Relation between myofibre retraction and dedifferentiation phenotype. (a-j) During regeneration, "short" myofibres change their shape from elongated (before amputation) to compact (1-4 dpa) and back to elongated (6-12 dpa) (a-e, e corresponds to 12 dpa; n = 94 fibres). This change in shape is also observed in semithin sections of wild-type animals fixed at different days post amputation (f-j, j corresponds to 14 dpa; n ≥ six sections per animal, three animals per time point). At 1 dpa (b, g), "short" myofibres are retracted (b) and the dedifferentiation phenotype is occasionally observed in fibres with similar compact shapes (g, arrows point to disorganized sarcomeres, arrowheads point to normal sarcomeres. See also Figure 3c-e). At 3 dpa (c, h), the dedifferentiation phenotype is more common and is generally present in myofibres with irregular shapes (h, arrow), i.e., the ones that retracted (c). At 6 dpa (d, i), retracted myofibres start to have a more elongated shape (d, boxes correspond to different focal planes) and the dedifferentiation phenotype is now less common (i, arrows), with more myofibres showing normal sarcomeres (i, arrowheads). At 12-14 dpa (e, j), elongated shape is regained (e) and many myofibres show a normal internal structure (j, arrowheads), while few still have some sarcomeric disorganization (j, arrow). (k-n) The "long and thin" myofibres were less prone to retract (compare fibre "b" in k and l; n = 21 fibres), in contrast to "short" myofibres that become even shorter with amputation (compare fibre "c" in k and l). Comparatively, "long and thin" myofibres did not show the dedifferentiation phenotype at 3 dpa (arrowheads in m and n), while short myofibres did (arrows in m and n). Myofibre "a" died with the amputation (was not found at 1 dpa). Nt: notochord. Sk: skin. (o-t) Proximal amputations (done approximately at the middle of the tail) generally resulted in no or little myofibre retraction (o-q; n = 18 animals). Likewise, semithin sections (r-t) revealed that myofibres generally kept the organized internal structure (arrowheads), with some myofibres showing only a mild disorganization of the sarcomeric structure (arrows). Dashed lines: amputation planes.