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Skin Physiology 101

Updated: Sep 16, 2019

Knowing how skin works is essential to good health. Here are some quick facts...

The skin is composed of two primary layers: the epidermis and the dermis.

The epidermis is the outer section, varying in thickness from 0.04 mm (on the eyelids) to 1.6 mm (on the palms and soles of the feet).

Five epidermal layers make up the epidermis and act as a biological conveyor belt, bringing new cells to the surface. Within these various layers, the epidermis consists mostly of living and dead keratinocytes, the cells that make keratin, a protective protein. Keratin is important in creating barriers to environmental factors and toxins, in maintaining skin integrity, and in preventing water loss.

These epidermal layers are the:

  1. Stratum corneum: the outermost layer, and thickest layer of the epidermis, which is composed of dead keratinocytes.

  2. Stratum lucidum: the second deepest layer, consisting also of dead keratinocytes.

  3. Stratum granulosum: the next deepest layer is a three to five cell layer with increasing concentrations of keratin.

  4. Stratum spinosum: the penultimate layer, containing mature living keratinocytes.

  5. Stratum basale: the deepest layer, which consists mostly of keratin-producing keratinocytes.

Through the process of mitosis, cell division occurs at the lowest level (the stratum basale), pushing new cells up to the surface. 

The epidermis is separated from the dermis by an acellular basement membrane. The epidermis is an avascular structure which lacks its own blood supply; therefore, it receives nutrients through the basement membrane from the dermis. The lack of blood in the epidermis becomes important as we try to ascertain how to best moisturize the skin. The multiple layers of the skin are very important to recall, as it is in these layers where infections and rashes can begin.

In addition to keratinocytes, the epidermis also contains a variety of cell types: melanocytes, Merkel cells, and Langerhans’ cells. Melanocytes produce melanin, the pigment responsible for both the color in skin and for protecting the skin from ultraviolet radiation. Melanin is what gives an individual his or her complexion. Darker individuals have a greater concentration of melanin in their skin. Fair-skinned individuals have low melanin levels, and are therefore at higher risk for developing skin cancers.

The epidermis also contains Merkel cells (mechanoreceptors responsible for light touch sensation), and Langerhans’ cells (responsible for the immune response in the epidermis) which lie in the deepest layers of the epidermis.

The epidermis also contains appendages rooted within the dermis, the most notable of which are hair follicles, glands, and nails. Hair follicles are dynamic organs that regulate hair growth. Hair itself is primarily composed of soft keratin, and its main function is to regulate body temperature. The follicle itself also includes a sebaceous gland at the base of the follicle, which secretes sebum (an oily, waxy substance that lubricates and waterproofs the hair and skin). Nails, on the other hand, are composed of hard keratin.

The skin is functionally a protective interface between internal organs and the environment, and as a result, it constantly encounters a wide array of toxins, pathogenic organisms, and physical stresses. To combat these attacks, the skin functions as more than a physical barrier: it is an active immune organ.

Stay tuned for more Wound Care Insights - to read more about the skin and its care, check out prior installments in our series, including:


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