ONTOGENIC CHANGES IN THE REGENERATIVE ABILITY OF CHICK RETINAL GANGLION-CELLS AS REVEALED BY ORGAN EXPLANTS

Whereas mature neurons in the central nervous system (CNS) of birds lack the capability of regenerating axons after injury, embryonic nerve cells are able to do so. The time course of this decline of regenerative ability was investigated in ganglion cells from embryonic chick retinae. Retinal strips from 7- to 19-day-old embryos (E7-E19) were explanted and cultured in vitro. The numbers of retinal ganglion cell (RGC) neurites that had extended during the first 22-23 h incubation, their elongation rates, and morphometric parameters of the growth cones were measured to characterize the regenerative behavior. We observed two periods of decline in neuritic growth: the first from E7 to E9, and another from E14 to E19. The first decrease may reflect a gradually disappearing portion of neurons which produced their axons de novo. The second decline coincides with the major period of synaptogenesis by ganglion cell axons in ovo. The time required for initiation of axonal outgrowth increased, accordingly, from less than 3 h in explants from younger retinae (E7-E16) to 10-12 h for E17 and E19 explants. Axonal elongation rates ranged between 36-mu-m/h and 56-mu-m/h (mean values) for E7-E13 explants, but were significantly lower for cells from E14-E19 retinae (13-21-mu-m/h). Morphologically, neurites and growth cones for RGC explanted before E17 were characterized by their high variability. They possessed more filopodia than mature neurons (E17, E19), fasciculated to a higher degree and branched more frequently. In addition, older neurites appeared "stiffer" and were morphologically simpler. We propose that, during the embryogenesis of the chick retinofugal system, the changes that render mature ganglion cells incapable of axonal regeneration are mainly triggered in the period of establishing synaptic contacts in the optic tectum.