Huge numbers of peroxisomes are present in guinea pig duodenum, jejunum, and ileum, and in rat duodenum. The peroxisomes have been studied by light and electron microscopy, including visualization by incubation in a newly-developed alkaline 3,3' diaminobenzidine (DAB) medium. Electron micrographs of more than 3700 guinea pig peroxisomes have been studied. The diameter of most peroxisomes ranges from 0.15 µ. to 0.25 µ. They often appear in clusters, surrounded by and continuous, in numerous places, with smooth endoplasmic reticulum (ER). The ER is extremely tortuous in these regions. Serial sectioning is valuable for studying the ER-peroxisome relationships but viewing sections at different angles, tilted with a goniometer stage, is more informative. The intimate relations of the two organelles appear the same in tissue fixed in four different fixatives. The peroxisomes may be interpreted as localized dilatations of smooth ER retaining multiple membranous continuities. This interpretation is discussed in light of the turnover data on peroxisomal proteins of rat hepatocytes reported by Poole and colleagues. The very large numbers of peroxisomes in intestinal epithelium lead to speculations concerning their functional significance. They resemble the small peroxisomes described in many other cell types. Although the distinctive relationship of these peroxisomes to the ER is probably more significant than their small size, for practical purposes we propose the term "microperoxisomes" to distinguish these peroxisomes from the better-known larger peroxisomes of liver and kidney.

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