Sense organs originate in placodes, thickened   regions within the early ectoderm. The inner ear arises from the otic placode, a cluster of cells which, in a   process known as ‘invagination,’ recedes from the   ectodermal surface and is folded over, resulting in the formation of a segregated hollow called the otocyst   which ultimately gives rise to the mature inner ear.   We are studying the inductive factors that dictate the   location and timing of these developmental processes. In a series of tissue manipulation experiments   conducted previously, we identified a trio of signaling   factors –   FGF-8, FGF-19, and Wnt-8c – each of   which localizes in a separate germ layer, and which   cooperate to induce inner ear development. In this   network, it appears that FGF-8, expressed in the   endodermal layer, induces the expression of FGF-19   in the immediately overlying mesoderm. FGF-19 in   turn induces Wnt-8c in the neuroectoderm, triggering   a complex regulatory loop in which FGF-19 and Wnt8c maintain each other’s expression for the duration   of early ear development