Three monoclonal antibodies have been raised against the ganglion cell layer of the adult mouse retina. The first antibody, R3, labeled optic axons in the inner retina, and with colchicine pretreatment somata and dendrites of large ganglion cells could be seen. A small number of other processes, including fibers projecting to the retina from elsewhere (efferent fibers), were also labeled in the inner retina. In the outer plexiform layer R3 stained the axonless class of horizontal cells. R3 recognized a 185,000- to 200,000-dalton polypeptide which is most probably the heaviest of the neurofilament subunits. Antibodies R4 and R5 labeled filamentous components mainly in glia and cells of mesenchymal origin. The antigens appeared in most but not quite all locations morphologically closely related to the intermediate filament protein vimentin. In the retina both antibodies labeled strongly the regularly spaced Müller glia. The astroglia of the optic fiber layer was stained with R5 but not R4. Although the two antigens were in general not expressed in neurons, they were both present in axonless horizontal cells in the outer plexiform layer, coexisting with neurofilaments in this neuron.

The localization of calcium binding sites in eyes was determined autoradiographically after extracting endogenous Ca from tissue sections and replacing it with 45Ca.The strongest labeling was associated with pigmented tissues due to the high concentration of melanin, which was shown to bind Ca effectively and in a pH-dependent fashion. The second strongest binding was over the tapetum lucidum of the cat eye, and moderate labeling was associated with eye muscles and epithelium and endothelium of the cornea. The neural retina was generally more lightly labeled than the surrounding tissue of the eye; here the plexiform layers stood out in comparison to the nuclear layers, as did a band located internal to the photoreceptor outer segments. The possibility that the Ca buffering capacity of melanin may represent the common denominator for the various neurological defects found in hypopigmentation mutants is discussed.Hypopigmentation mutants in mammals have two types of visual abnormalities that cannot be explained by insufficient light screening but point to another, unidentified function of pigment. The first abnormality is due to miswiring of optic connections; it includes the extensively studied defect in optic nerve crossing (1, 2) and probably also the eye movement defects (3-5). The second type is a dynamic defect that depends on direct contact of the neural retina with the retinal pigment epithelium (RPE): in optic nerve recordings from intact pearl mice, a hypopigmentation mutant not allelic with the albino, visual thresholds were elevated about 100 times at dim backgrounds, as compared to fully pigmented mice, but were about normal at bright ambient illumination (6); when recordings were done from ganglion cells in isolated retinas, however, visual thresholds of normal and pearl mice were indistinguishable (7). Defects in optic nerve crossing and eye movements are caused by mutations at any of several genes involved in pigmentation; severity of defects correlates roughly with the degree of impairment in pigmentation of the RPE and not with pigment in any other tissues (8)(9)(10). This is consistent with the notion that the defects are expressed in the embryo, because the RPE becomes pigmented at early stages of eye development, long before any other pigment appears in the organism; the choroid of the eye, for instance, assumes pigment only several days after birth in mice, at a time when optic connections are for the most part well established. It is not known with which pigment, if any, the light sensitivity defect may correlate, as it has only been described in pearl mice; a similar defect does seem to be present in albino mice (unpublished observations) and it may also be reflected in visual abnormalities found in albinos of several species, including humans, which cannot be explained by light scatter or aberrant crossing (11, 12). METHODS AND RESULTSMice were perfused with paraformaldehyde and (in some cases) glutaraldehyde, and their eyes were dissected as eye cups, cut on a cryostat at 10 ,m, e...

Mice of the mutant strain pearl (pe/pe) differ from the wild strain by a single gene mutation, which leads to a lightening of the coat color. We tested this strain to see if this mutant gene also expressed itself in one or more visual abnormalities. Pearl mice were found to lack totally the optokinetic nystagmus reflex that was present in every normal mouse that we examined. This lack of optokinetic nystagmus was not due to oculomotor defects, since postrotatory nystagmus was normal. As described for other pigmentation mutants, we found that pearl mutants had a reduced ipsilateral projection to the lateral geniculate nucleus, superior colliculus, and visual cortex. We recorded from single cells in the superior colliculus and found response properties and light sensitivities to be normal over the luminance range at which optokinetic nystagmus was tested. However, at very dim backgrounds (scotopic levels), the incremental sensitivities of these cells in pearl mice were about 100 times lower than those of normal mice. This reduction in sensitivity was restricted to scotopic backgrounds and was not due to abnormalities in either the time course of dark adaptation or the receptive field sizes of single cells. In recordings of the electroretinographic response, both the waveforms and the normalized magnitudes of the A and B waves of pearl were indistinguishable from those of normal mice, which seems to indicate that the cause of pearl's sensitivity defect is located central to the main electrical events in the photoreceptors. The normality of many aspects of the visual system of pearl mice contrasts sharply with the complete absence of optokinetic nystagmus, with the reduced ipsilateral projection, and with the reduced dark sensitivity of the cells in the superior colliculus.