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Re: Embryological evidence that bird fingers are I, II, III



From the Discussion:

"The finding that the chick wing polarizing region only gives rise to soft tissues along the posterior margin of the most-posterior digit, but not the cartilage, is strong evidence that the bird wing digits are 1, 2 and 3, and not 2, 3 and 4. It is striking that the contribution the mouse forelimb polarizing region makes to digit 3 is also predominantly to soft tissues along its posterior margin^8. Therefore, because a primary axis of cartilage condensation running through the digit 4 position is not conserved in all vertebrate limbs, the primary axis cannot be used to assign digit identity, thus eliminating the requirement of a digit frame-shift in bird wing evolution (Fig. 1d,e). Our results support the fossil record, indicating that birds evolved from theropod dinosaurs that lost digits 4 and 5 (Fig. 1a) and is consistent with the overlooked 'axis-shift' hypothesis that proposes the primary axis runs through the digit 3 position in bird wings^19–21 (Figs 1f and g). We speculate that the inability of the chick wing polarizing region to form a digit in the wing, but not in the leg, is associated with cell death that is higher along the posterior margin of the wing bud compared with the leg bud^22. This cell death could account for the progressive decrease in the width of the stripe of cells derived from the chick wing polarizing region going from proximal to distal (Fig. 2d). We further speculate that this pattern of cell death evolved around 200 million years ago and eliminated posterior digits of the theropod hand/bird wing (Fig. 1a)."

"Our findings lead us to suggest that the digit 4 position in all vertebrate limbs is within the polarizing region, and this may be because three is the maximum number of different digits that can be specified by a concentration gradient of paracrine Shh signalling. This implies that additional digit types will always be derived from the polarizing region and specified by autocrine Shh signalling."