Sertoli cell cytoplasts in the semen of the spiny dogfish Squalus acanthias

The fine structure of Squalus acanthias (Elasmobranchii) semen was investigated to determine the cellular component responsible for the steroidogenic activity previously demonstrated in the seminal plasma of this species. Semen was found to consist of bundles of spermatozoa, many of which were encased in a sleeve of cytoplasm restricted to the tail region; large, dense bodies lacking a limiting membrane and numerous anuclear cytoplasmic remnants containing lipid droplets, mitochondria, areas of agranular reticulum, and possibly unreleased spermatozoa. These remnants, which we have termed cytoplasts, morphologically resemble in appearance Sertoli cells of S. acanthias at the time of spermiation. This structural similarity, plus the fact that many elasmobranch species slough the apical regions of Sertoli cells during the release of spermatozoa, indicates that the cytoplasts present in the semen of S. acanthias originate from Sertoli cells. Furthermore, the occurrence in these cytoplasts of organelles normally associated with steroid synthesis strongly suggests these structures are the source of steroidogenic enzymes in the semen of S. acanthias. The steroidal contribution to the semen by Sertoli cell cytoplasts may be necessary for either maturation or maintenance of spermatozoa in the excurrent reproductive ducts of S. acanthias.