Stratum corneum is the outermost of the five layers of the epidermis — the top layer of the skin. Also known as stratum corneum epidermidis, horny layer, keratin layer, and corneal layer, the stratum corneum is responsible for providing a protective barrier against environmental damage from sun, penetration, toxins, and microorganisms, and by retaining moisture and lubricants. The stratum corneum is composed of corneocytes, corneodesmosomes, keratinocytes, enzymes, lipids, and natural moisturizing factor (NMF), and plays a complicated and critical role in the health of the skin.
The five layers of the epidermis are, from inside to outside, the stratum basale, stratum spinosum, stratum granulosum, stratum licidum, and stratum corneum. Mainly, the stratum corneum consists of dead or dying keratin-containing cells. It generally is responsible for the look, feel, and health of the skin.
Strength of the stratum corneum comes from 12 to 16 layers of corneocytes, which are brick-shaped cells made of layers of keratin mesh that trap water. Each corneocyte is about one micron thick, which is about 1/25,000 of an inch (about .001 mm). Thickness can vary with age and location.
The cell wall of the corneocyte is primarily comprised of the proteins loricrin and involucrin. These proteins interlace, thereby creating strong connections between cells. The connections, called corneodesmosomes, add to the impermeability of the skin. Degradation of the corneodesmosomes leads to exfoliation, or skin sloughing — a process that is not well understood.
Keratin is made in cells called keratinocytes, which make up 90 percent of skin cells. As these cells mature, they push toward the surface of the skin, dry out, and slough off. Humans slough 40,000 to 50,000 skin cells every day. Keratin is a strong fibrous protein that gets its strength from the component cysteine disulphide — a compound that allows keratin to form disulfide bridges. The number of disulfide bridges formed determines whether the keratin layer is hard like a hoof or soft like skin.
Keratinocytes also contain lamellar bodies, which form in lower layers of the skin called the stratum spinosum and stratum granulosum. When the keratinocytes migrate to the stratus corneum, enzymes cause the lamellar bodies to release free fatty acids and ceramides. Ceramides regulate cell growth and change, and apoptosis — also called programmed cell death. The fatty acids and ceramides also combine to form a protective lipid barrier layer.
About 20 to 30 percent of the weight of the corneocyte consists of NMF. NMF are water-soluble compounds that occur only in the stratum corneum. Generally, NMF take water from the air and surrounding tissues, and assure that the outer layer of the skin remains hydrated despite the conditions it is exposed to. The lipid layer formed in the lamellar bodies helps seal the corneocyte to prevent moisture loss and keep the skin soft and supple.