Biology of the wool follicle: an excursion into a unique tissue interaction system waiting to be re-discovered

Wool fibres are hairs and the term 'wool' is usually restricted to describe the fine curly hairs that constitute the fleece produced by sheep. In a broader sense, it can be used to describe the fleeces produced by related species such as goat or yak. Research into the biology of wool growth and the structure of the wool fibre has been driven by the demands of the wool industry to improve both the efficiency of growing wool and the quality of the product. Well beyond this very applied perspective however, the wool follicle is a unique basic research model for the life sciences in general. These unique features include, to name just a few selected examples, accessibility for studying the molecular controls involved in branching of secondary epithelial-mesenchymal structures, the photoperiod-dependence of regenerating tissue interaction systems, the origin of fibre curliness and follicle wave pattern formation, and the effect of alterations in nutrient supply on epithelial growth and fibre structure. In this review, investigation of growth processes in the formation of the wool fibre is broadly surveyed. The relevance and potential for practical outcomes through characterization of wool follicle genes are discussed and particular features of the wool follicle contributing to our knowledge of the biology of hair growth are highlighted. The practical potential of gene discovery in wool research is the provision of molecular markers for selective breeding and for altering wool growth and wool structure by other biological pathways such as sheep transgenesis that could lead to novel wool properties. In this background, the current review attempts to revive general interest in the fascinating biology of the wool follicle which is not only of profound economic and practical importance but offers an exquisite, highly instructive research model for addressing key questions of modern biology.