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Fig. 11 Sagittal sections showing expression of the type II collagen gene in the median fins of zebrafish. In all panels, anterior is to the left and dorsal to the top. Black patches are melanophores. A: Early expression in the anal fin field (5.1 mm). Both prechondrogenic mesenchyme and radial chondrocytes (arrowheads) express type II collagen. B: Type II collagen is expressed throughout the fin field of both dorsal and anal fins (5.7 mm). Stain of mature chondrocytes in the radials is turquoise in color while interradial mesenchyme stains purple (this is loosely correlated to relative gene expression levels, with higher levels appearing as turquoise; R. Harland, personal communication). C: After radial segmentation is complete, type II collagen expression remains in the distal radials and the bands of cartilage that remain unossified in the distal portion of the proximal radials (arrowheads; 12.2 mm). This pattern is observed in the dorsal fin as well (data not shown). A few cells in the most proximal portion of the proximal radials (top arrowheads) also express type II collagen. D: Strong expression in mesenchymal condensations of the parhypural (p) and hypural one (1; 4.0 mm NL). E: Stain of mature chondrocytes in the hypurals is turquoise in color while inter-hypural mesenchyme stains purple (5.7 mm). A developing hemal arch expresses type II collagen as well (∧). Fin rays do not express the gene (arrows). F: Type II collagen expression becomes increasingly distally restricted as the hypurals continue to grow to adult size (11.3 mm). G: Sense probe does not exhibit any staining. Scale bars = 0.1 mm in A,D,G, 0.5 mm in B,C.