Dev Growth Differ
December 1, 2017;
Vitamin A induced homeotic hindlimb formation on dorsal and ventral sides of regenerating tissue of amputated tails of Japanese brown frog tadpoles.
When anuran tadpoles are treated with vitamin A after tail amputation, hindlimb-like structures can be generated instead of the lost tail part at the amputation site. This homeotic transformation was initially expected to be a key to understanding the body plan of vertebrates. Unfortunately, homeotic limb formation has been reproduced in only some Indian frog species and a European species, but not in experimental anurans such as Xenopus laevis or Rana catesbeiana. Consequently, this fascinating phenomenon has not been well analyzed, especially at the molecular level. In addition, the initial processes of ectopic limb development are also unclear because morphological changes in the early phases have not been analyzed in detail. In this study, we report the induction of homeotic transformation using Japanese brown frogs and present a detailed morphological analysis. Unexpectedly, the ectopic limbs developed not only at the ventral sites, but also at the dorsal sites of the tail regenerates of vitamin A-treated tadpoles. The relationship between position and axial orientation of ectopic limbs suggested the double duplication of positional value order along the rostral-caudal axis and the dorsal-ventral axis of the tail regenerates.
Dev Growth Differ
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Figure 1. (a, b) A pair of left [white arrowhead in (a) and (b)] and right [white arrow in (a) and (b)] type ectopic limbs on the dorsal side of a tail regenerate in R. ornativentris. Each of the two limbs has an independent knee (red arrowhead). The dorsal side of the tadpole is to the top. The rostral side of the tadpole is to the left in (a) and to the right in (b). (c) Two double posterior limbs in R. ornativentris. The limb-like organs in this panel are regarded as two limbs, and not as four in this study, because there are two knees (red arrowheads). The dorsal side of the tadpole is to the top. The rostral side of the tadpole is to the right. (d) Schematic illustration of the left lateral view of a regenerating tail to show the definition of position and proximodistal direction of ectopic limbs. The dorsal side of the tadpole is to the top. The rostral side of the tadpole is to the left. “O” is the original point of the x–y coordinates where the y-axis corresponds to the amputation line. The dorsoventral boundary of the tail is the horizontal plane containing the apexes of the chevron-shaped myomeres. The central position of the base of an ectopic limb (P) is determined by a relative length of the OP segment (a red arrow) to the length between the dorsal and ventral fin bases on the amputation line and by θa that is formed by the OP and the x-axis. The proximodistal direction of the ectopic limb is defined as θb that is generated by the thigh-foot line (a black arrow) of the limb and the x-axis. Scale bars = 1 mm.
Figure 2. Effect of retinoid treatment on tail regeneration in Xenopus laevis. The rostral side of the tadpole is to the left. The amputation lines are indicated by broken lines. (a) A tail regenerate with partial fin regeneration (brackets) 33 days after amputation without retinoid treatment. (b) A poorly regenerating tail 33 days after amputation following treatment with 1 nmol/L TTNPB for 1 h. Scale bars = 1 mm.