Organization of axoplasm in crayfish giant axons

Distributions of subcellular organelles in medial giant axons (MGAs) and segmental lateral giant axons (SLGAs) of crayfish were evaluated as part of an ongoing effort to understand and explain differences in distal stump survival following axonotomy. Both axons were able to endocytose tracer proteins placed extracellularly, and horseradish peroxidase injected by cannulation into MGAs could transfer into adaxonal glial cells. Concentrations of tubulovesicular organelles near axonal cell membranes were measured as a possible index of the relative level of axonal endo- and exocytosis, and concentrations in MGAs were found to be twice those in SLGAs. In both axons, microtubule concentrations were highest near the axolemma and lowest in the central core of axoplasm. In thoracic and abdominal regions of MGAs, microtubules and other organelles were located only in a thin layer of subaxolemmal axoplasm. Overall, MGAs contained fewer microtubules per cross-section than did SLGAs, although MGAs are five to ten times as long as SLGAs and support more synapses. Total numbers of microtubules per cross-section varied with distance from the cell body of an MGA, whereas microtubule numbers were similar in proximal and distal cross-sections of SLGAs. In addition to a layer of subaxolemmal mitochondria which was observed in MGAs and SLGAs and which is characteristic of crayfish axons, mitochondria were also concentrated in the central core of SLGA axoplasm.