Fig. 3. Mesenchymal cells derived from ventral somite contribute to vertebrae. Cells of the ventromedial cell cluster of each somite migrate dorsally and spread along the AP axis (Morin-Kensicki and Eisen, 1997). (A) Shown is a single cell (arrow) labeled at 18 h and the distribution of its progeny (arrows) at 24 h (B) and 48 h (C). Arrowheads mark myotomal boundaries. (D,E) Serving as an early marker for vertebral column components, -coll2a1 is localized to anterior neural arches (arrows) by 10 d in the side view in D (compare with Fig. 2C) and in transverse section (E). (F,G) Progeny derived from a single, labeled ventromedial cluster cell of an anterior trunk somite contributed to vertebral components when visualized at 10 d. (F) Cells visualized with an antibody to fluorescein;. The arrow indicates a cell that is incorporated into a neural arch and the arrowheads indicate cells in other regions of the developing vertebra. (G) The same section as in F labeled with Xylenol Orange to reveal developing bone; the cells indicated in F are also positive for this bone marker. Asterisk in E marks pigment cells. n, notochord; nt, neural tube. Scale bar: 10 µm in A; 15 µm in B,D,F,G; 20 µm in C; 35 µm in E.