A Comparative View of Sperm Ultrastructure1

The relatively primitive spermatozoa of animals with external fertilization have an uncomplicated locomotor apparatus consisting of a simple 9 + 2 flagellum with a single ring of mitochondria around its base. But at the same time, these sperm often have highly specialized acrosomes that undergo complex reactions to insure that the sperm contacts and penetrates the protective coatings of the egg. In animals with internal fertilization, such as the mammals, the acrosome is less highly specialized and proximity to the egg elicits a response that is apparently limited to release of lytic substances by a process akin to secretion. The locomotor apparatus, on the other hand, has become adapted for progression through the female reproductive tract by acquiring an additional outer row of fibers, resulting in the familiar 9 + 9 + 2 pattern. Accompanying this change is an increase in number of mitochondria and their association to form a long middle piece, apparently to provide for the greater energy requirements of locomotion in a more viscous medium. The increase in power achieved by these additional components must be offset to some extent by the greater internal resistance to bending that would inevitably result from the structures added and from the overall thickening of the proximal segments of the sperm. The comparative studies of mammalian sperm described here reveal a common architectural plan but surprising species differences in thickness of the outer fibers, length of middle piece, and overall diameter of the tail. There is suggestive morphological evidence that both external retarding forces and internal resistance to bending are overcome by increasing the diameter of the outer fibers; by moving them farther from the axis of bending to increase their mechanical advantage; by addition of a third order of motor elements, the satellite fibers; or by various combinations of these devices. Much more might be learned about the functional significance of the observed differences in sperm from cinematographic analysis of unusual patterns of locomotion that may be correlated with exceptional development of particular structural components. Investigation of the fine structure of a urodele spermatozoon suggests that its axial fiber is the homolog of one of the outer dense 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 rather 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.