Fertility of stallions with abnormalities of the sperm acrosome

During a 2-year period, 7 stallions were identified as having ejaculated spermatozoa characterized by a high incidence (27-74%) of acrosomal abnormalities. The most frequent abnormality of the acrosome was the 'knobbed sperm' defect which was observed in nigrosin--eosin, Giemsa, and haematoxylin and eosin stained semen smears under light microscopy, in buffered formal--saline and in glutaraldehyde-fixed wet mounts under phase contrast and differential interference microscopy, and in glutaraldehyde-fixed spermatozoa with electron microscopy and scanning electron microscopy. The defect was visible as a flattened apical ridge, tongue-like flap at the apical surface of the acrosome, or a bead-like protrusion from the apical ridge. A cystic swelling on the acrosomal surface was also seen in some electron micrographs of knobbed spermatozoa. Crater defects of the sperm nucleus were also present in the spermatozoa of 3 of 4 stallions examined by phase contrast, differential interference and electron microscopy. All the stallions sired foals; they achieved a pregnancy rate that ranged from 12.5 to 100% per oestrous cycle.