The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. There has been a long-standing belief in dermatology that the stratum corneum consisted of dead cells (corneocytes), devoid of biological activity and function. The stratum corneum is now understood to be live tissue that performs protective and adaptive physiological functions including mechanical shear, impact resistance, water flux and hydration regulation, microbial proliferation and invasion regulation, initiation of inflammation through cytokine activation and dendritic cell activity, and selective permeability to exclude toxins, irritants, and allergens. This layer is composed of 15–20 layers of flattened cells with no nuclei or cell organelles. Their cytoplasm shows filamentous keratin. These corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids.