Macromolecular structure of axonal membrane in the optic nerve of the jimpy mouse.

The macromolecular structure of the axon membrane in 26-28-day-old Jimpy mice and control optic nerve were examined with quantitative freeze-fracture electron microscopy. Premyelinated and myelinated axons were observed in control optic nerves, with axonal diameters of premyelinated axons being generally smaller than that of myelinated axons (approximately 0.2-0.4 micron vs approximately 0.5-1.5 micron, respectively). Axon membrane from control optic nerves exhibited an asymmetrical partitioning of intramembranous particles (IMP). P-faces of internodal membrane displayed nearly twice as many IMP as the premyelinated axolemma (1,731 vs 893 micron-2, respectively). E-faces of internodal and premyelinated axolemma exhibited IMP densities of 124 and 157 micron-2, respectively. Few myelinated axons were apparent in optic nerves from Jimpy mice. The amyelinated axons of Jimpy mice displayed a spectrum of axonal diameters, ranging from approximately 0.2 to 1.5 micron. P-face densities of amyelinated axons, considered as a group, exhibited a wide range (600-2,100 micron-2). However, large diameter (greater than or equal to 0.5 micron) axons exhibited a significantly greater P-face IMP density than that of small caliber (greater than 0.5 micron) axons (1,525 vs 1,032 micron-2, respectively). Aggregations of E-face IMP were not observed along amyelinated axons of Jimpy optic nerves. The results demonstrate that the changes in P-face IMP density that occur during development of normal myelinated axons also occur in developing axons of Jimpy optic nerve, irrespective of a lack of normal glial cell association, and provide further evidence that the primary defect of hypomyelination within Jimpy mice is not attributed to the neuron.