In situ hybridization showing expression of chordin during median fin development in zebrafish. Arrows mark fin rays (except in A,B), arrowheads mark radials, and hypurals are numbered. Black patches are melanophores. Anterior is to the left and dorsal to the top, except in A and B, which are dorsal views (after Miller-Bertoglio et al.,[1997]). A: Strong chordin expression in future dorsal cells (arrow) during the shield stage (Miller-Bertoglio et al.,[1997]) serves as a positive control (∼6 hours postfertilization [hpf]). B,C: Sense probe does not detect chordin early (∼ 6 hpf, B) or late (5.6 mm, C), but does provide relative levels of background staining. D: The chondrocytes of the forming radials (arrowheads) do not express chordin, but the gene can be detected in the mesenchyme between the radials. Expression is not strong, but the stain is distinctly above background levels (5.6 mm). E: As segmentation progresses and fin rays are formed, expression of chordin is observed in the fin rays (arrows), and in the zone of segmentation (ZS; distal radials not present in this plane of section, but note expression at arrowhead tips, which point at the distal edges of the proximal radials) (5.6 mm). F: Expression of chordin in the ZS (asterisks) is clearly visible (5.8 mm). G: Mesenchyme distal to the hypurals expresses chordin (see hypural 5), although not as strongly as we have observed for other genes (6.2 mm). H: The highest levels of chordin expression are detected in the mesenchyme (∧) between the forming posterior neural (shown) and hemal (not shown) arches. Note absence of expression in the arch cartilages (cells to the right of the patches of expression) themselves. Scale bars = 0.1 mm in A,B, 0.05 mm in C,H.
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