Fig. 1

(A, B) Overview of the median fin fold in a 42 hpf zebrafish embryo from the lateral view (A) and higher magnification of anterior trunk region (B). The continuous midline fin fold (mff, median fin fold (MFF)) surrounds the tail and trunk region from the cloaca (arrow in panel A) to the rostral edge at the 8th somite level (arrow in panel B). paff, pre-anal fin fold; y, yolk; ye, yolk extension. Numbers indicate the somite number. (C–H, a–l) Histological analysis of median fin fold. (C–H) 15–24 hpf zebrafish embryos from the lateral view. (a–l) HE staining of transverse sections at the level of each bar in panels C–H. At 15 hpf (C, a) and 16 hpf (D, b), the presumptive fin fold epidermis (indicated by the black broken lines in a and b that are under the epidermal cell layer) was still connected to the neural ectoderm in the tail bud. (E, c) Epidermis covered the midline of the tail and trunk at 17 hpf (indicated by the black broken lines in c). ep, epidermis; nc, notochord; nt, neural tube; pe, peridermis; pnt, prospective neural tube; so, somite. (F) At 18 hpf, wedge-shaped epidermal cells were distinguishable at the midline of the tail bud (e, f; indicated by brackets), but these cells were not observed at a more rostral region (d). (G, H) Then the area in which wedge-shaped cells were seen (h, i for 21 hpf, and k, l for 25 hpf; indicated by brackets) expanded rostrally and caudally as the tail elongated. The dots indicate the final position of the anterior boundary of the fin fold (8th somite level). (j2, f2) High magnifications of j and f. Typical cuboidal-shaped epidermal cells and wedge-shaped epidermal cells indicated by the blue (in j2) and red (in f2) broken lines, respectively. Scale bars in panels A–C, F, and (a) are 500 μm, 100 μm, 500 μm, 200 μm, and 50 μm, respectively. The panel width of (d–l) is 100 μm.