Development-Associated Genes of the Epidermal Differentiation Complex (EDC)

Abstract

The epidermal differentiation complex (EDC) is a cluster of genes that encode protein components of the outermost layers of the epidermis in mammals, reptiles and birds. The development of the stratified epidermis from a single-layered ectoderm involves an embryo-specific superficial cell layer, the periderm. An additional layer, the subperiderm, develops in crocodilians and over scutate scales of birds. Here, we review the expression of EDC genes during embryonic development. Several EDC genes are expressed predominantly or exclusively in embryo-specific cell layers, whereas others are confined to the epidermal layers that are maintained in postnatal skin. The S100 fused-type proteins scaffoldin and trichohyalin are expressed in the avian and mammalian periderm, respectively. Scaffoldin forms the so-called periderm granules, which are histological markers of the periderm in birds. Epidermal differentiation cysteine-rich protein (EDCRP) and epidermal differentiation protein containing DPCC motifs (EDDM) are expressed in the avian subperiderm where they are supposed to undergo cross-linking via disulfide bonds. Furthermore, a histidine-rich epidermal differentiation protein and feather-type corneous beta-proteins, also known as beta-keratins, are expressed in the subperiderm. The accumulating evidence for roles of EDC genes in the development of the epidermis has implications on the evolutionary diversification of the skin in amniotes.

Keywords: chicken; cornification; epidermis; evolution; keratinocytes; loricrin; periderm; skin barrier; stratum corneum; subperiderm.

Figure 2

Genes of the epidermal differentiation complex (EDC). (a) Exon–intron structure of the main types of EDC genes. The basic structure of the encoded proteins and their functions are schematically depicted below. C-term., carboxy-terminal; SEDC, single-coding exon EDC gene; SFTP, S100 fused-type protein. (b) Arrangement of genes in the EDC. The relative arrangement of EDC gene types (indicated by arrows of different color, labeled at the top) in amniotes is depicted schematically. The number of genes in the diagram does not represent a particular species. The EDC contains 39 SEDC and 7 SFTP genes in humans [72] and 100 SEDC and 2 SFTP genes in the chicken [35]. The expression profile of SEDC and SFTP genes in the embryonic periderm and subperiderm is indicated below the schematic. See the main text for information on individual genes.

Figure 1

Embryonic development of the epidermis in amniotes. The periderm is a transient layer of epithelial cells covering the epidermis during its initial stratification in the embryo. Upon formation of the cornified layer of the epidermis, peridermal cells also undergo cornification. Subsequently, the periderm is shed from the surface. The periderm is either single-layered (a), two-layered (b) or multi-layered (not shown). Developing scutate scales of birds are covered by a subperiderm in addition to the bilayered periderm (c) [51,58]. The timing (days of embryonic development) of epithelial stratification events in selected species [5,44,51,59] is shown in each panel. Colors indicate the differentiation states of epithelial cells: green, basal layer keratinocytes; yellow suprabasal keratinocytes; orange, cornified keratinocytes of the stratum corneum; red, cornified keratinocytes of cornified scales; light blue, periderm; dark blue, subperiderm.