The epidermis is 50–100 μm thick, and is composed of a stratum corneum of flat keratin cells, that is 10–20 μm thick; it protects the underlying viable epidermis, which is composed of keratinizing epithelial cells. The dermis is mostly collagen and elastin fibers embedded to a viscous water and glycoprotein medium. The fibers of the upper dermis ("papillary dermis") are thinner than the fibers of the deep dermis, thus the skin envelope is 1–3 mm thick.