Fate of microinjected sperm components in the mouse oocyte and embryo

Intact mouse sperm or mouse sperm tails alone, labelled with MitoTracker Green FM(R) fluorochrome, were injected into mouse oocytes and the cells cultured in vitro for up to 5 days. The dye stained midpiece mitochondria, the sperm tail coarse fibres and the sperm perforatorium. Intact sperm (or tails injected with separated heads) induced normal embryonic development. The mitochondria could be identified in embryos up to the 4-cell stage, remaining associated with the sperm tail. They largely disappeared by the 8-cell stage, when only a minority of embryos (6/43) could be found with small patches of mitochondria. Axonemal elements could be identified coiled up in single external blastomeres as late as day 5 blastocysts. By contrast, mitochondria as well as tail components could be identified up to 5 days after injection of sperm tails alone into non-activated oocytes and also in embryos that arrested development before the 8-cell stage. We conclude that disappearance of the labelled sperm mitochondria in normally cleaving embryos is not due to fading or inactivation of the fluorochrome marker, but is rather an event specifically tied to cell cycle activities around the second cell division.