Human hepatic peroxisomes with crystalloid cores associated with urate oxidase activity

Biogenesis of peroxisomes in fetal liver

Peroxisomes are single membrane-limited cell organelles that are involved in numerous metabolic functions. Peroxisomes do not contain DNA; the matrix and membrane proteins are encoded by the nuclear genome. It is assumed that new peroxisomes are formed by division of existing organelles. The present article gives an overview of microscopic studies and recent unpublished results dealing with peroxisome biogenesis in mammalian fetal liver and presents data on peroxisomes in oocytes. Cytochemical (catalase and D-aminoacid oxidase activity) and immunocytochemical data in rat and human liver (antigens of catalase, the three peroxisomal beta-oxidation enzymes, alanine: glyoxylate aminotransferase, peroxisomal membrane proteins with molecular weights of 42 and 70 kDa) indicate that during embryonic and fetal development the peroxisomal population undergoes a differentiation with respect to the composition of the matrix and to the size and number of the organelles. In the youngest stages, rare and small peroxisomes are present, into which the matrix components are imported in a sequential way. The import seems asynchronous in peroxisomes of the same hepatocyte. The size and number of the peroxisomes increase during liver development. In rat and human liver, no morphological or immunocytochemical evidence for an elaborate network of interconnected peroxisomes ("reticulum") was found. Instead, peroxisomes presented as individual organelles, which occasionally show membrane extensions. The importance of the metabolic functions of peroxisomes in human liver is emphasized by the peroxisomal disorders. In the liver of affected fetuses, the microscopic features associated with the defect can already be recognized; i.e., either catalase containing peroxisomes are absent and catalase is localized in the cytoplasm (in fetuses affected with Zellweger syndrome or with infantile Refsum disease) or peroxisomes are present but they are abnormally enlarged (e.g., a fetus affected with acyl-CoA oxidase deficiency). In the quail ovary, numerous peroxisomes are observed in the oocyte and in the granulosa cells during follicle maturation, but not in the full-grown egg. Thus, the mechanism of peroxisome inheritance remains unresolved.

Secondary alterations of human hepatocellular peroxisomes

The morphological and morphometric characteristics of peroxisomes in normal human liver and the peroxisomal alterations in the liver of patients with acquired or congenital non-peroxisomal diseases are reviewed. Secondary peroxisomal changes are observed in steatosis, hepatitis and cirrhosis induced by various agents (viruses, alcohol, drugs, etc.), in cholestasis, in hepatomas, in extra-hepatic cancer with or without liver metastasis, in extrahepatic inflammatory processes, in metabolic disorders affecting metabolism of carbohydrates, lipids and lipoproteins, glycoproteins, amino acids, bilirubin or copper, and in altered thyroid hormone levels. They are recognized as a proliferation of peroxisomes (increased in number and to a lesser extent in surface density and volume density) often accompanied by a minor reduction in size (at most to 68% of the mean diameter in control livers) but very rarely by an increase in mean peroxisomal diameter, and as proliferation-related changes in shape (tails, gastruloid cisternae, funnel-like constrictions, elongation, protrusions) in at least a few of the peroxisomes. These secondary alterations of the peroxisomes are clearly distinguishable from the primary changes in peroxisomes observed in the liver of patients with congenital peroxisomal disorders.