Control of neurite extension by embryonic heart explants

The dynamics of neurite outgrowth elicited by embryonic chick heart explants in sympathetic, spinal, ciliary and Remak's ganglia were investigated in collagen gel cocultures. Neurites emerged preferentially on the side facing the heart explants even after only 6 h and continued to increase in density and length for the next 2 days. Removal of the heart explants after only initial stimulation resulted in less-dense neurite outgrowth. Washing of such cultures led to retraction or degeneration of neurites, effects which could be countered by again adding heart explants. Addition of a second set of heart explants on the back of ganglia initiated a second wave of neuritic outgrowth locally. Ganglia extracted from gels separate from their fibre halos and transferred to a second gel did not regenerate neurites unless again stimulated by heart explants. Neurites from additional, distally positioned ganglia failed to advance into parts of the gel shadowed from the heart explants by proximal ganglia. The asymmetry of neurite outgrowths may be explained by local chemokinetic stimulation of extension, possibly in combination with chemotactic orientation of fibre tips up concentration gradients. The results show that the extension of several categories of ganglionic neurites was reversible, being controlled by the concentration of a soluble neuronotrophic factor released from a developing end organ.