The stratum corneum is the outer layer of the skin (epidermis). It serves as the primary barrier between the body and the environment.

The epidermis is made up of five layers:

  • stratum basale: the deepest layer of epidermis, made up of cuboidal and columnar cells
  • stratum spinosum: made up of spined polyhedral cells that are connected by desmosomes
  • stratum granulosum: made up of diamond-shaped cells and contains keratin
  • stratum lucidum: thin, clear layer usually present in thicker skin
  • stratum corneum: the outermost layer of skin, made up of layers of dead cells and keratin

The stratum corneum consists of a series of layers of dead cells which are continuously shedding. It’s also called the horny layer, as the cells are tougher than most (like an animal’s horn). The stratum corneum exists to protect the inner layers of skin.

Most areas of the stratum corneum are about 12 to 16 layers of cells thick, but areas of skin that are thin (like your eyelids) are only about 9 cell layers thick. On the other end of the spectrum, calloused parts of your skin such as your hands and heels can contain up to 50 layers of skin cells.

Your skin is the biggest organ system in your body. The most important function of the skin is to protect the body from things in the environment that may cause it harm. The skin helps to keep your body temperature at a healthy level. It also works to prevent water loss or absorption.

The stratum corneum is sometimes described as a brick wall. The corneocytes that make up the cell envelope are layers, like bricks, mortared together by lipids, which creates the outer water barrier.

If everything is working properly in the stratum corneum, the skin layer will help defend you against dehydration, toxins, and bacteria, while protecting the skin layers underneath.

Unfortunately, some of the products that you use to clean your body can do harm to the stratum corneum.

Surfactants, such as hand soap, will bind to the proteins in the skin and allow water loss through the skin as well as weaken the barrier created. Therefore, using natural soap and not over-washing is suggested. Using moisturizers can help to prevent dryness of the skin as well.

The stratum corneum is made up of many parts to help protect your lower skin layers. Although the structure could be described in vastly greater detail, for ease of basic understanding, you can focus on three primary categories.

The bricks

The bricks, also called corneocytes, are mostly made up of keratin. Keratin is a protein also found in in hair and nails.

Keratinocytes are created in the lower layers of the epidermis and operate with a phospholipid cell membrane, which is very permeable. When the keratinocytes are pushed to the stratum corneum, they are transformed into corneocytes with a more durable cell envelope.

A healthy stratum corneum will shed approximately one layer of corneocytes each day. The corneocytes will then be replaced with new keratinocytes from a lower layer of the epidermis called the stratum granulosum.

The desmosomes

The desmosomes serve to connect the bricks by joining the corneocytes together. These are made out of glycoproteins and an additional protein called corneodesmosin.

In order for the bricks to shed at a healthy rate, the desmosomes must be dissolved by enzymes. If the enzymes are not present or overabundant, it can be the source of skin conditions such as psoriasis.

The mortar

The mortar that secures everything in place is made out of lipids that have been released from tiny lamellar bodies that are present in the stratum granulosum. The lipids float into the space between the bricks and between the layers of corneocytes.

The mortar is very important in protecting the lower layers of the skin. It creates the barrier that keeps out bacteria and toxins.

The mortar and whole of the stratum corneum is slightly acidic due to cellular processes that work to produce the lipids. The stratum corneum has a pH of around 4 to 5.5. The acidity helps to prevent bacteria growth.

The stratum corneum is the outer layer of your epidermis (skin). It’s made up of mostly keratin and lipids. The cells that are visible shed and are replaced from the lower epidermal layers.

The cells have a cycle in the stratum corneum of two weeks, meaning that when a keratinocyte enters the stratum corneum, it is changed into a corneocyte and shed within a two-week period.

If you’re concerned about your stratum corneum or general skin health, you should talk with your doctor about a skin care regimen that will help your skin’s bricks and mortar stay intact.