The Effects of Dilution, Egg Yolk and Cooling to 5° C on the Ultrastructure of Ram Spermatozoa

Summary. A transmission electron microscope was used to examine samples of ram semen after 30-fold dilution in a buffered glucose solution and storage at either 35° C or 5° C. The appearance of sections of heads and mid-pieces of spermatozoa was systematically scored so that the frequency of occurrence of several changes in ultrastructure in specimens could be compared between treatments. Incubation of diluted semen at 35° C for 1 or 2 hr did not cause significant changes in acrosomes but a significant proportion of spermatozoa showed condensation of mitochondria or loss of material. Although the acrosomes of spermatozoa in undiluted semen were unaffected by incubation at 35° C for 2 hr, mitochondria had lost material and were scored as 'pale' in 80·5% of spermatozoa; only 13·5% of spermatozoa in the control samples were scored in this category. Acrosomes and mid-pieces were affected by cooling to 5° C and storage at this temperature for up to 72 hr. The acrosome changes involved swelling and vacuolation or, in a small proportion of spermatozoa, swelling and vesiculation of the outer acrosomal membrane. Mid-pieces showed condensation of mitochondria and loss of material. The inclusion of 3% (v/v) egg yolk in the diluent reduced the frequency of these changes.