Axonal branch shapes.

To distinguish the relative roles of the intrinsic and the extrinsic determinants of axonal branching shapes, a number of key branch parameters were measured under a variety of conditions. Branch shapes of frog and of chick axons were analyzed in tissue culture, and these in vitro patterns were compared with in vivo branch patterns of axons in tadpole tail fins. The shape of a branch junction can be characterized by the sizes of its branch angles. In all cases, branch junctions had only two branches (3 branch angles), and the bifurcation angle between them was usually the smallest. The shape of the branch junction was constant in a wide variety of environments, but the exact branch angles, as well as the numbers of branches per axon and the numbers of axons per neuron, could be modulated by changes in the substrate adhesivity.