Factors determining the migration of astrocytes into the developing retina: migration does not depend on intact axons or patent vessels.

Astrocytes migrate into the cat retina from the optic nerve, beginning from embryonic day (E) 52. Once they have entered the retina they concentrate along major axon bundles and fail to enter regions of the retina with high densities of neurones, in particular the area centralis region of the ganglion cell layer. These nonuniformities appear as the astrocytes spread over the retina during development, and in this study we have examined factors that might control their spread. First we examined astrocytes in a retina in which the axon bundles had degenerated following an optic nerve lesion at birth. The area over which astrocytes had spread was normal, suggesting that their spread does not depend on the presence of intact axons. Second, we noted that, despite the degeneration of all ganglion cells following the nerve lesion, astrocytes still did not spread over the area centralis. Their spread is apparently not inhibited by concentrations of neurones. Third, we examined astrocytes in retinas of animals raised in an atmosphere containing 70-80% oxygen, which prevents the formation of retinal vessels. Again, the area over which the astrocytes had spread was normal, suggesting that their spread does not depend on the presence of patent blood vessels. These negative findings led us to compare the distribution of spindle cells (precursors of retinal vasculature) and astrocytes in the cat during development. The close correspondence in their topographical distribution and the earlier spread of the spindle cells lead us to suggest that spindle cells provide a basal lamina component that may guide the migration of astrocytes.