Stages in axon formation: observations of growth of Aplysia axons in culture using video-enhanced contrast-differential interference contrast microscopy

The regenerative growth in culture of the axons of two giant identified neurons from the central nervous system of Aplysia californica was observed using video-enhanced contrast-differential interference contrast microscopy. This technique allowed the visualization in living cells of the membranous organelles of the growth cone. Elongation of axonal branches always occurred through the same sequence of events: A flat organelle-free veil protruded from the front of the growth cone, gradually filled with vesicles that entered by fast axonal transport and Brownian motion from the main body of the growth cone, became more voluminous and engorged with organelles (vesicles, mitochondria, and one or two large, irregular, refractile bodies), and, finally, assumed the cylindrical shape of the axon branch with the organelles predominantly moving by bidirectional fast axonal transport. The veil is thus the nascent axon. Because veils appear to be initially free of membranous organelles, addition of membrane to the plasmalemma by exocytosis is likely to occur in the main body of the growth cone rather than at the leading edge. Veils almost always formed with filopodial borders, protruding between either fully extended or growing filopodia. Therefore, one function of the filopodia is to direct elongation by demarcating the pathway along which axolemma flows. Models of axon growth in which the body of the growth cone is pulled forward, or in which advance of the leading edge is achieved by filopodial shortening or contraction against an adhesion to the substrate, are inconsistent with our observations. We suggest that, during the elongation phase of growth, filopodia may act as structural supports.