FATE OF THE SPERM MITOCHONDRIA, AND THE INCORPORATION, CONVERSION, AND DISASSEMBLY OF THE SPERM TAIL STRUCTURES DURING BOVINE FERTILIZATION

Sperm incorporation and the conversion of the sperm-derived components into zygotic structures during in vitro fertilization of bovine oocyt es was explored by combining ultrastructural studies with observations of the fertilizing sperm tagged with a mitochondrion-specific vital d ye Mito Tracker green FM. The zygotes fertilized by the Mito Tracker-l abeled sperm were fixed at various times after fertilization and then processed for immunocytochemistry to examine the distribution of DNA, microtubules, and sperm tail components, including the fibrous sheath and axonemal microtubules. We show here that the complete incorporatio n of the sperm, but not sperm-oocyte binding and oocyte activation, de pends upon the integrity of oocyte microfilaments and is inhibited by the microfilament disrupter cytochalasin B. After sperm incorporation, the mitochondria are displaced from the sperm's connecting piece, and the sperm centriole is exposed to the egg cytoplasm. This event is fo llowed by the formation of the microtubule-based sperm aster, which is responsible for the union of male and female pronuclei. Concomitantly , the major structure of the sperm principal piece, the fibrous sheath , disappears. After the first mitosis, the compact mitochondrial sheat h can be seen in one of the blastomeres. An aggregate of the sperm mit ochondria is observed at the entry of the second mitosis, although the y remain in the vicinity of the nucleus and can later be seen at one p ole of the metaphase spindle. The mitochondrial cluster is occasionall y found in one of the blastomeres in the early-stage four-cell embryos , but it is no longer detected by the beginning of the third mitotic c ycle. These data suggest that the disassembly of the sperm tail during bovine fertilization occurs as a series of precisely orchestrated eve nts involving the destruction (fibrous sheath and mitochondrial sheath ) and transformation (DNA, sperm centriole) of particular sperm struct ures into zygotic and embryonic components.