Skin is the body’s largest organ and acts as a protective barrier from infection, injury, light, and heat. It also regulates body temperature and stores water, fat and vitamin D. A basic understanding of skin anatomy and physiology is an essential part of improving skin health.

There are three structural layers of the skin: the epidermis, the dermis, and the hypodermis. Skin varies in thickness depending on the area of the body and age.

The epidermis is the thin outer layer of skin and is made up of five sub-layers. The deepest layer, the basal layer, has cells shaped like columns that divide and push already-formed cells into higher layers. As the cells move upwards, they flatten and eventually die and get sloughed off the skin’s surface.

Epidermal skin cells are known as keratinocytes because they synthesize and contain the waterproof protein keratin. Protein bridges called desmosomes connect the keratinocytes, which are in a constant state of upward motion from the deeper to the superficial layers.

The stratum corneum is the most superficial layer of the epidermis. At this level, the cells are called corneocytes; between the corneocytes are lipids (fats). The stratum corneum is the skin’s natural physical and water-retaining barrier.

It takes approximately 28 days for a new epidermal cell to move up through the layers of the epidermis and become a corneocyte. This is called the epidermal transit time and it becomes prolonged with age.

The epidermis also contains specialized cells. For example, melanocytes in the basal layer produce pigment (melanin), and Langerhan’s cells are considered the first line of defense of the immune system in the skin.

The dermal-epidermal junction (DEJ) is the area of tissue that joins the dermis and epidermis. It contains blood vessels that deliver nutrients and oxygen from the dermis to the epidermis.

The dermis is the middle layer of skin that contains elements such as collagen, elastin and hyaluronic acid, which are produced by fibroblasts. Collagens make up 70% of the dermis providing structure and strength. Elastin lends elasticity and flexibility, while other materials, including hyaluronic acid, maintain hydration and viscosity. Blood vessels, lymphatics, immune cells, nerve tissue, hair follicles, and sweat glands are also found in the dermis. Sebaceous glands are attached to hair follicles and release oil (sebum), which helps lubricate and waterproof the skin. In humans, sebaceous glands are most numerous on the face and scalp and are found everywhere except the palms and soles.

The hypodermis is the innermost layer of the skin and lies beneath the dermis. It is made up of fat, connective tissue, blood vessels, and nerves, and serves to cushion and insulate the body.