Fate of sperm tail components after incorporation into the hamster egg

The changes in the cytoplasmic organelles of sperm tail in golden hamster eggs fertilized in vivo were observed by electron microscopy. Eggs were obtained from oviducts of hamsters that had been superovulated and inseminated by injection of cauda epididymal spermatozoa into the uteri. In the egg cytoplasm 10 hours after insemination, some of the mitochondria of the spermatozoon midpiece had begun to swell, and a number of multivesicular bodies were observed surrounding the midpiece. The fibrous sheath of the principal piece quickly disappeared prior to the first cleavage, whereas the axoneme and outer dense fibers were unaltered. During the two-cell stage, numerous multivesicular bodies gathered around the midpiece and fused with the mitochondria. The heterophagic vacuoles thus formed then gradually separated from the axial fibers. The outer dense fibers were disarranged and partially torn into small segments; then they seemed to dissociate into substructural granular components. The axonemal microtubules had begun to swell but were still present in the two blastomeres. It is indicated from these observations that at least the mitochondria of the tail constituents carried into the oocyte are digested into small molecular elements by the multivesicular bodies and are possibly distributed as nutrients for the blastomeres during the early stage of development.