The tridimensional structure of Nissl bodies: a stereoscopic study in ventral horn cells of rat spinal cord.

The tridimensional structure of rough endoplasmic reticulum was examined with both high and low voltage electron microscopes in large ventral horn cells of rat spinal cord, by combining stereoscopic techniques with the use of thick sections selectively impregnated with heavy metal salts. In all neurons examined Nissl bodies appeared as well defined clusters of densely stained and profusely anastomosed plate-, ribbon-, and thread-like cisternae. Plate-like cisternae were variable in size, often showed a shallow curvature, and usually ran in short parallel arrays, separated from one another by fairly constant intervals. All gave rise at their edges to several ribbon-like extensions which occasionally decreased in width distally, turning into thin, thread-like cisternae. Characteristically, these ribbon-like structures would emerge at an angle from their plate of origin and smoothly curve away from the plane of the plate to merge with ribbons or threads arising from adjacent or more distant plates. Most plate-like cisternae were found at the periphery of Nissl bodies and tended to be oriented parallel to their surface. In contrast, the center of Nissl bodies was almost exclusively occupied by a complex network of ribbon- and thread-like cisternae. It is suggested that the basic plate/ribbon association here described in spinal motoneurons might be a constant feature of Nissl body architecture in various neuronal types, while the size, orientation, and relative proportion of plate-like cisternae may vary according to the metabolic state and/or functional specialization of the cells.