Peroxisomes are ubiquitous organelles that contain catalase and oxidative enzymes producing H2O2. Depending on cell type, their number, shape, and size vary. By electron microscopy, peroxisomes have a single membrane that encloses a dense matrix that 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 they differ in their fine structure from mitochondria (M). Cisternae of the endoplasmic reticulum (ER) can be closely associated with peroxisomes and with mitochondria (cf. Fig. 97). Usually, peroxisomes are spherical, with a diameter as large as 1 μm in hepatocytes and as small as 0.1 μm in fibroblasts. However, in kidney tubular cells they may be angular. In specialized mammalian cells, proliferating hepatocytes after partial hepatectomy and some yeast, peroxisomes may form an interconnected network of tubules and cup-shaped structures, which are referred to as peroxisomal networks.