Integration of tissue mechanics and cell signalling in developmental patterning of the skin

The project aims to understand how tissue mechanical and cell signalling processes are integrated to produce repeating patterns of cell fates in embryonic development. The focus is on the skin, an organ with two constituents: a continuous sheet of cells called the epidermis covering a looser arrangement of cells called the dermis. This type of composite structure that involves a sheet of cells on top of a matrix containing scattered and motile cells is found in a wide range of organs, including the intestine, lung, kidney, and mammary and salivary glands. Such organs are almost invariably composed of an intricate pattern of small repeating structural elements. This proposal focuses on the repeated elements in the avian skin, the feather follicles, while in other organs the repetition can produce a branched tree, or fingerlike projections, to attain a greater surface area. There is emerging evidence that these and other structures are formed not only from cell signalling interactions and local cell movement, but also by poorly-characterised mechanical and physical inputs which are generated by and responded to by cells. Much of our knowledge on mechanical influences on cell behaviours comes from culture experiments performed on isolated cells, absent the context of a complex tissue environment. This project will use the rapidly developing chicken embryonic skin to define the physical differences in local tissue environment, identify how these physical differences arise and are regulated, and determine their effects on intercellular communication, fate determination and pattern formation.

We will use chicken embryonic skin for the project due to its large size, ease of manipulation, the availability of suitable genetic lines for tracking and modulating developmental processes, and the early embryonic stage at which developmental events take place, avoiding use of animals in experimentation. From experiments done to date we know that cell motion, cell signalling, and tissue stretching/compression are together key to the production of feathers and their specific spacing arrangement. We will use recently generated lines of chicken allowing visualisation and manipulation of cells and engineering approaches to ask how these processes are integrated with the mechanical/physical changes that cooperate to drive embryonic pattern formation.

This project aims to understand how mechanical forces are integrated into cellular and molecular events that drive the emergence of repeated anatomical structures in vertebrate development, in this case the longstanding model of feather patterning. The skin, like many organs, is a composite between an epithelial sheet overlying a mesenchyme with fewer cells scattered within. The cells in the epithelium have limited ability to move relative to one another, while the cells in the mesenchyme can move much more freely. We have recently shown that coordinated cell signalling and movement generate the feather pattern, with a key role for mechanical events.

A range of developmental processes are now thought to include mechanical aspects not merely as outputs of prior cell signalling decisions, but as a core elements in defining tissue organisation and cell fate. The skin, particularly the chicken skin, represents an excellent experimental model to investigate these processes as it is large, readily and cheaply available without employing a Protected Animal species, the molecular processes taking place to generate feathers are now quite well defined, genetic models of great utility in visualising and manipulating cell number have been generated and characterised, and it is easily manipulated, and undergoes development in a sequence, providing a range of dynamic developmental stages on each sample of skin. We will map the physical characteristics of the developing skin across its different regions and through the dynamic events that pattern it, determine the effects of mechanical events on cell behaviour, signal transduction, gene expression, and cell fate determination, and determine the cell and molecular source of physical differentiation of the skin. The conclusions gained will advance understanding of vertebrate development and cell biology, as well as providing insight into the means to achieve the engineering of complex tissues.

This project will illuminate mechanisms of embryonic development and pattern formation, achieved by integrating cell motility, signalling and tissue mechanical influences. The immediate impact will chiefly be upon the academic beneficiaries and in engaging the general public, with future underpinning of understanding of the determination of tissue structure and potential for tissue engineering. The project is particularly relevant to tissue engineering of the skin, a major area of interest for wound healing therapeutics and in industry for cosmetic development and testing in vitro.

The general public has an interest in the beautiful patterns seen in the anatomy and in the origins of these patterns. The impact of this project upon the public will be enhanced by the 'real-world' nature of the model system studied; the arrangement of feathers on birds. This model is easily appreciated by a lay audience and, combined with the striking moving images produced of cells undergoing spatial patterning, this area is ideal for public engagement activities. In this regard the use of cultured skins, rather than experimentation on intact animals, will aid public engagement and acceptance.

Academic beneficiaries will gain new insights into the basis of vertebrate development and the way in which cell signalling and tissue mechanics are integrated to break the symmetry of tissues and generate pattern de novo. In addition, the specific signalling pathways under study - those of the WNT, BMP and FGF pathways - are relevant to the development of many vertebrate organs and will inform studies regarding the effects of these signals on cell behaviour and patterning in a range of tissues, and in efforts to guide and control self-organisation in cultured organoids.

The practical implications arising from this work lie in understanding tissue development for repair and regeneration, and for design of tissue engineering approaches in which physical manipulations can be employed to improve the properties of constructed tissues.