[Ultrastructure of frozen and thawed ram sperm]

With the aid of a transmission electron microscope we studied the ultrastructure of frozen and thawed ram sperm. The cryoprotective agents do not completely prevent the occurrence of structural changes in spermatozoa. The sperm membrane system is affected. The greatest and most frequent damage is to the acrosome, the cell membrane and mitochondria are less injured. We observed regional differences in the damage to the cell membrane. It is most damaged above the acrosome, least in the postacrosomal area and in the principal piece of the tail. We found considerable variability in the changes among individual spermatozoa from the same ejaculate. The changes are not in the nature of mechanical damage as a result of the occurrence of ice microcrystals. We therefore think that the occurrence of structural changes is dependent on damage at molecular level and consists of a change in the physical-chemical properties of membranes.