Fig. 1. tsh expression in the eye disc starts during L2 and is restricted to the columnar ME. In all images, anterior is towards the left. (A) Dorsal view of a late ey-Z embryo. The eye disc primordium, visualized with an ey-Z reporter (anti-ß-galactosidase, red) does not express tsh (anti-Tsh, green). (B-E) vertical optical sections of L1 (B), L2 (C) and early L3 (D,E) eye discs; the PE is marked by an arrow. (D,E) Region where the retina is already differentiating is marked by the curved line. Merged images and separate channels are shown. Ey (anti-Ey, red) is expressed in both eye disc layers during the whole development of the disc (B,C,E). tsh, which is monitored by a tsh-Z reporter (anti-ß-galactosidase, green), is not expressed in L1 discs (B), but becomes upregulated in L2 discs in the columnar layer (C; arrowhead) and maintained there in early L3 discs (E). (D) Tsh expression overlaps Eya in the ME (anti-Eya, red). (F,G) Confocal images through the ME (F) or PE (G) layers of early L3 discs, stained for Hth (blue), Tsh (green) and Eya (red). (F) tsh is expressed only in the main epithelium (F), where it is co-expressed with Hth in an anterior subdomain (cyan) and with Eya in a posterior one (yellow-orange). In the scheme on the right, the domain of tsh expression is outlined in green, and the anterior and posterior subdomains are colored in cyan and orange, respectively. The approximate domains of wg and dpp expression at this stage are depicted as the blue and red bars, respectively. (G) In the PE of a similar stage disc, neither tsh nor eya is expressed. This is represented in the scheme on the right. (H) Idealized vertical cross-section through the line in F and G. The color codes as in F and G. pe, peripodial epithelium; me, main epithelium. (I) tsh-expressing clones in the disc margin or PE induce the columnarization of the cells. Tangential confocal section through a disc, showing a tsh+ clone (marked with anti-Tsh, green) in the dorsal PE. The membranes are labeled with Arm (red). tsh+ cells overproliferate and exhibit a columnar morphology, contrasting with the more squamous morphology of neighboring, Tsh non-expressing, cells. The broken line in the merged image delineates the tissue.