The Anatomy of Skin
The largest organ of your body is your skin (known as integument in the world of clinical anatomy). It includes the outer covering that protects your inside parts from the elements and from viruses and bacteria. The skin is also necessary for heat regulation, sensation, and making vitamin D.
The skin can be a good indicator of health. A person who is in shock may have pale skin and goose bumps, and someone with a fever may feel warm to the touch.
The skin has two layers, called the epidermis and the dermis:
Epidermis: This tough layer of cells is the outermost layer of skin. It gets its toughness from a protein called keratin. The epidermis has five layers:
Stratum corneum is made up of dead, mature skin cells called keratinocytes. These cells are constantly shed and replaced by cells from the lower layers of the epidermis. These cells have lost most of their internal structures and organelles.
Stratum lucidum is found in thicker skin and helps reduce friction between the stratum corneum and the stratum granulosum. It’s composed of dead, flattened cells.
The skin is thicker in some areas (like the soles of your feet) and thinner in others; women also tend to have thinner skin than men do.
Stratum granulosum is where keratin is formed. The cells in this layer also produce materials that prevent evaporation, which helps waterproof the skin.
Stratum spinosum contains the keratin-producing cells that were formed in the stratum basale. Keratin is a major structural component of the outer layers of skin.
Stratum basale forms the deepest layer. The cells of this layer continuously divide and form new keratinocytes to replace the ones that are constantly shed. This layer also contains melanocytes, which are the cells that produce skin coloring.
Dermis: This lower layer of the skin contains collagen and elastic fibers that give strength to the skin. This layer is also where the vasculature and nerves live.
Together the epidermis and dermis form the cutaneous layer. The subcutaneous layer (area below the skin) lies underneath the cutaneous layer and is sometimes called the hypodermis or superficial fascia. It holds most of the body’s fat, so it varies in thickness from one person to another.
Creases form over joints because the skin always folds the same way as the joints bend. The skin is thinner in those areas and is firmly attached to the underlying structures by connective tissue.
The integument also includes the structures that grow out of the skin, plus a couple of glands:
Hair: The protein keratin forms hair. Hair has an inner layer (the cortex), which contains pigments that give it color, and an outer layer (the cuticle). It grows out of follicles, which are little pockets of epidermis in the dermis. The shape of the follicle determines whether hair is curly or straight. Each follicle contains a hair bulb from which the hair develops. Arrector pili muscles connect the hair follicle to the skin.
Nails: Keratin shows up again in the form of plates found on ends of the fingers and toes. Underneath each nail is a nail bed with a root at the proximal end (closer to the rest of the body).
Sebaceous glands: These glands are connected to the hair follicles. They produce sebum, which is an oily substance that helps keeps the hair flexible.
Sweat glands: Sweat glands are coiled tubular glands found in most of the skin. The secretory portion (the part that secretes the sweat) of each gland lies in the fascia with a duct that runs up to the surface of the skin.
The fascia is divided into two types: the superficial fascia and the deep fascia, which forms the following structures:
Investing fascia: This part of the fascia covers deeper structures, such as muscles and ligaments.
Intermuscular septa: The septa divide muscles into various groups.
Subserous fascia: This part of the fascia lies between the body walls such as the thoracic wall (or abdominal wall and the membranes that line corresponding body cavities.
Retinacula: These band-like structures hold tendons in place while joints move.
Bursae: These fluid-filled sacs are made of fibrous tissue; they’re lined with membranes and contain a bit of viscous fluid. They reduce friction between body tissues. Major bursae are located near joints between the tendons and bone.
Bursitis is an inflammation of a bursa, which is usually the result of repetitive motion injuries. Fasciitis is pain and inflammation of the fascial tissues. Plantar fasciitis occurs in the fascia that runs along the bottom of the foot from the heel bone to the toes. It’s a common cause of heel pain.