Gene expression of potential morphogens during hair follicle and tooth formation: a review

Early work on the morphogenesis of hairs and teeth was largely descriptive histology and established the times and order of visible initiation of anlagen and their patterns of development. However, in the last 30 years, many growth factors have been discovered; more recently, their expression during morphogenesis has been determined and immunohistochemistry has enabled the visualization of structural elements of organs. This review is concerned primarily with aspects of these recent phases of research with respect to the formation of hairs and, to a lesser degree, teeth. The expression of several growth factors including bone morphogenetic proteins 2 and 4, the glycoprotein tenascin, the proteoglycan syndecan, and the expression of the mammalian homologue of Notch, cadherins and epimorphin is examined here during the early stages of organogenesis, primarily to review the type of research that should be extended to the organogenesis of wool fibres in Merino sheep. Signal transduction, the third and increasingly complex phase of research that is now rapidly developing, follows the establishment of ligand-receptor complexes during morphogenesis and is included here in a preliminary way.