Ganglion cells in the juvenile chick retina and their ability to regenerate axons in vitro.

Ganglion cells in the chicken retina fail to regenerate their axons upon mechanical injury. In order to determine whether this failure to regenerate axons is intrinsic to the neurons or is mediated by the environment, we asked whether ganglion cells possess an ability to regrow their injured axons in the absence of their natural environment, namely in vitro. Since the retina contains morphologically different types of ganglion cells, it became desirable to investigate whether all types of ganglion cells contribute to regeneration of axons. Ganglion cells were labelled post-mortem with the fluorescent dye DiI and described morphologically. Morphometric parameters like the sizes of their perikarya, their dendrites, and the patterns of dendritic ramification and stratification were considered for grouping cells. Although a strong classification of the cells could not be achieved because of the high diversity among this population of neurons, the chick retinal ganglion cells could be separated into seven regular groups which have their somata located within the ganglion cell layer and into one group with the somata located within the inner nuclear layer (displaced ganglion cells). The experimental procedure for regeneration combines crush injury of the chick optic nerve in situ with explanation of retinal pieces 1 week later for organ cultures in a serum-free medium. Under these conditions, the ganglion cells extended axons 1 day after explanation on polylysine/laminin. The densities of ganglion cells contributing to the axonal regrowth reached up to 1447 cells mm-2 (mean 1028 cells mm-2; S.D. 237). This density corresponds to 13% of the ganglion cell density in the normal retina, averaged across the total retina area. Although the dendrites of some cells whose axons had regenerated were altered in comparison with the normal ganglion cells, all morphological types of ganglion cells including those with displaced cell bodies contributed proportionally to the regrowth of axons from the explants.