Characterization of the antigenic phenotype of B-crystallin-expressing peripapillary glial cells in the developing chick retina

Radial glia are transdifferentiated into astrocytes within the developing brain and spinal cord. The neural retina contains M$\ddot{u}$ller cells, which are retinal radial glia. Some of the cells that surround the optic nerve head among M$\ddot{u}$ller cells in the chicken retina are called peripapillary glial cells (PPGCs). PPGCs express different molecules compared to typical M$\ddot{u}$ller cells. However, an antigenic PPGC phenotype has not yet been clearly established. In this study, we classified the antigenic PPGC phenotypes and identified the differentiation stages of these cells. At embryonic day (E)8, $\alpha$B-crystallin-positive PPGCs had a bipolar shape with long processes that traversed entire layers of the retina. Pax2 and vimentin were expressed in $\alpha$B-crystallin-positive PPGCs. Glial fibrillary acidic protein (GFAP) immunoreactivity was not observed in PPGCs. At E18, $\alpha$B-crystallin immunoreactivity disappeared from the vitread processes of PPGCs. However, the PPGC cell bodies and ventricular processes contained $\alpha$B-crystallin protein, and the PPGCs retained the same Pax2-positive/vimentin-positive/GFAP-negative profile as that seen at E8. At post-hatch day 120, $\alpha$B-crystallin and Pax2 immunoreactivity was not observed, but vimentin and GFAP expression was clearly observed in the presumptive location of the PPGCs. Furthermore, these two proteins overlapped within that location. Considering that vimentin expression is prolonged until the post-hatching period in chicken brain, these findings suggest that Pax2-negative/vimentin-positive/GFAP-positive PPGCs are phenotypically identical to mature astrocytes in this avian species.