Endomembrane Systems

Abstract

The endomembrane system consists of a series of membrane-enclosed compartments that segregate internal, cisternal space from die cytosol of the cell. These compartments are divided into a number of categories according to their morphology: nuclear envelope; rough and smooth endoplasmic reticulum, which may consist of flattened and extensive cisternae, or of interconnecting and anastomosing tubules; the Golgi apparatus; and finally the secretory vesicles. In plants the endomembrane system is a highly complex reticulate structure that penetrates all parts of the cytoplasm. While individual elements of the Golgi, the dictyosomes, are reasonably conspicuous in thin sections of cells viewed by electron microscopy, the endoplasmic reticulum often appears to be relatively insignificant. It is more effectively visualized by selective staining followed by viewing thick sections in high (1000 kV) or medium voltage (200 kV) electron microscopes (Hawes et al. 1981; Juniper et al. 1982; Barlow et al. 1984) or living cells in a fluorescence light microscope (Quader and Schnepf 1986; Quader et al. 1987). These methods have focused attention on subplasma membrane cortical endoplasmic reticulum (Hepler et al. 1990). Morphometric analysis has revealed the true extent of both cisternal (sheetlike) and tubular elements of the endoplasmic reticulum, with typical volumes of several 100 μm³ per cell and surface areas of the order of 2-15000 μm²/cell (Stephenson and Hawes 1986).