Ultrastructural features and pathogenesis of knobbed spermatozoa in a boar

Ultrastructures of knobbed spermatozoa in a boar were observed. The knobs, found at the apex of the spermatozoa, were spherical swellings of the acrosome; vacuoles were found in the swellings. According to the contents, 2 types of the vacuoles were recognized: a vacuole containing cell debris that was surrounded by 2 or 3 layers of membranes, and a vacuole containing an amorphous material that was surrounded by a single membrane. Several vacuoles might be observed in a knob. Observations of both testes indicated that the cell debris in the vacuole of the knob was derived from the cytoplasm of the Sertoli cell, which evaginated into the spermatid. Origin of the amorphous material in the other type of knob is not known.