Induction of shh during regeneration of the caudal fin. shh expression was determined (A,E) 2 days; (B,D,F-I) 4 days, (C) 12 days after amputation. Two fin rays are shown in A and D, three fin rays in B and branches of three fin rays are shown in C. Shh is confined to one or two subsets of cells in individual fin rays on each side of the fin (BD). Shh is expressed in only one subset of cells as soon as a bifurcation is formed (C) or in the lateral-most fin rays of a caudal fin which never form branches (marked with an asterisk in B). (D) The distal part of the fin is slightly tilted; the open triangles and the filled triangles indicate the two subsets of shh-expressing cells on each side of the ray. (E-G) Proximodistal and (H,I) transverse sections of regenerating caudal fins, 2 days (E) and 4 days (F-I) after amputation. The planes of sections are shown in B. (G) Higher magnification of a section comparable to that shown in F with shh-expressing cells delimited by brackets. The scleroblasts can be distinguished from the rest of the blastemal cells by their location and by their more condensed appearance. The region containing shh-expressing cells seems to overlap the limit where lepidotrichial material can be observed in the epidermal-blastemal interface. (H) shh is expressed in a unique broad domain when no bifurcation is forming. (I) shh expression in two subsets of cells on one side of the fin ray precedes the formation of a bifurcation of the lepidotrichia. The large arrows in A, B and E indicate the level of amputation. P (in B) indicates the proximal direction; b, blastema; l, lepidotrichia; e, epidermis; s, scleroblasts. Scale bars, A-D, 80 μm; E,F,H,I, 30 μm; G, 4 μm.
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