Expression of an intrinsic growth strategy by mammalian retinal neurons

Postnatal cat retinal ganglion cells (RGCs) were retrogradely labeled with fluorescent microspheres, dissociated from the retina using a peeling procedure, and monitored in cell culture with a time-lapse video microscopy system. The spatial patterns formed by the growing neurites were analyzed using conventional and fractal measures (Hausdorff dimension, H) of their extent and complexity. The results presented were obtained from the arborizations formed by the neurites of 48 labeled and isolated ganglion cells growing separate from each other and separate from a feeder layer of astrocytes. Cells were obtained from animals when the RGCs were postmitotic and after dendritic differentiation in vivo at age 0-1 week (4/48), 2-5 weeks (35/48), or 6-8 weeks (9/48). By 48 hr after plating, the number of surviving labeled RGCs was reduced to 22-28% of its initial value. After removal of all processes and isolation in vitro, these RGCs expressed neurite patterns strikingly similar to those seen in the intact retina, although the RGCs had been deprived of potential cues from the intact retina and target tissue. Self crossings of the growing neurites were rare (less than 0.5%, 20 cells, n = 2500 neurites). Calculation of the Hausdorff dimension, a metric for the space-filling capacity of the neurite patterns, revealed that after 3-day culture 77% (n = 56) of the RGCs achieved relatively uniform coverage of territory (1.6 less than H less than 1.9). This coverage was independent of the number of interbranchpoint segments and/or the total neurite length of a particular neurite pattern. A sample of dendritic arbors from RGCs in intact retina yielded similar values for the Hausdorff dimension (H = 1.73, SD = 0.12, n = 18, range 1.54-1.94). These results reveal that a mammalian central nervous system neuron, for at least 8 postnatal weeks, has the intrinsic capacity for reexpression of in vivo structure characteristic of that cell type in the absence of interaction with neighboring neurons, afferent input, and target tissue. These neurons exhibit stereotyped growth resulting in uniform coverage of a restricted territory by the strategic selection of the length, location, and orientation of interbranchpoint segments.