The mammalian epidermis is the first line of defense against external environmental challenges including dehydration. The epidermis undergoes a highly intricate developmental program in utero, transforming from a simple to a complex stratified epithelium. During this process of stratification and differentiation, epidermal keratinocytes express a defined set of structural proteins, mainly keratins, whose expression is controlled by largely unknown mechanisms. In order to identify novel factors contributing to epidermal morphogenesis, we performed a global transcriptional analysis of the developing mouse epidermis after separating it from the underlying dermis (E12.5-E15.5). Unexpectedly, the recently identified genes encoding secreted peptides dermokine (Dmkn), keratinocyte differentiation-associated protein (krtdap), and suprabasin (Sbsn) as well as a largely uncharacterized embryonic keratin (Krt77), were among the most highly differentially expressed genes. The three genes encoding the secreted proteins form a cluster in an approximately 40-Kb locus on human chromosome 19 and the syntenic region on mouse chromosome 7 known as the stratified epithelium secreted peptides complex (SSC). Using whole mount in situ hybridization, we show that these genes show a coordinated spatio-temporal expression pattern during epidermal morphogenesis. The expression of these genes initiates in the nasal epithelium and correlates with the initiation of other epidermal differentiation markers such as K1 and loricrin (Byrne et al.  Development 120:2369-2383), as well as the initiation of barrier formation. Our observations reveal a coordinated mode of expression of the SSC genes as well as the correlation of their initiation in the nasal epithelium with the initiation of barrier formation at this site.
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