AIM:To investigate the ultrastructure of abnormal hepatocyte mitochondria,including their cellular and hepatic zonal distribution,in bioptates in pediatric nonalcoholic steatohepatitis(NASH).METHODS:Ultrastructural investigations were conducted on biopsy liver specimens obtained from 10children(6 boys and 4 girls)aged 2-14 years with previously clinicopathologically diagnosed NASH.The disease was diagnosed if liver biopsy revealed steatosis,inflammation,ballooned hepatocytes,Mallory hyaline,or focal necrosis,varying degrees of fibrosis in the absence of clinical,serological,or histological findings of infectious liver diseases,autoimmune hepatitis,metabolic liver diseases,or celiac disease.For ultrastructural analysis,fresh small liver blocks(1 mm3 volume)were fixed in a solution containing 2%paraformaldehyde and 2.5%glutaraldehyde in 0.1 mol/L cacodylate buffer.The specimens were postfixed in osmium tetroxide,subsequently dehydrated through a graded series of ethanols and propylene oxide,and embedded in Epon812.The material was sectioned on a Reichert ultramicrotome to obtain semithin sections,which were stained with methylene blue in sodium borate.Ultrathin sections were contrasted with uranyl acetate and lead citrate,and examined using an Opton EM 900 transmission electron microscope.RESULTS:Ultrastructural analysis of bioptates obtained from children with non-alcoholic steatohepatitis revealed characteristic repetitive mitochondrial abnormalities within hepatocytes;mainly mitochondrial polymorphisms such as megamitochondria,loss of mitochondrial cristae,and the presence of linear crystalline inclusions within the mitochondrial matrix of an increased electron density.The crystalline inclusions were particularly evident within megamitochondria(MMC),which seemed to be distributed randomly both within the hepatic parenchymal cell and the zones of hepatic lobule,without special variations in abundance.The inclusions appeared as bundles viewed longitudinally,or as an evenly spaced matrix in cross section,and frequently caused mitochondrial deformation.The average diameter of these linear structures was 10 nm and the average space between them 20 nm.Sometimes enlarged intramitochondrial granules were seen in their vicinity.Foamy cytoplasm of hepatocytes was found,resulting from the proliferation of smooth endoplasmic reticulum and glycogen accumulation.The perivascular space of Disse was frequently dilated,and contained transitional hepatic stellate cells,as well as mature and/or newly forming collagen fiber bundles.CONCLUSION:Marked ultrastructural abnormalities observed in hepatocyte mitochondria,especially their polymorphism in the form of MMC and loss of mitochondrial cristae,accompanied by foamy cytoplasm,clearly indicate a major role of these organelles in the morphogenesis of pediatric NASH.Our findings seem to prove the high effectiveness of electron microscopy in the diagnosis of the disease. AIM: To investigate the ultrastructure of abnormal hepatocyte mitochondria, including their cellular and hepatic zonal distribution, in bioptates in pediatric nonalcoholic steatohepatitis (NASH). METHODS: Ultrastructural investigations were conducted on biopsy liver specimens obtained from 10 children (6 boys and 4 girls) aged 2-14 years with previously clinicopathologically diagnosed NASH. The disease was diagnosed if liver biopsy revealed steatosis, inflammation, ballooned hepatocytes, Mallory hyaline, or focal necrosis, varying degrees of fibrosis in the absence of clinical, serological, or histological findings of infectious liver diseases, autoimmune hepatitis, metabolic liver diseases, or celiac disease. For ultrastructural analysis, fresh small liver blocks (1 mm3 volume) were fixed in a solution containing 2% paraformaldehyde and 2.5% glutaraldehyde in 0.1 mol / L cacodylate buffer. postfixed in osmium tetroxide, subsequently dehydrated through a graded series of ethanols and propylene ox ide, and embedded in Epon 812. The material was sectioned on a Reichert ultramicrotome to obtain semithin sections, which were stained with methylene blue in sodium borate. Ultrathin sections were contrasted with uranyl acetate and lead citrate, and examined using an Opton EM 900 transmission electron Ultrastructural analysis of bioptates obtained from children with non-alcoholic steatohepatitis revealed characteristic repetitive mitochondrial abnormalities within hepatocytes; mainly mitochondrial polymorphisms such as megamitochondria, loss of mitochondrial cristae, and the presence of linear crystalline inclusions within the mitochondrial matrix of an increased electron density.The crystalline inclusions were particularly manifested in megamitochondria (MMC), which seemed to be distributed randomly both within the hepatic parenchymal cell and the zones of hepatic lobule, without special variations in abundance.The inclusions were as bundles viewed longitudinally, or as anevenly spaced matrix in cross section, and frequently caused mitochondrial deformation. The average diameter of these linear structures was 10 nm and the average space between them 20 nm. from the proliferation of smooth endoplasmic reticulum and glycogen accumulation. The perivascular space of Disse was frequently dilated, and contained transitional hepatic stellate cells, as well as mature and / or newly formed collagen fiber bundles. CONCLUSION: Marked ultrastructural abnormalities observed in hepatocyte mitochondria, especially their polymorphism in the form of MMC and loss of mitochondrial cristae, accompanied by foamy cytoplasm, clearly indicate a major role of these organelles in the morphogenesis of pediatric NASH. Our findings seem to prove the high effectiveness of electron microscopy in the diagnosis of the disease.