The phagocytosis of rabbit spermatozoa has been studied with the electron microscope, and the process of ingestion of both intact and damaged spermatozoa by leucocytes in the oestrous uterus is described. The cell membrane and acrosome cap remained intact in many sperm heads wholly ingested by uterine leucocytes up to 12 hr after mating. However, only spermatozoa with damaged acrosomes and/ or damaged head membranes were ingested to any extent in the pseudopregnant uterus, in the pleural cavity, or following in-vitro incubation with leucocytes obtained from the oestrous uterus, intact sperm heads merely adhering to the leucocyte surface. It is suggested that some change occurs in the surface membrane of spermatozoa in the oestrous uterus which renders the intact sperm head acceptable to uterine leucocytes. Such surface changes may be associated with the process of sperm capacitation.
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