STUDIES ON MICROPEROXISOMES V. ARE MICROPEROXISOMES UBIQUITOUS IN MAMMALIAN CELLS?

A variety of mammalian cell types has been studied by electron microscopy following incubation in a 3,3'-diaminobenzidine medium at pH 9.7 and containing a high H 2 O 2 concentration. This medium visualizes the recently described anucleoid microperoxisomes as well as the nucleoid-containing peroxisomes. All 24 cell types contain 3,3'-diaminobenzidine-positive microperoxisomes but none shows nucleoid-containing peroxisomes. The number of microperoxisomes within a cell varies greatly among different cell types. There are huge numbers in some cell types; in others microperoxisomes are common, few or rare. Such differences imply varying functional significance of these organelles in the metabolism of different cell types. Whether the endoplasmic reticulum (ER) is abundant or scarce or whether its membrane is studded with numerous ribosomes or not, ribosomes are lacking where the ER is connected to the microperoxisomes by slender channels. It may be presumed that molecular interchange occurs between these two organelles. Such interchange may occur between microperoxisomes, ER and lipid droplets, as previously suggested, and between zymogen granules of guinea pig pancreas and ER and microperoxisomes. Two rapidly growing malignant cell types were studied (HeLa and Novikoff hepatoma) and both show moderate numbers of microperoxisomes.