Fig. 7. Models illustrate how Tcf3 function shapes gene expression in the neurectoderm. (A) By antagonizing Tcf3-mediated repression, a gradient of Wnt/ß-catenin signaling (blue line) transforms early broad expression of Hdl/Tcf3b (unbroken black line) to a rostrocaudal gradient of effective repression (broken black line). Progressive loss of Hdl/Tcf3b (unbroken gray lines) results in changes in the effective repression gradient (broken gray lines). (B) The gradient of Tcf3-mediated repression (broken black line) alters the efficacy of caudalizing factors that are distributed in a gradient (unbroken purple line) to define a gradient of effective caudalizing activity (broken purple line). Discrete windows of effective caudalizing activity regulate the expression of genes that define blue, green, yellow and red compartments. Moderate (C) to severe (D) reduction in Tcf3 function alters the gradient of effective caudalizing activity and expands caudal domains at the cost of rostral domains. (E) A ventrolateral source of Wnts and other caudalizing factors establishes a caudalizing activity gradient with its low end just dorsal to the animal pole. Darker shades of purple represents lower levels of caudalizing activity. (F) A gradient of BMP activity [black (highest) to white (lowest)] is established by the shield (orange) and determines the neurectoderm (broken line). (G,H) In the neurectoderm the gradient of caudalizing activity is interpreted to define discrete neural compartments (blue, green, yellow and red). (I-L) Illustrations of how gene expression domains would be altered with progressively higher effective caudalizing activity. AP, animal pole; V, ventral.