Transcriptional control of the differentiation program of interfollicular epidermal keratinocytes

Mammalian epidermis is a stratified squamous epithelium that serves as a protective barrier against external harmful elements. The development of the epidermis is a highly regulated process that begins by the commitment of a single layer of multipotent ectodermal cells to a keratinocyte cell fate. This is followed by stratification and a subsequent elaborate program of differentiation leading to the generation of a multilayered epidermis and patterned cutaneous appendages such as the hair follicles and sebaceous glands. The stratified epidermis occupying the space between skin appendages is referred to as the interfollicular epidermis (IFE) and is the focus of this review. Within the IFE, keratinocytes in the innermost basal layer are mitotically active. Upon specific cues, these cells undergo cell cycle arrest and execute a terminal differentiation program as they progress through spinous, granular, and cornified layers. This program operates continually throughout the life of an organism; dead cells sloughed off from the skin surface are replenished by basal cells moving outward in a highly synchronized fashion. Not surprisingly, at the heart of the control process is a dedicated group of transcription factors that ensure the integrity of the keratinocyte differentiation program by regulating gene expression in a temporally and spatially coordinated manner. Here we review the transcription factors that play important roles in the development and maintenance of IFE as evidenced by biochemical and genetic studies.