Anatomy of the Skin
A good understanding of skin anatomy is necessary to be able to evaluate wound depth and the extent of damage to the tissues, as well as in the assessment of wound healing. How much do you remember (or how much have you forgotten!) about the anatomy of the skin?
The skin varies in thickness from .05 mm to 6 mm, with the thinnest layer located on the eyelids and the thickest on the palms and soles of the feet. Your skin weighs approximately one-sixth of your total body weight, making it the largest organ in (or, more accurately on) your body. It receives one-third of your cardiac output when you are at rest.
The skin is composed of two layers, the epidermis, and the dermis. Under the dermis is subcutaneous tissue. Deeper structures located under the subcutaneous tissues include muscle, bone and other important structures that you must be able to recognize.
The epidermis is the outermost layer of your skin. It is the layer that is visible to the world and thus is important to your sense of self. The skin is constantly growing. The epidermis is comprised of five layers. New cells (keratinocytes, which produce keratin) are formed in the bottom layer and make their way up to the top layer, where they are shed. This process takes two to three weeks in total. The layers of the skin, from bottom to top, include:
- stratum basale– a single row of newly formed keratinocytes
- stratum spinosum– so named because of the spiny appearance of keratin filaments under the microscope, this layer is composed of several rows of maturing keratinocytes
- stratum granulosum– this layer consists of three to five rows of keratinocytes with increasingly higher concentrations of keratin
- stratum lucidum– consists of a few layers of dead cells which are clear when viewed under the microscope (hence the name)
- stratum corneum– this is the outermost “horny” layer of the epidermis, consisting entirely of dead cells waiting to be shed; this layer can be up to 30 cells thick, making it the thickest layer of the epidermis by far
Several cells within the epidermis perform specialized functions. Merkel cells provide information to your brain regarding light touch. Melanocytes are responsible for the color, or pigment, of your skin and protect your skin from radiation effects. Langerhans’ cells help to fight infection and dwell in the deeper layers of the epidermis. In addition to these specialized cells, several appendages are also present in the epidermis, including hair follicles containing sebaceous glands. Sebaceous glands secrete sebum, which helps to lubricate our skin and hair. Our hair contributes to cosmesis. Sudoriferous glands are also present in the epidermis and secrete sweat containing metabolic waste products, salt, and water. These waste products make their way to the skin surface via sudoriferous ducts. Nails consisting of hardened keratin are also present in the epidermis and help to protect the ends of our fingers and toes.
The epidermis is not just a pretty face, or the face you present to the world! It performs many vital functions. It provides a barrier to microorganisms and other harmful substances. It plays a role in converting sunlight into vitamin D by converting 7-dehydrocholesterol into cholecalciferol when your skin is exposed to the sun. The skin also helps to regulate fluid, assists us in gathering information that is sent to the brain regarding light touch, helps to regulate our body temperature and assists with excretion of waste products.
Next, we will be looking at the dermis, subcutaneous tissues, and deeper tissues.
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Editors Note: This post was originally published in March 2014 and has been revamped and updated for accuracy and comprehensiveness.