Mobile and immobile endoplasmic reticulum in onion bulb epidermis cells: short- and long-term observations with a confocal laser scanning microscope.

The endoplasmic reticulum (ER) of onion bulb scale epidermis cells consists of long, tubular strands lying deep in the cytoplasm which move quickly and a less mobile peripheral network of tubules and cisternae that change in position, shape and size but that also have immobile, fixed, sites (IFSs). IFSs occur in junctions, at vertexes and at blind endings of tubules as well as at the edges and the surface of cisternae. They are regularly arranged in helicoidal rows and may be knot- or ring-like in structure. They become enlarged by treatment with oryzalin but not with colchicine. They persist for long times (for more than 30 min); together with pulling forces, the surface tension and other factors, they determine the configuration and motion of the peripheral network. New polygons of the network are mainly formed by the development of new tubules that become joined with other parts of the network. Polygons disappear by contraction and fusion of tubules. The inner, rapidly moving ER tubules remain connected with the peripheral network over longer distances by sliding junctions. Cytochalasin D causes an accumulation of the ER into patches, a fusion of tubules into cisternae and changes in shape, which indicate the loss of pulling forces. In contrast to animal cells (but like the movement of the inner tubular strands), the latter is dependent upon the actomyosin system; microtubules are not involved. Despite the differences in the organizing components, the peripheral ER in onion bulb scale epidermis cells and that of the borders of cultured animal cells are similar in morphology and motility.