Size‐dependent distribution of axoplasm, schwann cell cytoplasm, and mitochondria in the peripheral nerve fibers of mouse

Electron micrographs of vagus and sciatic nerve fibers of different size from adult male mice were examined by planimetry to determine the distribution of axoplasm, Schwann cell cytoplasm and mitochondrial density. The findings revealed that the ratio of axoplasm to Schwann cell cytoplasm in normal mature fibers increased with fiber size. The increase appeared curvilinear from 61% axoplasm in nonmyelinated fibers to 90% in the thickest myelinated fibers. Schwann cell cytoplasm extended along the sheaths in longitudinal and interconnected strands. The number of strands varied between 2 and 5, depending on fiber size. An analysis of mitochondrial density in axoplasm and Schwann cell cytoplasm of nonmyelinated and myelinated fibers revealed a greater concentration of mitochondria in the Schwann cells with increasing fiber size. The mean ratio of mitochondrial density between axoplasm and Schwann cell cytoplasm was 1:0.24 for nonmyelinated fibers, 1:2.54 for fibers with less than 30 lamellae and 1:5.21 for fibers with 30 or more lamellae. The distribution of mitochondria observed in electron micrographs was confirmed histochemically by localization of NAD‐diaphorase activity.