An intrinsic cell cycle timer terminates limb bud outgrowth.

Joseph Pickering,Constance A Rich,Holly Stainton,Matthew Towers,Cristina Aceituno,Patricia Saiz-Lopez,Marian A Ros,Kavitha Chinnaiya

doi:10.7554/elife.37429

Abstract

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.

Highlights

Detailed models of pattern formation often involve cells being informed of their position in the embryo by interpreting gradients of extrinsic signals, or by measuring intrinsic time
To gain insights into how intrinsic cell cycle timing is maintained in limb development, we used RNAseq to determine how the transcriptome changes over time in the distal part of the wing bud during normal development and compared this to the HH24g transcriptome
We performed RNA-seq on pooled blocks of distal cells from HH24, HH24g, HH27 and HH29 chick wing buds

Summary

Introduction

Detailed models of pattern formation often involve cells being informed of their position in the embryo by interpreting gradients of extrinsic signals, or by measuring intrinsic time. Once distal mesenchyme cells of the wing bud are displaced by growth away from proximal signals, there is a switch to an intrinsic timing mechanism that specifies distal identity (elbow to digits) (Saiz-Lopez et al, 2015). This intrinsic timer involves cells executing a programme of proliferation, expressing regulators of distal position (i.e. Hoxa13) and altering their cell surface properties, which are considered to encode position along the proximo-distal axis.

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Conclusion