The peripapillary glia of the optic nerve head in the chicken retina

The eye of reptiles and birds is characterized by an avascular retina and a vascular convolute called conus papillaris in reptiles and pecten oculi in birds which arises from the papilla nervi optici (PNO) or optic nerve head into the vitreous. At least in birds, this central part of the retina is the site of a heterogeneous population of glial cells. Müller cells reside in the retina, astrocytes in the optic nerve, and pecteneal glial cells in the pecten. The latter are developmentally related to the pigment epithelial cells. In addition to these established types of cells, there is a population of glial cells lining the base of the pecten oculi. In the present study, we investigated both the morphology and the development of these glial cells of the PNO in a series of chicken embryos. These cells were called peripapillary glial cells. They were characterized by their morphology and by their spatiotemporal expression of antigens typical of glial cells (intermediate filaments and glutamine synthetase). They reside at the border between the retina and the optic nerve and at the innermost border of the ventricular cleft representing transitional forms among Müller cells, astrocytes, and pigment epithelial cells. The developmental data suggest a migration of the perikarya of the peripapillary glia in vitread direction, which may coincide with that of the pecteneal glia. Whereas the pecteneal glial cells differentiate morphologically from E16 on, the peripapillary glia retain characteristics of radial glia by spanning the distance from the vitreous to the ventricular cleft. Blood vessels only occurred in the optic nerve head and the pecten oculi. No capillaries were found in the retinal tissue, beyond the peripapillary glia, leading us to suggest that these cells may play a role in demarcating the outer limit of vascularization. The functional properties of these cells are unknown but were discussed to include prevention of vessel growth into the avascular retina and/or axonal guidance during development.