Splitting Hairs: Dissecting Minireview Roles of Signaling Systems in Epidermal Development

creating the hair. FGFs, BMPs, and Shh are secreted proteins whose binding to receptors on susceptible cells triggers signal transduction changes and transcription Hairs in mammals and feathers in birds are two of the of target genes. Notch receptor, in contrast, is activated smallest and most fascinating organs in the body. These by membrane-bound ligands such as Delta-1 on the keratinized skin appendages serve the purposes of ther-sending cell, resulting in transcriptional changes and moregulation, sensation, and social interaction, and in induction of Notch target genes. the case of feathers, flight. Early stages of feather and Feather placode formation appears to be controlled hair development have both common and distinct fea-by a competition between FGF4 and BMP2 or BMP4 tures, so comparative studies provide useful ideas about (Figure 1A). Transcription of all three of these genes is how both structures form (thoroughly reviewed in Chu-induced in the epithelium by the underlying mesoderm ong, 1998). The variation of these organs in nature is early in hair and feather placode formation. Retroviruses impressive. Otter skin has 125,000 hairs per square cen-expressing FGF in unpatterned epithelium induce ec-timeter to provide insulation and a waterproof exterior, topic feather buds, whereas viruses driving BMP2 or whereas human hair is relatively sparse and mouse foot BMP4 production dramatically inhibit placode formation pads are devoid of hair. Peacock feathers, with their The role extraordinary colors and patterns, make up about 17% of FGF was further shown using scaleless mutants, of the birds' body weight. How is all this controlled? featherless chickens that have a defect in the epidermis. The first recognizable harbinger of the site of the future FGF-coated beads restore wild-type feather density, in-hair or feather is the placode. The placode consists of a dicating that FGF2 can stimulate feather formation (Song clump of mesoderm cells underneath a small symmetric et al., 1996). In contrast, increasing doses of BMP2 or invagination of epidermis. Hair and feather development BMP4 applied on beads correlated with increasing dis-can be thought of as a two-step process. First, the num-tances between placodes (Jung et al., 1998). These ex-ber and spacing of the placodes are determined. Sec-periments suggest that placodes induced by activators ond, cells in the placode differentiate into several types like FGF are surrounded by cells prevented from adopt-that grow into all the structures of the mature hair or ing a placode fate by the overriding activity of inhibitors feather. …