The longstanding view of how proliferative outgrowth terminates following the patterning phase of limb development involves the breakdown of reciprocal extrinsic signalling between the distal mesenchyme and the overlying epithelium (e-m signalling). However, by grafting distal mesenchyme cells from late stage chick wing buds to the epithelial environment of younger wing buds, we show that this mechanism is not required. RNA sequencing reveals that distal mesenchyme cells complete proliferative outgrowth by an intrinsic cell cycle timer in the presence of e-m signalling. In this process, e-m signalling is required permissively to allow the intrinsic cell cycle timer to run its course. We provide evidence that a temporal switch from BMP antagonism to BMP signalling controls the intrinsic cell cycle timer during limb outgrowth. Our findings have general implications for other patterning systems in which extrinsic signals and intrinsic timers are integrated.

Summary A fundamental question in biology is how embryonic development is timed between different species. To address this problem, we compared wing development in the quail and the larger chick. We reveal that pattern formation is faster in the quail as determined by the earlier activation of 5′ Hox genes, termination of developmental organizers ( Shh and Fgf8 ), and the laying down of the skeleton ( Sox9 ). Using interspecies tissue grafts, we show that developmental timing can be reset during a critical window of retinoic acid signaling. Accordingly, extending the duration of retinoic acid signaling switches developmental timing between the quail and the chick and the chick and the larger turkey. However, the incremental growth rate is comparable between all three species, suggesting that the pace of development primarily governs differences in the expansion of the skeletal pattern. The widespread distribution of retinoic acid could coordinate developmental timing throughout the embryo.

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How development is timed between differently sized species is a fundamental question in biology. To address this problem, we compared wing development in the quail and the larger chick. We reveal that developmental timing is faster in the quail than in the chick, and is associated with pattern specification, proliferation, organiser duration, differentiation and apoptosis. However, developmental timing is independent of the growth rate, which is equivalent between both species, and therefore scales pattern to the size of the wing. We reveal that developmental timing can be either maintained or reset in interspecies tissue grafts, and we implicate retinoic acid as the resetting signal. Accordingly, retinoic acid can switch species developmental timing and rescale pattern, both between the quail and chick, and the chick and the larger turkey. We suggest that the scaling of pattern to wing bud size is achieved by the modulation of developmental timing against a comparable rate of growth.

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