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It has been known since the 1930s that the dorsal fin is induced by the underlying neural crest. The inducer of the ventralfin, however, has remained elusive. We have investigated the source of the inducer of the ventralfin in Xenopus and show that it is the ventral mesoderm and not the neural crest. This induction takes place during mid-neurula stages and is completed by late neurulation. In terms of cell composition, the dorsal finmesenchyme core arises from neural crest cells, while the mesenchyme of the ventralfin has a dual origin. The ventralfin contains neural crest cells that migrate in from the dorsal side of the embryo, but a contribution is also made by cells from the ventral mesoderm.
Fig. 1. Diagram representing the stages of fin induction and formation. A: Stage 25.
B: Stage 28. C: Stage 31. D: Stage 32.
Fig. 2. Role of the neural crest. A: Schematic of neurulaembryo stage 13. Dorsal and side view. Red areas represent the posterior neural crest ablated in C. Green areas represent the posterior dorsal finepidermis ablated in D. Blue areas represent the ventral region. Pink areas represent the lateral region. B: Control embryo stage 41. C: Ablation of the posterior neural crest at stage 13. Embryo cultured to stage 41. Embryo shows loss of the dorsal fin from arrow, except at the very tip. The ventralfin remains unaffected. D: Ablation of the dorsal finepidermis at stage 13. Embryos cultured to stage 41. No fin defect is observed. n 35 for C, n 30 for D.
Fig. 3. Explant cocultures. A: Uninduced epidermis from alongside the neural folds and under the blastopore, extirpated at stage 13 and cultured until controls reached stage 40. See green and blue areas in Figure 2A. B: Lateral, ventral, and dorsal epidermis cocultured as a sandwich with neural folds. See green, blue, and pink areas, plus red area, in Figure 2A. Note the ruffles of fin induced around the explant. C: Lateral, ventral, and dorsal epidermis cocultured with lateralmesoderm and endoderm. See green, blue, and pink areas, plus underlying pink area in Figure 2A. No fin induction is observed. D: Fluorescein dextran amineabeled non-finepidermis grafted at stage 20 above the posterior neural tube and incorporated into the development of a normal dorsal fin by stage 40. Fin is outlined by dashed white line. E,F: Ventraltissue explanted at stage 13 and cultured until controls reached stage 40. See blue area in Figure 2A. Arrow in E points to fin-like structures. n 20 for A and B, n 30 for C and E.
Fig. 4. Location of the ventralfin inducer. A: Stage 41 embryos. A,C,E,G,I,K,M,O: Side views of fins. B,D,F,H,J,L,N,P: Frontal sections through developing tails. A,B: Operations carried out at stage 13. C,M: Operations carried out at stage 15. K,L: Operations carried out at stage 20. A,B: Square of tissueventral to blastopore was ablated, resulting in reduction of the ventralfin. C,D: Similar ablation at stage 15, resulting in complete loss of the ventralfin. E,F: Similar-sized piece of lateraltissue ablated. No effect on fin development. G,H: Ventralepidermis only ablated, resulting in no fin defect. I,J: Ventralmesoderm and endoderm ablated, resulting in defects in the ventralfin. K,L: Ventralmesoderm and endoderm ablated at late neurula stage, showing only limited defect in the ventralfin. M,N: Ventralendoderm alone ablated, resulting in no ventralfin defect. O,P: Ventralmesoderm alone ablated, resulting in ventralfin defects. N 300 for each extirpation.
