Developmental regulation of chick embryo retina neurite extension by extrinsic factors.

Developmental changes in the control of neurite extension by extracellular factors can be examined by utilizing cultures of neurons from various aged embryos. Several conditioned media and tissue extracts were added to cultures of chick embryo retinal neurons on collagen substrates for 1, 3 and 5 days in vitro. Neurite outgrowth, measured as the percentage of neurons with neurites and the length of neurites, was promoted by optic tectal extract and cornea conditioned medium in retina neurons from younger ages (6- to 12-day embryos), but not from older ages (14- and 16-day embryos). The promotion of neurite outgrowth by optic tectal extracts may be mediated by a promotion of glial cell growth. The developmental changes in neurite extension may be due to either an altered sensitivity to the neurite promoting factors or by an altered intrinsic ability of retinal neurons to extend processes.