The skin, the largest and primary protective organ in the body, covers the body's entire external surface and serves as a first-order physical barrier against the environment. It has many functions including protection against ultraviolet (UV) light, temperature regulation, trauma, pathogens, microorganisms, and toxins, among others. Also, the skin plays a role in immunologic surveillance, sensory perception, control of insensible fluid loss, and homeostasis in general.
The skin has multidimensional functions. Its most obvious role is protection since it is the first physical barrier the human body has against the potential harms of the external environment. The skin is responsible for the initiation of the biochemical processes that lead to the production of vitamin D, which is essential for calcium absorption and normal bone metabolism. It is also a thermal regulator that helps humans conserve energy in the form of heat. Thus it helps maintain our normal (basal) body temperature and helps to maintain the body’s water homeostatic balance. The skin produces hormones and growth factors. It serves as a nervous transducer, conveying sensory information from the external environment to the nervous system. It also is involved in the secretion of exocrine products like sebum, sweat and pheromones, and exerts important immunologic functions via the secretion of bioactive substances such as cytokines.
The skin is made up of the epidermis and dermis. The dermis is the deeper layer and rests on a subcutaneous layer of fat (panniculus adiposis). Embryologically, the epidermis originates from the surface ectoderm. It is infiltrated with pigment-producing cells known as melanocytes, which originate from the neural crest. Other cell types present in the epidermis include keratinocytes, antigen processing Langerhans cells, and Merkel cells, tactile receptors that sense pressure changes at the bottom of the epidermis. The dermis is embryologically derived from the mesoderm and contains connective tissue macromolecular components and cells. It includes elastic fibers, collagen, nerves, blood vessels, adipocytes (fat cells), and fibroblasts.
The skin is highly vascularized. There is an extensive network of larger blood vessels and capillaries that extend from regional branches of the systemic circulation to local sites throughout subcutaneous tissue and dermis, respectively. In addition, there is an extensive lymphatic framework that runs alongside many of the skin’s blood vessels, particularly those attached to the venous end of the capillary networks.
There are several skin receptors that play specific roles in our ability to physically perceive the changes of external environment. The Meissner receptors help us detect light touch. Pacinian corpuscles perceive deep pressure and vibrational changes. Ruffini endings also detect deep pressure and stretching of the skin’s collagen fibers. Free nerve endings located in the epidermis respond to pain, light touch, and temperature variations. Merkel receptors associated with the Merkel cells respond to sustained light touch induction over the skin. 
In all areas of the skin where there are hair follicles, abundant arrector pili muscles are found — the smallest skeletal muscles of the body. These tiny muscular structures control the positioning of hairs and the activity of certain glands in response to environmental induction, such as heat and abrasion. In addition, the skin over certain mucosal surfaces contains a layer underneath the epithelium called the muscularis mucosa. This is composed of smooth muscle and is typically responsible for the epithelial topical contour and positioning. In the esophagus, the inner lining skin is made of a continuation of the oral mucosa epithelium (non-keratinized stratified squamous epithelium) with an underlining muscularis mucosa that aids peristalsis, allowing a bolus to move down to the stomach in a safe and timely manner.
The multiple layers of the skin are dynamic, shedding and replacing old inner layers. Although the thickness of skin varies based on its location, there are additional factors such as age, gender, and health that affect the skin’s density or thickness. The varying thickness is mainly due to changes in the dermis; the epidermis is of relatively uniform thickness, irrespective of where it is located on the body, except where marked keratinization is typical of thick skin on the soles and palms. Thinner skin is found elsewhere, especially on the mucosal surfaces exposed to the external environment such as oral mucosa, vaginal canal, and other selected internal body surfaces.
Mostly due to the effects of androgens, adult males typically have thicker skin than females on most areas of the body. Children tend to have thin skin, which gradually thickens until the fourth decade of life, affected by the concentration of sex steroids, general health, and hydration. The skin begins to thin again during the fifth decade of life, primarily due to changes in the dermis with loss of epithelial appendages, elastic fibers, and ground substance, among others. Genetic and environmental factors also affect the skin’s fullness. For example, a person with an occupation requiring much outdoor exposure to the sun and ultraviolet radiation will tend to show premature skin aging signs sooner than a person working indoors. Genetics also influence the natural skin contour; for example, people of African-American descent typically exhibit fuller and more lustrous skin compared to their Anglo-Saxon counterparts.
Surgical incisions usually are made along the relaxed skin tension lines to improve healing and reduce scarring. These lines are commonly referred as “cleavage lines,” which closely match the alignment of bundles of collagen fibers within the dermis. Their discovery followed the investigations of the Austrian anatomist Karl Langer, who noticed a consistent pattern of ellipsoidal lines over the skin surface of the studied specimens (cadavers) after repetitively puncturing their skin with a circular tool. His reports referred to the lines as elongated creases following singular ellipsoidal patterns in different directions based on the specific body areas. These lines are known as the “Langer’s lines.” Surgical incisions are best made parallel to the direction of langer’s lines to reduce scarring and improve healing, especially during cosmetic surgical procedures. Langer’s lines have a completely different functional rationale and anatomical etiology from dermatomal lines.
Lines and creases develop over the bony articulations (joints) and high friction surface areas, such as the knees and elbows. Contraction of skin causes wrinkles that lie perpendicular to the underneath skeletal muscles which act as vectors of physical tension or stress points. Relaxed skin tension lines form during relaxation and often follow a different direction than those from age and contracting wrinkles. The relaxed skin tension lines are caused by natural tension on the skin from the deeper skin structures.
Dermatomes and Referred Pain
In the human body, the sensory fibers carrying pain stimuli are arranged into dermatomes, which are segmentally distributed across the entire body surface (illustration below). Dermatomes develop embryologically, but functionally represent how sensory information (i.e., pain) travels from a particular skin receptor type (i.e., nociceptor) to the corresponding peripheral nerves that, in turn, connect to specific spinal nerves (cervical; C1–C8, thoracic: T1–T12, lumbar: L1–L5 and sacral: S1–S5), reaching the spinal cord where the signals ultimately ascend to the brain.
|||Maranduca MA,Branisteanu D,Serban DN,Branisteanu DC,Stoleriu G,Manolache N,Serban IL, Synthesis and physiological implications of melanic pigments. Oncology letters. 2019 May; [PubMed PMID: 30944614]|
|||Someya T,Amagai M, Toward a new generation of smart skins. Nature biotechnology. 2019 Apr; [PubMed PMID: 30940942]|
|||Wade NJ, Microscopic anatomy of sensory receptors. Journal of the history of the neurosciences. 2019 Mar 11; [PubMed PMID: 30856054]|
|||Poirot K,Le Roy B,Badrikian L,Slim K, Skin preparation for abdominal surgery. Journal of visceral surgery. 2018 Jun; [PubMed PMID: 29631947]|
|||Whitman PA,Adigun OO, Anatomy, Skin, Dermatomes 2019 Jan; [PubMed PMID: 30571022]|
|||Dayan RR,Peleg R, Herpes zoster - typical and atypical presentations. Postgraduate medicine. 2017 Aug; [PubMed PMID: 28540752]|
|||Choi I,Jeon SR, Neuralgias of the Head: Occipital Neuralgia. Journal of Korean medical science. 2016 Apr; [PubMed PMID: 27051229]|