Functional Ultrastructure

Abstract

Peroxisomes (formerly called microbodies) are ubiquitous organelles that contain catalase and oxidative enzymes producing H 2 O 2 . Depending on cell type their number, shape, and size vary. By electron microscopy, peroxisomes have a single membrane that encloses a dense matrix, which contains a crystalloid core in some species (e.g., rat hepatocytes) but not in others (e.g., human hepatocytes). The typical fine structure of peroxisomes (Po) in rat liver hepatocytes with a dense crystalloid core (asterisk) is shown in panel A, which also illustrates how different they are from mitochondria (M). Cisternae of the endoplasmic reticulum (ER) can be closely associated with peroxisomes, alike as with mitochondria. Usually, peroxisomes are spherical, with a diameter as large as 1 μm in hepatocytes or as small as 0.1 μm in fibroblasts. However, in kidney tubular cells they may be angular. In specialized mammalian cells, such as proliferating hepatocytes after partial hepatectomy and some yeasts, peroxisomes may form an interconnected network of tubules and cup shaped structures, which have been referred to as peroxisomal networks.