Hair Development, Growth, and Loss 44 hair follicles are uniformly orientated in an anterior-posterior direction, which are determined at hair peg stage.16 Each murine hair follicle development can be divided into three phases: induction, organogenesis, and differentiation. Each phase is characterized by the expression of specific molecules and can be subdivided into morphologically distinguishable developmental stages.2 Embryonic murine skin begins as a single layer of progenitor cells derived from ectoderm at around E12.5 when mesenchymal cells of different sources populate the skin to form the underlying collagenous dermis. Prior to hair follicle placode formation, the dermis is the most likely source of an unknown first hair-inducing signal. The overlying epidermal keratinocytes that respond to this specific cue will assume an upright position and specify to adopt a hair follicle fate, followed by proliferation. This becomes obvious when a small epithelial structure is formed and invades the dermis. This structure is referred to as local epithelial thickening or the hair placode. The canonical Wnt/β-catenin signaling in the epidermis has been demonstrated to function as the master switch for hair follicle fate. Ectopic expression of Wnt inhibitor Dickkopf-related protein 1 (Dkk1) or ablation of β-catenin expression in the epidermis results in the absence of hair follicle.17 Conversely, forced expression of a stabilized form of β-catenin in epidermal cells induces a global Figure 3 The diagram of hair follicle morphogenesis image courtesy of Dr. Sarah Miller