Basic Facts about the Skin: Structure and Function

The skin is the body’s largest organ. It is the outer covering of the entire body and is continuous with the mucous membrane of the mouth, nose, urogenital organs and the anus. In an adult, the skin measures 1.5 to 2 sq.m., while the thickness varies from a fraction of a millimeter (eyelids, external acoustic meatus) to 4 mm. on the palms and soles.

The skin has a matt tinge due and peculiar color due to the color of its component tissues, the thickness of the granular and horny layers of the epidermis, the blood vessels visible through the skin, and the presence of the pigment melanin. The color of skin may change because the amount of pigment in it varies due to internal and external factors.

The skin performs various functions

• Protective barrier
•   Organ of sense
• Thermoregulatory function (regulates body temperature)
• Secretory and excretory function  (production of sweat and sebum and elimination of water, some chemicals and drugs through the skin)
• Respiratory function (absorption of oxygen and elimination of carbon dioxide
•  Metabolism of water, minerals, hormones and vitamins

The skin has three layers: the epidermis, dermis and subcutaneous tissue.

Epidermis

The epidermis is the most superficial of the layers of the skin. It is relatively thin, averaging in thickness to about less than 1 mm, yet tough. It is composed mainly of keratinocytes, which develop into 5 layers:

(1)  germinative layer (stratum basale)

(2)  prickle-cell layer (stratum spinosum)

(3)  granular layer (stratum granulosum)

(4)  stratum lucidum

(5)  horny layer (stratum corneum).

Stratum basale is the innermost and contains the youngest, differentiating cells, while stratum corneum is the outermost , made up of dead cells that are constantly shedding. The chemical protein in these cells is called keratin which is capable of absorbing vast amount of water. This is readily seen during bathing, when the skin of the palms and soles become white and wrinkled; albeit the cornified layer provides a major barrier of protection for the body by preventing bacteria, viruses and other foreign substances from entering the body. The epidermis (along with the other layers of the skin) protects the internal organs , muscles, nerves and blood vessels against trauma. In certain areas which require greater protection, such as the palms and soles, the horny layer of the epidermis is much thicker.

Melanocytes, which are the pigment-producing cells of the epidermis can be found in the basal layer.  Melanin’s primary role is to filter out ultraviolet radiation from the sun. That is why incidence of skin cancer is higher in Caucasians because the amount of pigment in their skin is low. It is interesting to note that the number of melanocytes in both dark and fair-skinned individuals is the same.  It is now accepted that the degree of skin pigmentation is determined by the functional capacity of the these cells and not by their amount.

The epidermis also contains Langerhan’s cells, which are part of the skin’s immune system. However, these cells are also responsible for the development of skin allergies.

Another group of cells that can be found in the epidermis and is primarily located in the basal layer are Merkel’s cells. They are assumed to function as touch receptors.

The epidermis is devoid of blood vessels.

Dermis

The dermis is located between the epidermis and the subcutaneous tissue. It consists of connective tissue, cellular elements and ground substance.It has a rich blood and nerve supply and contains sweat and oil glands and hair follicles.

Two layers are distinguished in the dermis: the papillary and reticular layer. The papillary layer are composed of thin bundles of collagen fibers, while in the reticular layer the collagen bundles are more compact and thick and intertwine into a thick network of loops.

Collagen, reticulin and elastic fibers comprise the connective tissue component of the dermis. They contribute to the flexibility and strength of the skin.

The cellular elements of the dermis include fibroblasts, endothelial cells, mast cells, histiocytes (macrophages), lymphocytes and plasma cells. Histiocytes play a predominant role in phagocytosis of bacteria and particulate matter in pathologic condtions and also of antigens in immune processes.

Lymphocytes and plasma cells are found only in a small number in normal skin, but their number significantly increase under pathologic conditions.

The ground substance of the dermis is a gel-like amorphous matrix, consisting of proteins, mucopolysaccharides, soluble collagens, enzyme, immune bodies, metabolites and many other substances.

The nerve endings sense pain, touch, pressure, and temperature. Some areas of the skin contain more nerve endings than others. For example, the fingertips and toes contain many nerves and are extremely sensitive to touch.

The sweat glands produce sweat in response to heat and stress. Sweat is composed of water, salt, and other chemicals. As sweat evaporates off the skin, it helps cool the body. Specialized sweat glands in the armpits and the genital region (apocrine sweat glands) secrete a thick, oily sweat that produces a characteristic body odor when the sweat is digested by the skin bacteria in those areas.  Eccrine sweat glands produce sweat which has a weak acid reaction. This helps to neutralize the damaging effect of chemical substances and prevents penetration of micro-organisms into the skin.

The sebaceous glands secrete sebum into hair follicles. Sebum is an oil, that keeps the skin moist and soft and acts as a barrier against foreign substances. The chemical compostion of the sebum contributes greatly to the bactericidal properties of the skin.

The hair follicles produce the various types of hair found throughout the body. Hair not only contributes to a person's appearance but has a number of important physical roles including regulating body temperature, providing protection from injury, and enhancing sensation. A portion of the follicle also contains stem cells capable of regrowing damaged epidermis.

The muscles connected to the hair follicles (mm. arrectores pilorum) and muscle fibers in the walls of blood vessels and sweat glands are the smooth voluntary muscles of the skin. When the arrectores pilorum muscle contracts, it raises the hair and squeezes out the secretion from the sebaceous glands. Smooth muscle fibers which are not connected to the hair follicles are present in the skin of the scalp, forehead, cheeks, and dorsal surfaces of the hands and feet.

The blood vessels of the dermis provide nutrients to the skin and help regulate body temperature. Heat makes the blood vessels enlarge (dilate), allowing large amounts of blood to circulate near the skin surface, where the heat can be released. Cold makes the blood vessels narrow (constrict), retaining the body's heat.

Over different parts of the body, the number of nerve endings, sweat glands and sebaceous glands, hair follicles, and blood vessels varies. The top of the head, for example, has many hair follicles, whereas the soles of the feet have none.

Subcutaneous layer

The subcutaneous layer serves as a receptacle for the formation and storage of fat. It supports the blood vessels and the nerves that pass from the dermis above to the tissue beneath. This layer constitutes the largest volume of adipose tissue in the body. The subcutaneous tissue protects the body from mechanical injuries and cooling

The thickness of the subcutaneous layer varies in different areas, as well as in different individuals. The amount of fatty tissue is practically nil on the eyelids, under the nails, on the prepuce and scrotum and there is very little of it on the nose, ears and vermillion border of the lips.

Notes:

1. Itching is a mild sensation of pain because of its lower-frequency of stimuli.
2. Shaving of excess hair does not promote more rapid growth of coarse hair. If allowed to grown normally, the hairs appear  and feel no different than before 
3. The value of intermittent massage on the scalp hair growth has not been proved.
4. Hair cannot turn gray overnight. The melanin pigmentation, which is distributed throughout the length of the hair shaft, takes weeks to be shed through the slow process of hair growth.
5. Heredity is the greatest factor predisposing to baldness, and an excess of male hormones may contribute to it.

 References: 

1.   Yu K. Skripkin and M.V. Milich, Skin and Venereal Diseases, pp.27-51 
2.    John C.Hall, MD, Sauer’s Manual of Skin Diseases, pp.1-5
3.      University of Maryland Medical Center http://www.ummm.edu/dermatology-info/directory.htm
4.    Merck Manual Home Edition http://merckmanuals.com/home/sec18/ch201b.html

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