Ultrastructure of boar testis: spindle shape body of spermatid.

The fine structure of the spindle shape body (SSB) of boar spermatid was studied using testes samples fixed by perfusion. This structure appeared on the middle piece which served as the upper border of the fibrous sheath of principal piece during the transition period between the late acrosome phase to the maturation phase of spermiogenesis. The formation of this thread-like spindle form coincided with the development of the postnuclear sheath with perinuclear ring just prior to the growth of the equatorial segment of the acrosome. Likewise, the ribs of the fibrous sheath on the principal piece were observed to have already formed but have not yet completed the mitochondrial sheath. The total size of the SSB and its consisting microtubule were measured. The functional meaning of this transitory construction may involve a threshold condition on the sperm middle piece.