Histochemistry of Peroxisomes: Overview.

Peroxisomes are present in virtually all eukaryotic cells and known to contain more than fifty enzymes. Their size, number and composition of enzymes vary considerably between different species and between different cell types within the same species. During the past two decades, there has been an explosion in the amount of information about their biogenesis, peroxisomal targeting signals, the role of these organelles in lipid metabolism, in the synthesis of cholesterol, bile acids and glycerolipids, and in the degradation of uric acid. The importance of peroxisomes in mammalian metabolism is underscored by the discovery of carcinogenic peroxisomes proliferators and by the occurrence of peroxisomal genetic diseases. Of considerable importance is the implication of induction of hepatic peroxisome proliferation and the peroxisomal fatty acid β-oxidation enzyme system in the development of hepatocellular carcinomas in rats and mice by structurally diverse peroxisome proliferators. The immunocytochemical and in situ hybridization procedures are extremely valuable in identifying qualitative and quantitative changes of specific enzymes in animals exposed to peroxisome proliferators. Peroxisome proliferators appear to exert their pleiotropic responses by activating a receptor known as peroxisome proliferatoractivated receptor. An understanding of the ontogeny of this receptor in relation to the expression of peroxisomal genes is an important consideration in assessing the implications of peroxisome proliferatorinduced toxicity and the role of peroxisomes in genetic diseases.