The ultrastructure of a characteristic spermhead-defect in the boar: the SME-defect.

Semen samples from an eight-month-old landrace boar showing poor sperm motility were investigated at the boars' arrival to an A.I. centre. Sperm morphology was found abnormal in +/- 30% of the sperm heads, which contained a rounded, often circular hypochromatic body (eosin-nigrosin stain) about 2 mum in diameter. The body was located in the front half of the sperm head, i.e. within the region of the acrosomal cap. Later a similar defect (+/- 50%) was found in an imported 2 1/2-year-old Yorkshire boar with decreased fertility. Electron microscopy of ejaculates and testis tissue showed that the hypochromatic body was a cyst containing a body of variable size but of moderate electron density. Usually the cyst was located within the nucleus and sometimes with direct communication to the acrosonal system. During spermiogenesis the abnormal spermids may show a deep invagination into the nucleus of the acrosomal granule. Inbreeding experiments are planned in order to determine if a possible hereditary factor is involved in the defect.