A comparative view of sperm ultrastructure.

A comparative study of mammalian sperm showed common structure but species differences in the thickness of the outer fibers, length of the midpiece and overall diameter of the tail. Both external retarding forces and internal resistance to bending may be overcome by the increased diameter of the outer fibers, moving them farther from the axis of bending to increase their mechanical advantage and by addition of a third order of motor elements, the satellite fibers. Cinematographic analysis of unusual patterns of locomotion that may be correlated with exceptional development of particular structural components may shed some light on the functional significance of these differences. The fine structure of urodele spermatozoa suggested that its axial fiber is the homolog of one of the outer fibers of the mammalian sperm tail and that the marginal fiber is probably homologous with one of the longitudinal columns of the fibrous sheath. The close association of the mitochondria with the axial fiber in urodele sperm, instead of with the axonemal complex is consistent with the interpretation that the axoneme is capable of functioning independently of a mitochondrial energy source, but that the long middle piece of vertebrate sperm has evolved to meet the energy requirements of the accessory outer fibers.