Fig. 6. An endogenous growth gradient in the spinal cord. The rates of proliferation and differentiation were determined by BrdU cumulative labeling (1, 2, 3, 4, 5 and 6 hours) and N-tubulin staining for 5 bins across the DV axis at HH stages 11, 14, 17, 21 and 25. (a-e) Representative images from each stage, labeled for 4 hours with BrdU and immunostained for BrdU and N-tubulin. A dorsal to ventral gradient of proliferation and a ventral to dorsal gradient of differentiation are apparent. The rates across space and time are represented by color intensity for (f) proliferation (green) and (g) differentiation (red). (h) The pattern of Wnt signaling determined by medial cyclin D1 expression is shown in blue. (i) The proliferation rate, differentiation rate, and amount of Wnt signaling are merged to show the positive correlation between the proliferation rate and Wnt signaling and the inverse correlation between the differentiation rate and Wnt signaling. There is a growth gradient from early to late and dorsal to ventral that correlates with the pattern of Wnt signaling.