Origin of retinal astrocytes in the rat: evidence of migration from the optic nerve.

To test recent ideas on the origin of retinal astrocytes, we have studied the spread of astrocytes in the developing retina of the albino rat. Astrocytes were identified with antibodies to their intermediate filaments, glial fibrillary acidic protein (GFAP). Astrocytes were first detected at E(embryonic day) 18, forming a corona of processes around the optic disc. Over subsequent days, astrocytes extended over the retina, covering approximately 35% of the retina at birth (typically E21-22) and reaching the edge of the retina by P(postnatal day)8. As they spread, astrocytes were closely associated with the developing vasculature, spreading ahead of patent vessels by a small but distinct margin. The most peripheral astrocytes assumed a bipolar morphology and extended processes towards the margin of the retina. Astrocytes nearer the optic disc showed the stellate shape characteristic of mature cells. The appearance of astrocytes at the optic disc at E18, 2 days after the appearance of type-1 astrocytes in the optic nerve (Miller et al.: Dev. Biol. 111:35-41, '85), suggests that retinal astrocytes may be type-1 astrocytes generated in the optic nerve. Watanabe and Raff (Nature 332:834-837, '88) have recently reported an independent study supporting the same conclusions.