Transferrin binding protein is expressed by oligodendrocytes in the avian retina

It has been documented that some axons of ganglion cells in the nerve fiber layer of avian retina are wrapped in a myelin sheath. However, the identity of myelin-forming cells has not been established. In this study we demonstrated immunohistochemical evidence for the existence of a large population of oligodendrocytes in avian retina, using an antiserum against transferrin binding protein (TfBP), the avian homologue of the mammalian GRP 94 family of stress-regulated proteins. TfBP⁺ cells were mostly confined to the ganglion cell and optic nerve fiber layers of the retina, in which they were closely associated with the soma and axons of ganglion cells. The double-labeling experiments clearly show that TfBP is specific to oligodendrocytes. The morphology, distribution, and antigenic properties indicated by our findings suggest that TfBP⁺ cells are retinal oligodendrocytes that may be responsible for the myelination of ganglion cell axons in avian retina. A putative tropic role of TfBP⁺ oligodendrocytes to the ganglion cells is also discussed in conjunction with the physical properties of TfBP and avascular retinae of birds.