Fig. 6. Nerve fiber outgrowth and innervation of the myotome. Explants were taken from HH 14 (A-C) and HH 23 (D) embryos, and stained with antibodies that identify neurons (red) and desmin (C,D) (green). (A) After 4 hours of incubation, no nerve fiber outgrowth was detected in explants of a HH 14 embryo. (B) Nerve fibers can be seen to extend laterally from the neural tube of explants of HH 14 embryos after 18 hours of incubation. (C) When explants were stained for both desmin and neurofilament protein, it became apparent that nerve fibers were extending to innervate the developing myotomal regions of the somite. (D) In an explant taken of the wing-level somites of a HH 23 embryo and incubated for 18 hours, it can be seen that mature nerve fibers enter the myotomal regions of the somite at two discrete sites (arrows).