Differentiation of a unique macroglial cell type in the pecten oculi of the chicken

Glial cells in the CNS of vertebrates serve specialized functions in close interaction with surrounding neurons and blood vessels. In the avian eye, t he neural tissue (retina) and the supporting vascular structure (pecten ocu li) are spatially separated and comprise distinct glial cell types, i.e., t he Muller glia and the pecteneal glia, respectively. In the present study w e combined morphological and immunocytochemical investigations on the diffe rentiation of the pecteneal glia in comparison to the retinal Muller glia, the retinal pigment epithelium, and the astrocytic cells of the optic nerve head in order to elucidate the nature, origin, and function of the pectene al glia. Conventional transmission electron microscopy and freeze-fracture imaging revealed striking similarities between the pecteneal glia and retin al pigment epithelial cells at the transition zone to the optic nerve head. Immunofluorescence investigation identified specific labeling for vimentin and glutamine synthetase (GS) but not for glial fibrillary acidic protein (GFAP) in the mature pecteneal glia. Immunogold labeling confirmed the cell ular specificity. GS labeling was weak during embryonic development but inc reasingly strong after hatching. Surprisingly, the intraneuroectodermal end othelial cells were highly immunopositive for GS throughout embryonic devel opment and lost GS expression after hatching. CTS expression in the pectene al glia may participate in pH-regulation of the avian eye. Endothelial GS e xpression in the developing CNS may detoxify detrimental ammonium concentra tions resulting from egg yolk degradation. (C) 1999 Wiley-Liss, Inc.