Membrane fusion in mitochondria. II. An ultrastructural interpretation of cristae-enriched megamitochondria induced by cuprizone.

Ultrastructure of megamitochondria in mouse hepatocytes induced by cuprizone was analyzed in the light of the fusion of inner mitochondrial membranes. Hepatic parenchymal cells of mice fed with a 0.5% cuprizone-diet for 7-8 days were characterized by megamitochondria with numerous cristae including disoriented, shorter ones than those of the control and those in vesicular configuration. After feeding the animals with cuprizone for 15-16 days, megamitochondria increased their sizes further and the number of cristae decreased drastically. Numbers of cristae based on a unit-mitochondrial area were 41.3 +/- 10.8 for the former type of cristae-enriched megamitochondria and 3.8 +/- 1.8 for the latter type of megamitochondria with the value of 23.9 +/- 6.2 for the control. Considering the content of cytochromes in megamitochondria previously reported, these results might suggest that the fusion and defusion of inner mitochondrial membrane take place along with the fusion of outer membranes during the megamitochondrial formation.