Prior to branchial arch outgrowth, Foxi3 is expressed broadly in the pharyngeal region in both ectoderm and endoderm. As arch development progresses, Foxi3 expression is refined to the clefts between the arches (arrowheads). Foxi3 is expressed in both ectoderm and endoderm, but is not expressed in the mesoderm or neural crest cells that make up the mesenchyme of the arches. A1 – branchial arch 1, A2 – branchial arch 2, P1 – pharyngeal pouch 1, P2 – pharyngeal pouch 2, ss – somite stage of development. Scale bars are 100μm.