The Axon: A Prototype for Studying Expressional Cytoplasm

Neurons extend their cytoplasm over long distances away from the cell body into processes called axons; however, only a special fraction of the neuronal cytoplasm is extended into these processes. In particular, the protein-synthesis machinery of the neuron does not enter the axon. Thus, the neuron illustrates a natural division of the cytoplasm into two regions or compartments. One region can be called translational cytoplasm, because it contains the polysomes and is responsible for protein synthesis (translation). The other region can be called expressional cytoplasm, because it contains the expressed products of transcription and translation. This division of neuronal cytoplasm into two major compartments arises from the selective affinity of the elements that comprise each of these regions for nondiffusing structures located exclusively in these regions. This selective-affinity compartmentation can be distinguished from the more traditional view that cytoplasmic compartmentation is due principally to lipid bilayers which provide an actual...