Fine Structural Organization of the Lateral Horn of the Rat Spinal Cord as Studied by Glyoxylic Acid-Permanganate Fixation

A method combining glyoxylic acid perfusion with potassium permanganate postfixation revealed a moderate number of characteristic axon terminals including noradrenaline storage granules in the rat thoracic segmental lateral horn (Th₁, Th₃, Th₆, Th₉, Th₁₂). The region showed two kinds of cells: the one (principal cell) was characterized by a remarkably indented nucleus, many mitochondria and long endoplasmic reticulum, and the other by markedly developed short endoplasmic reticulum to increase its cytoplasmic density. The axon terminals observed on lateral horn cells could be roughly divided into four types depending upon the shape and size of the synaptic vesicles included and their contents : type I contained many small and a few large granular vesicles. Type II mainly had many large vesicles with diffuse contents and hardly noticeable cores or granules; type III included only small spherical vesicles without any granules; the rare type IV had flattened vesicles with small and large granular vesicles. Type I, type II and type III contacted not only the soma and dendrites of the principal cells but also each other, forming a so-called “cluster,” which suggested an axo-axonic contact. No difference could be observed between each level of the thoracic lateral horn (Th₁, Th₃, Th₆, Th₉, Th₁₂) in terms of noradrenaline granules and synaptic organization. Occasionally, the contacts of a certain process were observed which resembled a dendrite, including short endoplasmic reticulum, lysosomal dense bodies and large and small vesicles with a dendrite of the principal cell. These findings corresponded to the present data of innervation of abundant amine fibers descending through the dorsal half of the lateral funiculus to accumulate around lateral horn cells by the histofluorescent method.