Hisham Bazzi

Summary Ectodermal appendages in mammals, such as teeth, mammary glands, sweat glands and hair follicles, are generated during embryogenesis through a series of mesenchymal–epithelial interactions. Canonical Wnt signaling and its inhibitors are implicated in the early steps of ectodermal appendage development and patterning. To study the activation dynamics of the Wnt target and inhibitor Dickkopf4 ( Dkk4 ) in ectodermal appendages, we used CRSIPR/Cas9 to generate a Dkk4‐Cre knock‐in mouse ( Mus musculus ) line, where the Cre recombinase cDNA replaces the expression of endogenous Dkk4 . Using Cre reporters, the Dkk4‐Cre activity was evident at the prospective sites of ectodermal appendages, overlapping with the Dkk4 mRNA expression. Unexpectedly, a predominantly mesenchymal cell population in the embryo posterior also showed Dkk4‐Cre activity. Lineage‐tracing suggested that these cells are likely derived from a few Dkk4‐Cre ‐expressing cells in the epiblast at early gastrulation. Finally, our analyses of Dkk4‐Cre ‐expressing cells in developing hair follicle epithelial placodes revealed intra‐ and inter‐placodal cellular heterogeneity, supporting emerging data on the positional and transcriptional cellular variability in placodes. Collectively, we propose the new Dkk4‐Cre knock‐in mouse line as a suitable model to study Wnt and DKK4 inhibitor dynamics in early mouse development and ectodermal appendage morphogenesis.