Structures in the embryonic vertebrate tail. (A) Three-dimensional (3-D) reconstruction of an extending vertebrate embryo tail. Axial structures include the NT and Nc; lateral to these are the paraxial somites and PSM. Somites are the embryonic precursors to skeletal muscle, ribs, and bony vertebrae; motor and interneurons are derived from the NT; the CNH is the remnant of Hensen's node and contains pluripotent cells; the PSM is the source of cells from which somites arise; and mesenchyme cells (M) at the distal tip of the tail feed into the CNH. Not shown: neural crest and ventral structures. Axis indicates Anterior, A; Posterior, P; Dorsal, D; and Ventral, V. (B) Lateral schematic of tail structures. The axial NT and Nc and paraxial somites and PSM lie dorsal to the TG, which in turn is dorsal to the VER. The VER is the remnant of the Hensen's node and a source of growth-promoting signals. Not shown: neural crest and PSM. (C) Chick embryo tail stage HH23 stained for somites with FITC-phalloidin. Abbreviations: CNH, chordoneural hinge; M, mesenchyme, Nc, notochord; NT, neural tube; PSM, presomitic mesoderm; S, somite; TG, tailgut; VER, ventral ectodermal ridge.