6.2—
Structure and Occurrence

Microbodies are usually spherical, but can be ellipsoidal or dumbell shaped, and range from 0.2 to 1.5 µm in cross sectional diameter. Because of the low contrast between the matrix of the microbody and the cytosol they are not detectable by light microscopy but in electron micrographs they can be seen to have a single limiting membrane enclosing a granular matrix of moderate electron density (Fig. 6.1). The core of the microbody commonly contains

Figure 6.1a
A portion of a tobacco leaf cell showing a microbody with a crystalline
inclusion appressed to two chloroplasts. A mitochondrion lies
to the right of the microbody. (Magnification × 33,000).

b. A portion of a tobacco leaf cell, incubated in DAB medium, showing a heavy
deposition of osmium throughout the crystalline inclusion of the microbody.
(Magnification × 30,500). (Reproduced with permission from Frederick and
Newcomb (1969). Original prints supplied by Professor E. H. Newcomb.)

fibrillar inclusions or a single large crystalline inclusion which may be formed by a reorganization of the fibrillar material. These crystalline bodies have been shown to have catalase activity (Frederick & Newcomb, 1969). No ribosomes or any form of nucleic acids have been detected in the microbody and hence they are not considered to be capable of self-replication. Since they are found in many tissues in association with the endoplasmic reticulum they are thought to be formed by this structure. Microbodies are therefore structurally very simple and cannot be confused in electron micrographs with any organelle except perhaps lysosomes from which they can be distinguished by cytochemical techniques.

The presence of catalase is a distinguishing feature of microbodies and this can be detected cytochemically using the dye 3,3'-diaminobenzidine (DAB). In fixed sections, catalase remains active and in the presence of hydrogen peroxide the dye is oxidized to give an osmiophyllic electron-dense product in the core of the microbody.

Microbodies have been seen in electron micrographs of many plant tissues. They have been found in the leaves of angiosperms, gymnosperms and bryophytes where they are numerous and are invariably located near, or appressed to, chloroplasts. Microbodies have also been found in yeast and hyphal fungi and in species representing a wide range of algal phyla. In the fat-storing seeds of higher plants they are numerous at certain stages of germination and are found in the cells in close association with spherosomes which are large lipid storage bodies. In the roots of higher plants a catalase-containing microbody has been found in association with the endoplasmic reticulum but since it does not contain either glycollate oxidase or enzymes of the glyoxyllate cycle, its function in the cell is not clear.