Figure 1.

Three models of mass addition during axonal elongation. The vertical line denotes the initial position of the distal tip. (A) Diagram of an axon before elongation. (B) Elongation by tip growth. The shaded area denotes the location of the addition of new mass. (C) Elongation by stretching in the presence of adhesions. Forces generated at the growth cone are dissipated because of interactions with the substrate. The force gradient results in nonuniform thinning of the axon. The unshaded area distal to the vertical line shows the portion of the axon that has been pulled forward by stretching. (D) Towed growth and extreme stretch growth. Axial tension is the same at all points, so that thinning and mass addition occur uniformly along the axon.