Single cell transcriptomics defines keratinocyte differentiation in avian scutate scales

Epidermal keratinocytes form cornified skin appendages such as scutate scales on the legs of birds. Here, we investigated the molecular pathways of keratinocyte differentiation in chicken scutate scales by single cell transcriptomics. We identified two distinct populations of differentiated keratinocytes. The first type of differentiated keratinocytes is characterized by mRNAs encoding scale-type corneous beta-proteins (CBPs), also known as beta-keratins and cysteine-rich keratins, indicating that these cells form hard scales. The second type of differentiated keratinocytes contains mRNAs encoding keratinocyte-type CBPs and cysteine-poor keratins, indicating that these cells form the soft interscale epidermis. Immunostaining with a newly raised antibody confirmed that keratin 9-like cysteine-rich 2 (KRT9LC2) or Hard Acid Sauropsid-specific 2 (HAS2) keratin, which is a marker of the first type of keratinocytes, is expressed in the suprabasal epidermal layers of scutate scales but not in interscale epidermis. Furthermore, mRNA of CTNN1B, previously implicated in scale placode formation, was enriched in differentiated scale keratinocytes, whereas genes involved in lipid metabolism, such as ELOVL4 and FADS1 were enriched in keratinocytes of the interscale epidermis. In conclusion, this study defines the gene expression programs that build the scutate scales and interscale epidermis of birds. Overall design: Comparision of gene expression in various keratinocyte differentiation stages