How cells measure time during pattern formation

I am interested in how complex structures are correctly formed in the body. The crucial events that control how we develop occur during the earliest stages of life in the embryo. In particular, my research is centred on how limbs develop and I mainly work on chicken embryos because we can look directly at how they develop by opening a small window in the egg. The specific question I want to understand is how cells divide for the correct numbers of time in order to generate correctly proportioned limbs. This research is important because it can give insights into the mechanisms that cause cells to lose control over the numbers of times they divide and turn into cancerous tumours. In addition, by revealing how digits develop we can use this knowledge to understand the causes of birth defects that affect the limb and other structures in the body.

The integration of growth and patterning is critical during embryogenesis to ensure the specification of complex structures. This was the focus of my previous work that revealed the link between digit number and digit identity in the chick wing. I showed that both these processes are controlled in the digit-forming field by the same signalling molecule - Sonic hedgehog (Shh) - produced by the adjacent polarizing region at the posterior margin of the limb bud. Now I will explore how the control of digit number and identity is integrated over time. Most knowledge of developmental timing comes from in vitro work and it is unclear how this relates to tissue development. To address this, I have performed a series of in vivo experiments in the chicken by grafting wing bud tissue that constitutively expresses Green Fluorescent Protein into normal buds. My preliminary data suggests that both the polarizing region and digit-forming field cell populations proliferate for a defined time. However, polarizing region cells have self-renewing properties similar to a stem cell population, whilst digit-forming field cells have a limited potential to proliferate similar to a transit-amplifying cell population. My preliminary data suggests that both cell proliferation timers are set by Shh signalling and this could integrate the extent of digit field expansion with the duration of Shh transcription. This self-terminating mechanism can be applied to other systems such as the neural tube. I have identified putative regulators of cell proliferation timers in the chick wing. I will elucidate how cell proliferation timers function using molecular, cellular and genetic approaches as well as mathematical modelling in both the chick and mouse limb. I will also identify the factors that limit or permit the self-renewal of putative polarizing region stem cells by microarray analysis. In the longer term, I will try to identify polarizing region stem cells by looking for stem cell markers and this could have major implications for tissue repair and limb regeneration. This work will also have implications for cancer biology and congenital disorders caused by temporally deregulated growth and patterning.