Organotypic cultures of neural retina: neurite outgrowth stimulated by brain extracts.

A bioassay for the growth of retinal neurites was designed. Circular plugs 0.5-1.5 mm in diameter were excised from the chick neural retina at embryonic day 6 and cultured as organotypic explants on a collagen gel. After addition of medium or various tissue extracts cultures were incubated and the length of extending neurites measured. In control medium with 10% serum, neurites were sparse and reached less than 0.2 mm after 4 days. Extract of the optic lobe from 18-day-old chick embryos distinctly increased the density and length of neurites in a dose-dependent manner reaching 1.2 mm after 4 days. A unit of outgrowth activity corresponding to half the maximum length of fibers was defined. The activity of the optic lobe extracts was retained in fractions with nominal molecular weights over 100,000 daltons upon pressure dialysis. Extract from the forebrain hemispheres had nearly the same effect on retinal neurite growth. Mouse submandibular gland NGF failed to evoke retinal fiber growth. Our data imply that macromolecules of the developing brain can support the growth of optic axons.