Reconnection of axotomized axon fragments is promoted by Ca²⁺ and cAMP and involves EFF-1. A, Types of reconnection or contact observed between regrowing axons and distal fragments (examples from pde-4(lf) zdIs5 background; confocal projections). Type 1 contact, Proximal and distal stumps reconnect without formation of a growth cone; Type 2 contact, the proximal stump forms a new growth cone that extends before turning (2a) or branching (2b) to fuse collaterally with the distal fragment. The dotted red line indicates the region of an EM section in C. Scale bar, 10 μm. B, Type 2 reconnection occurs in <5% of zdIs5 control animals and is elevated in egl-19(gf), pde-4(lf), and kin-2(lf) mutants (Fisher's exact test; number of axons are shown in bars). Reconnection occurs in ∼25% of control muIs32 animals, is slightly elevated in pde-4(lf), and is significantly inhibited by touch neuron-specific expression of PDE-4 (Pmec-4-PDE-4). C, Electron micrographs of the junction of the distal fragment (blue shading) and regrowing axon (yellow) from the animal shown in A; the inset shows continuity of cell membranes at fusion (arrow). Scale bar, 100 nm (inset). D, Type 2 contacts in muIs32 background are reduced in animals lacking eff-1 and are unaffected by loss of function in aff-1 or nsf-1. *p < 0.05, ***p < 0.01, Fisher's exact test. ns, Not significant.