The epidermis is a stratified squamous epithelium that provides the protective layer of the skin. The mammalian epidermis is derived from embryonic ectoderm, and this layer eventually gives rise to a very early epithelial cell that further commits to become epidermal tissue (for review see refs. 1,2). A single layer of proliferating cuboidal cells (stratum germinatium) represents the putative epidermis in the mouse up to E8.5–E12.5 d of gestation. The stratum germinatium resides on a basement membrane and expresses markers characteristic of simple epithelial cells (i.e., keratins 8 and 18 [K8/K18]). At E12.5–E14.5, an intermediate layer (the stratum intermedium) develops, which is relatively undifferentiated and is able to proliferate. However at E15.5, the stratum intermedium begins to differentiate into the spinous layer and starts to lose its proliferative ability with all mitotic activity disappearing at about E16.5, at which time the granular layer appears followed by the stratum corneum at E16.5–E17.5. By E17.5–E18.5, the mouse epidermis is fully differentiated (3–7).