Fine structure of the neural fat‐body sheath in the stick insect and its physiological significance

Abstract

AbstractThe interganglionic connectives of the stick insect ventral nerve cord are surrounded by a fat‐body sheath, additional to the glial layer, perineurium and neural lamella found in other insects. The sheath is composed of a series of single‐cell lobes, bound together by an extracellular lamella of collagen fibrils in a mucopolysaccharide matrix. Each sheath cell contains a storage area of glycogen granules, fat‐droplets and protein secretory granules, and a secretory area with abundant granular endoplasmic reticulum which produces the protein granules. The fat‐body sheath appears to actively regulate the electrolytes in the extraneural space within the sheath. The low sodium and high potassium of the haemolymph, which are unfavourable for normal central nervous system activity, appear to be regulated in the extraneural space to surround the nerve elements with a fluid more favourable for impulse conduction. The presence of the fat‐body sheath is almost certainly related to the unusual haemolymph composition in the stick insect, and similar diffusion barriers may well be present in other herbivorous insects of unusual haemolymph composition.