Cytoplasmic Structure in Rapid-frozen Axons

Turtle optic nerves were rapid-frozen from the living state, fractured, etched, and rotary shadowed. Stereo views of fractured axons show that axoplasm consists of three types of longitudinally oriented domains. One type consists of neurofilament bundles in which individual filaments are interconnected by a cross-bridging network. Contiguous to neurofilament domains are domains containing microtubules suspended in a loose, granular matrix. A third domain is confined to a zone, 80-100 nm wide, next to the axonal membrane and consists of a dense filamentous network connecting the longitudinal elements of the axonal cytoskeleton to particles on the inner surface of the axolemma. Three classes of membrane-limited organelles are distinguished: axoplasmic reticulum, mitochondria, and discrete vesicular organelles. The vesicular organelles must include lysosomes, multivesicular bodies, and vesicles which are retrogradely transported in axons, though some vesicular organelles may be components of the axoplasmic reticulum. Organelles in each class have a characteristic relationship to the axonal cytoskeleton. The axoplasmic reticulum enters all three domains of axoplasm, but mitochondria and vesicular organelles are excluded from the neurofilament bundles, a distribution confirmed in thin sections of cryoembedded axons. Vesicular organelles differ from mitochondria in at least three ways with respect to their relationships to adjacent axoplasm: (a) one, or sometimes both, of their ends are associated with a gap in the surrounding granular axoplasm; (b) an appendage is typically associated with one of their ends; and (c) they are not attached or closely apposed to microtubules. Mitochondria, on the other hand, are only rarely associated with gaps in the axoplasm, do not have an appendage, and are virtually always attached to one or more microtubules by an irregular array of side-arms. We propose that the longitudinally oriented microtubule domains are channels within which organelles are transported. We also propose that the granular material in these channels may constitute the myriad enzymes and other nonfibrous components that slowly move down the axon.