Alteration in surface charge of chimpanzee sperm during epididymal transit and at ejaculation

To further investigate changes in the sperm surface occurring during epididymal transit and after ejaculation, the surface negative charge on the head of chimpanzee sperm recovered from the caput (n = 4) and cauda epididymis (n = 3) and from the ejaculate (n = 4) was measured. Washed sperm were exposed to colloidal iron at pH 1.6, washed, mounted on carbon plates, and examined in a scanning electron microscope equipped with an energy dispersive X-ray analyzer. This technique permits evaluation of individual sperm in a population and provides information not available when the entire population is measured as a whole. The results demonstrate a net increase in negative charge during epididymal transit and after ejaculation and a change in charge distribution within the population from a unimodal to a tetra modal distribution. Caput sperm showed a unimodal distribution with a mode of 20 +/- 2%. Cauda sperm showed three modes at 23 +/- 1, 34 +/- 3, and 41 +/- 3%. Ejaculated sperm showed four modes at 21 +/- 3, 31 +/- 4, and 40 +/- 2, and 46 +/- 2%. The sperm population in the chimpanzee epididymis and ejaculate is not homogeneous, and this technique will aid in future measurement of fertility of subpopulations of sperm in the ejaculate.