Division of Giant Mitochondria during Recovery from Cuprizone Intoxication

Giant mitochondria may be induced in mouse hepatocytes either by riboflavin deficiency (30) or by addition of the copper-chelating agent, cupri-zone (27, 28), to the diet. In both cases, the mitochondria attain enormous size, often exceeding the cell nuclei in diameter. It has recently been shown that the megam3tochondria resulting from riboflavin deficiency could be restored to normal dimensions within a relatively short period of time by injection of vitamin Be into the deficient animals (29). Mitochondrial normalization was brought about by division of the enlarged orga-nelles. This process involved the formation of a membranous partition, which separated the mitochondrion into two distinct chambers. Progressive constriction of the organdie at the level of the partition ultimately resulted in formation of two daughter mitochondria. In the present study, it was found that cupri-zone-induced giant mitochondria could be returned to normal size in an astonishingly short time simply by removing the noxious agent from the diet. Mitochondrial recovery involved division, but proceeded by a totally different route than in recovery from riboflavin deficiency. placed on a powdered complete diet containing 0.5% cuprizone (biscyclohexanone oxaldihydrazone) (G. Frederick Smith Chemical Go., Columbus, Ohio). After 9-14 days, depending on the clinical appearance of the animals, the cuprizone-containlng diet was replaced by Purina lab chow (Ralston Purina Go., St. Louis, Mo.). Animals were sacrificed by decapitation at various intervals after the normal diet was instituted (Table I). Specimens of liver were fixed in 2% OsO4 buffered with phosphate (20) or collidine (5) After rinsing in saline, the tissue blocks were soaked in 0.5% uranyl acetate made up in saline. The blocks were again rinsed in saline, dehydrated in graded ethanol, and embedded in Maraglas-DER 732 (Polysciences, Inc, Rydal, Pa.) (8). Thin sections were stained with uranyl acetate (26) and lead tartrate (21), and examined in a Siemens Elmiskop Ia electron microscope. Hepatic cells in mice exposed to cuprizone were similar to those described by Suzuki and Kikkawa (28). The cells were characterized by the presence of giant mitochondria as well as mitoehondria of normal size, and by a paucity of granular endo-plasmic reticulum. Inspection of 1 #m thick sections in the light microscope revealed that enlarged mitochondria were evenly distributed throughout the liver lobules. At the uhrastructural level, the megamitochondria had a characteristic morphology (Fig. 1). They were predominantly spherical to ovoid, with few surface irregularities. Irrespective of organdie shape, the cristae were not increased in …