Ultrastructural features of human spermiogenesis

The ultrastructural features of human spermiogenesis have been correlated with the light microscopic appearance of spermatids. The development of the acrosome together with the changes in nuclear shape, position, size and chromatin configuration are the most useful criteria for ultrastructural staging of spermiogenesis. The axial filament develops early in spermiogenesis from the longitudinal centriole and migrates from the acrosomal to the abacrosomal pole of the nucleus to lodge in the articular fossa. The transverse centriole forms a microtubular extension of 2–3 μ which passes through a deficiency in the segmented connecting piece. The latter develops around the centrioles from electron-dense precursors and can be identified at the stage of centriolar migration. In the region of the principal piece, the ribs of the fibrous sheath develop from microtubules which surround the axial filament. The ribs appear to develop before the two longitudinal columns of the sheath which connect the ribs. The aggregation of mitochondria around the axial filament to form the middle piece occurs late in spermiogenesis and is associated with the loss of a portion of the spermatid cytoplasm as the residual body, a process probably due to the activity of the adjacent Sertoli cells.