A newly identified population of presumptive microneurones in the cat retinal ganglion cell layer

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Abstract
A large population of microneurones has recently been discovered in the rabbit retinal ganglion cell layer1,2. These ‘coronate’ cells may represent a class of displaced amacrine cells1,2. Like conventional amacrine cells3 they are swiftly and selectively destroyed4 by low concentrations of kainic acid, a neurotoxin5. No similar cell has been described in the cat retina, which is reported to contain 217–260,000 neurones of classical appearance6. These outnumber the 128–180,000 optic nerve fibres7,8 but it has been suggested that the excess comprise Nissl-staining glial cells9. We report here that, using the neuro-toxic effects of kainic acid to test and confirm the neuronal nature of the classic neurone excess, a large additional population of at least 730, 000 presumptive microneurones was revealed. They resemble rabbit coronate cells, do not project into the optic nerve and have been previously identified as presumed glia6,10,11. Various lines of evidence for the neuronal nature of these cells is presented below, but synapses have not been demonstrated; subsequent reference to microneurones must therefore be regarded as presumptive.