Vestibular ganglion of the squirrel monkey.

The vestibular ganglia of adult squirrel monkeys (Saimiri sciureus) were studied with the aid of light and electron microscopy. The vestibular ganglia are formed by small and large neurons (perikarya). The perikarya's outermost surface is surrounded by 2-20 lamellae of compact myelin. The compact myelin sheath varies its thickness around one perikaryon. The sheath generally thins out i the vicinity of the axon hillock. Occasionally, the compact myelin transforms completely into loose myelin for a length of 2-5 mu. Two to eight lamellae of loose myelin usually face the cytoplasm of the neurons. This myelin arrangement is constant in the majority of the cells examined regardless of shape and/or size. Large myelinated perikarya form the majority of the ganglion cells. There are, however, a few neurons that lack a true compact myelin sheath. Others are surrounded by very few lamellae of loose myelin. The general morphology of the neurons was compared to the available literature on other species. Possible variation on the myelination pattern due to fixation differences, and variation on the perikaryal size due to the angle of cut irregular shape of the cells are also discussed.