Fig. 7.

Scheme illustrating the molecules that constitute the kidney morphogenetic field in a stage 8 chick embryo. The overlap between three different morphogen gradients determines the domain of kidney gene expression. Red illustrates the lateral-medial gradient of BMP signaling, yellow the mediolateral gradient of activin-like signaling, and blue the posterior-anterior gradient of RA that generates the expression domain of Hoxb4 (dark-blue domain on the left side, arrowhead). The kidney morphogenetic field (dotted line, arrow) is established in the overlapping domain of BMP and activin-like signaling and Hoxb4 expression (dashed line on the right).