GLIAL-CELLS OF THE PARIETAL EYE - STRUCTURAL AND BIOCHEMICAL SIMILARITIES TO RETINAL MULLER CELLS

The parietal eye of lizards is a relatively simple yet highly structured visual organ. Cone-like photoreceptors synapse directly onto ganglion cells that project to the brain. Golgi staining confirmed the existence of glial cells spanning the sensory epithelium in a manner analogous to the Muller cells of the vertebrate lateral eye retina. These parietal eye glial cells bear expanded end feet at the basal border of the eye. Electron microscopic examination revealed some ultrastructural similarity to Muller cells. Mitochondria, osmiophilic and transparent vesicles, and the Golgi apparatus are found in the apical end of the parietal eye glial cell. Junctional complexes join adjacent parietal eye glial cells and their neighboring photoreceptor cells. Bundles of filaments are found in the basal end of the cell and the plasma membranes of adjacent cells often intertwine tightly in this region. The parietal eye glial cells are immunoreactive to antibodies against glutamine synthetase, also characteristic of Muller cells. Some differences between parietal eye glial cells and Muller cells also are evident. The parietal eye glial cells are not immunoreactive to antibodies against another Muller cell marker, carbonic anhydrase II, and many of them contain melanin granules, while Muller cells are not pigmented. In addition, we have found that the lens cells of the parietal eye and the Muller cells of the lateral eye retina of lizards are immunoreactive to antibodies against both glutamine synthetase and carbonic anhydrase II.