Ultrastructural evidence of a peripheral nervous system pattern of myelination in the avascular retina of the guinea pig

Summary Myelination of axons in the nerve fiber layer (NFL) of the retina occurs as a sporadic abnormality in several mammalian species including man, monkey, cat and rat. All of these species have vascularized retinae and, in the latter three, ultrastructural studies have demonstrated the pattern of medullation to be similar to Schwann cell myelination in the peripheral nervous system. This contrasts with an oligodendrocytic pattern of myelination normally present in the avascular retina of the rabbit. One possible explanation for this difference is that the pattern of myelination is related to the presence or absence of retinal blood vessels. The present investigation provides the first evidence of NFL myelination in another avascular retina, that of the guinea pig. Myelination in the guinea pig retina was observed in a single bundle of axons and involved only large diameter fibers. With several axons, myelin sheaths terminated at hemi-nodes of Ranvier and in all such cases this occurred in association with marked paranodal infolding. Morphological characteristics of the myelination include (1) a one to one relationship between axon and myelinating cell, (2) cytoplasm between myelin sheath and plasma membrane, (3) basal lamina surrounding the myelinating cell, (4) collagen fibers in the adjacent extracellular space and (5) double intraperiod lines. These morphological features are characteristic of peripheral nerve myelination by Schwann cells. Thus, in all species so far described in which retinal medullation is abnormally present, the pattern of myelination has been Schwann cell in nature rather than oligodendrocytic. The reasons for this Schwann cell predominance remain undefined.Supported by the Medical Research Council of Canada