should be examined to diagnose and treat the condition. In species such as primates and rodents, semen normally coagulates after ejaculation.

Evaluating Sperm Concentration

The most common means of determining sperm concentration is to simply count sperm under a microscope with the aid of a hemacytometer. A hemacytometer is a glass slide onto which a precision grid has been etched. They are sold for counting blood cells, but work equally well for counting sperm. Since the dimensions of the grid squares and depth of sample chamber are known, it is a simple matter to calculate cell concentration.

Prior to counting sperm, they need to be killed to prevent movement; this is often accomplished by adding semen to a diluent containing a small amount of formaldehyde.

Counting sperm using a hemacytometer is tedious. An alternative technique is to determine sperm concentration using a spectrophotometer. A spectrophotometer measures the amount of light absorbed by a sample, and the more sperm are in the sample, the more light is absorbed. By generating a standard curve of absorbance versus sperm numbers, one can quickly and accurately measure sperm concentration without directly counting them.

There are, of course, huge differences among species in sperm concentration and ejaculate volume. There is also considerable variability in these parameters among males of a single species. Finally, sperm numbers and ejaculate volume are affected by sexual activity; ejaculates will have fewer sperm when collected during a sexually active period compared to that collected after a period of sexual rest.