Fig. 1. Expression and activity of EphA receptors in the E10 chick IO. (A-C) Whole-mount RNA in situ hybridization of the brainstem. Ventral views are shown. Rostral is at the top. Arrowheads indicate expression in the IO. (D-O) Coronal sections through the IO subjected to RNA in situ hybridization (D-F, H-J, L-N) or affinity probe in situ (G,K,O). Dorsal is at the top. EphA3 is expressed in a narrow area of the medial IO, whereas EphA5 and EphA6 show broader distribution. White and black arrowheads in I indicate areas Mm and Ir, respectively, which show strong EphA5 expression. Total receptor activity detected with ephrin-A2-AP corresponds to the RNA expression patterns. Scale bars: 500 µm. (P-Q) Areas defined by EphA receptor expression are schematized in a ventral view of the brainstem (P) and serial coronal sections (Q, left side). Lamellar organization of the IO is shown on the right side of Q.