““The key to tooth development is that an interaction between these two layers of tissue, an outer sheet of cells and an inner loose layer of cells, causes the tissue to fold and makes both layers secrete the molecules that build the organ. It turns out that exactly the same process underlies the development of all the structures that develop within skin: scales, hair, feathers, sweat glands, even mammary glands. In each case, two layers come together, fold, and secrete proteins. Indeed, the batteries of the major genetic switches that are active in this process in each kind of tissue are largely similar.””
