Myelinated nerve cell bodies in the dorsal horn of the monkey (Saimiri sciureus)

While observing electronmicroscopic preparations of laminae I-III of Rexed ('52), taken from the lumbosacral region of squirrel monkey spinal cord, several small neuronal cell bodies were found which were partially or completely encircled by compact myelin sheaths of varying thickness. Though found in all three laminae, the occurrence of these perikaryal sheaths was less frequent in the "inner zone" of lamina II where there were few myelinated fibers. Perikaryal profiles which were completely surrounded by myelin exhibited meither internal mesaxons nor external tongue processes and the origin of their myelin is obscure. In cases of partially enveloped cells the myelin was often clearly derived from extensions of myelin sheaths surrounding small-diameter axons. These overgrowths of myelin extended away from their axons at a pole near their internal mesaxon and spread out across the surface of neighboring nerve cells. In some cases the extensions were derived from the entire axonal sheath while in others only the external lamellae were included. The external tongue process, when observed, was located at the distal end of the axonal myelin extension. Overgrowths of axonal myelin which were unrelated to neuronal cell bodies were also found but these formations were less extensive.