Changes in the mitochondria in the initial part of the axon during regeneration.

The initial part of the axon including the axon hellock, the initial unmyelimated segment and the beginning of myelinated axon was studied electron microscopically during regeneration, 1--30 days following a crush lesion of the rat hypoglossal nerve. Large mitochondria reaching 1.1 mum in diameter, with abundant cristae and dense granules in the matrix were observed between days 3--21. They formed clusters in the initial myelinated segment of the axon. End-to-end contacts and ribosomes around them were very often visible. The large mitochondria exhibited strong succinate dehydrogenase and NAD - H2 diaphorase activities. The relationship between the appearance of large and active mitochondria in the initial part of the axon and the elevated axonal transport during regeneration of the peripheral nerve is also discussed.