Fine structure of the down feather during its early development

Abstract The down feather of the chick embryo has been examined by electron microscopy during three distinct stages of its early development; the presumptive stage, represented by dorsal skin of an area from which the feather organ will arise; the thickening stage, during which areas of the basal epidermis form spurs projecting into the mesenchyme, and the latter condenses under a thickened area of the epidermis; the elevation stage , at which time the basal epidermis flattens, the entire epidermis increases in thickness, and the underlying mesenchyme becomes more compact. As development proceeds the rough endoplasmic reticulum of the epidermal cells dilates, but during the elevation stage begins to flatten, and Golgi is observed with increasing frequency. The mitochondria do not appear to differ except for those in the periderm during the presumptive stage, in which case they reveal a vacant matrix and irregular cristae. Evidence is presented for actual contact between basal epidermal spurs and filopodia of cells within the mesenchyme, some of which contain numerous vesicles. The basal epidermal spurs are also seen in intimate association with collagen and anchor filaments and a network of reticulin. Evidence is also presented for the presence of neuronal elements within the mesenchyme during the thickening stage. Cross sections of cell processes within the condensations of the mesenchyme resemble unmyelinated nerve fibers, and cross sections of filopodia similar to arborizing axons abound at and within the basal lamina of both the thickening and elevation stages. Further support for the presence of nerve fibers within the mesenchyme comes from positive staining results with Bodian's and Ungewitter's methods. This comparative study of three stages of early development of the feather organ serves as a basis for more detailed investigations of each stage.