Skin is comprised of the epidermis, dermis and superficial fascia. It is considered the largest organ of the body and it facilitates mobility. The skin is made up of the epidermis, dermis and superficial fascia.
The epidermis is the outermost layer that protects the body against dehydration, infection and other elements of the harsh external environment. It is made primarily of cells called keratinocytes that produce keratin. Keratin cells create the rigid fibrous outer layer of the skin and are continually expanding and sloughing off like tree bark. Melanocytes also exist within the epidermis and protect the underlying layers from solar damage by progressively darkening (Kusuma, 2010). Nerve fibers are found within all layers of the epidermis with approximately one thousand fibers per square millimeter (Metze, 2009). These nerve fibers provide sensation and also have a growth and repair functions.
The dermis that is made up of collagen fibers is the layer below the epidermis. The collagen, elastin, proteoglycans, water and salt within the dermis create the characteristic density of the skin which cushions the body. Blood vessels, nerves, and other glands also exist within this layer. Type I collagen comprises 80% of the dry weight of the skin with the remainder made up of type III, type V and VI collagen. Branches of myelinated sensory nerves which are the fast conducting A-beta fibers parallel the skin surface with upward projections creating our ability to sense the outside world (Kusuma, 2010, Metze, 2009). The autonomic system which consists of the slower conducting unmyelinated A-delta and c-fibers is also found within this layer, detect pressure and temperature, and is crucial for monitoring and maintaining the health of the tissue (Metze, 2009). There are about one thousand afferent cutaneous fibers per square centimeter of skin (Metze, 2009). The sensory nerves coalesce into dermatomes whereas the autonomic nerve supply terminates into a terminal autonomic plexus supplying the erector pili muscles, blood vessels and skin glands (Metze, 2009).
Superficial fascia (Hypodermis)
The superficial fascia, also known as the hypodermis, lies beneath the dermis and is primarily responsible for storing fat. The superficial fascia is connected to the overlying dermis and deep fascia which is intimate with muscle via fibrous septa (Stecco, 2015). The fibrous septa create a three-dimensional framework that encloses the lobules of fat, connects the superficial dermis to the deeper structures and creates a flexible, resilient structure that resists multidirectional forces (Stecco, 2015). The superficial fascia also acts as a conduit for nerves and blood vessels to pass through to the dermis and epidermis.
Kusuma S, Vuthoori RK, Piliang M, Zins JE. Skin Anatomy and physiology. In: Siemionow MZ, Eisenmann-Klein M, ed. Plastic and Reconstructive Surgery. Dordrecht, Netherlands: Springer; 2010;161-171.
Metze D. Neuroanatomy of the skin. In: Granstein RD, Luger TA, ed. Neuroimmunology of the Skin: Basic Science to Clinical Practice. Berlin, Germany: Springer; 2009:3-12.
Stecco, Carla, Warren I. Hammer, Andry Vleeming, and Raffaele De Caro. Functional Atlas of the Human Fascial System. Edinburgh: Churchill Livingstone Elsevier, 2015.