The epidermal proliferative unit, shown in cross section (a) and in an epidermal whole mount (b). In a, note the columnar arrangement of the differentiating suprabasal cells. The edges of the outermost suprabasal cell (arrows) correspond to the silver-stained boundaries shown below. Note the mitotic basal cell beneath the edge of the suprabasal column. Original magnification, ×1000. In b, a surface view of the dorsal epidermis of the mouse is shown, demonstrating the hexagonal arrangement of the platelike cells in the stratum corneum (arrows) following silver impregnation. The nuclei of underlying basal cells are stained with hematoxylin. Original magnification, ×630.