Structural and molecular polarity in retinal photoreceptor neurons: roles for the cytoskeleton.

The differentiation of neuronal cells involves the synthesis of cell-specific macromolecules, as well as the assembly of the latter into supramolecular entities responsible for complex patterns of cell organization. For example, axons are distinguished from dendrites by the presence or absence of ribosomes and by the specific distribution of molecules such as cytoskeletal proteins, a-bungarotoxin receptors and GAP-43, among others (Banker and Waxman, 1988; Goslin et al., 1988). Asymmetries in molecular distribution also occur within individual neuronal processes, as illustrated by the concentration of voltage-sensitive sodium channels at the nodes of Ranvier (Ritchie, 1977; Angelides, et al., 1988). Although all neurons seem to share a similar polarized pattern of organization, different types of neurons can be distinguished by unique variations of this common pattern. Cell-specific morphology and branching patterns of axons and dendrites provide clear examples of this type of diversity (i.e., Wuerker and Kirkpatrick, 1972; Peters et al., 1976; Bartlett and Banker, 1984).