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Each square inch of skin contains approximately 19,000,000 skin cells, as well as many other structures, such as nerve endings, hair follicles and glands. The skin consists of three layers: the epidermis, the dermis and the subcutaneous tissue.

The epidermis is the thin yet tough top layer of skin. Measuring as thick as a sheet of paper, the epidermis has a number of sublayers. The bottom sublayer is made up of column-shaped cells known as basal cells. As basal cells divide, they form flat, scale-like skin cells called squamous cells. Also known as keratinocytes, squamous cells manufacture keratin, a protein which protects the skin from damaging substances (e.g., chemicals) as well as viral, bacterial and fungal infections. During cell division, basal cells push newly formed squamous cells up into the higher sublayers of the epidermis. As these new cells move upward and away from their source of nourishment, the cells shrink, flatten out and ultimately die. 

The top sublayer of the epidermis is known as the stratum corneum or horny layer. When undamaged, this waterproof portion of the skin helps prevent bacteria, viruses and other foreign substances from entering the body.  It consists of dead squamous cells, which are mostly composed of keratin. These cells continuously flake off during friction and activities such as washing. Approximately 30,000 to 40,000 dead skin cells are shed each minute. The cells are then replaced by new cells. Due to this process, the skin is constantly being replenished.  The epidermis cells are replaced about every 28 days.

Although basal cells and squamous cells comprise the majority of the epidermis, other cells are also present in the layer. Melanocytes are cells that manufacture melanin, a dark brown pigment that filters out ultraviolet radiation from sunlight and provides color to the skin and the hair. The more melanin that is produced in a person’s body, the darker the skin. Langerhans’ cells are also present in the epidermis. Part of the immune system, these cells help protect the body from infection. They also play a part in the development of skin allergies.

Melanocytes and Langerhans’ cells are both concentrated at the basal layer of the epidermis. However, they both contain dendritic processes, long, thin, arm-like structures that interact with squamous cells. These structures enable them to transfer pigment and monitor the immune status of the skin.

The epidermis also has supplemental areas that extend into the dermis and subcutaneous tissue. These areas are lined with epithelial cells, which have the potential for cell division and can become any type of epidermal tissue. The areas include the follicles, sebaceous glands and sweat glands. If the epidermis is removed or destroyed, the epithelial cells can basically reconstruct the epidermis.

The thick middle layer of the skin is called the dermis. It constitutes the majority of the skin. It contains strong white fibers (collagen) and elastic yellow fibers (elastin), which intertwine to provide the skin with its strength and elasticity. A number of other structures are also located in the dermis, including:

• Blood vessels. These structures provide the skin with oxygen and nutrients and help regulate body temperature. They also remove waste from skin cells.
  
  
• Nerve endings. These structures sense pain, touch, pressure and temperature. Some areas of skin, such as the fingertips and toes, have more nerve endings than other areas.
  
  
• Hair follicles. Tubular sacs located throughout the skin, with the exception of the lips, palms and soles. Each living follicle produces one strand of hair. They develop in the basal cell area of the epidermis and are rooted in the subcutaneous tissue. During periods of active growth, the follicles extend into the dermis.
  
  
• Oil glands. These structures branch off the walls of the hair follicles. Also known as sebaceous glands, oil glands secrete an oily substance called sebum. The sebum, which rises up through the hair follicle and out through a pore, provides lubrication for the skin and hair and keeps the skin soft and moist. This lubrication is important because it provides protection against friction. It also gives the skin and hair more of a “waterproof” quality and acts as a barrier against foreign substances. Oil glands are located throughout the skin, but large numbers are found in the scalp, face, midchest and genitals. Overactive oil glands can cause excessively oily skin (seborrhea).
  
  
• Sweat glands. Sweat is a liquid composed of salt, water and chemicals. There are two types of sweat-producing glands located in the skin – eccrine glands and apocrine glands. Eccrine glands are located throughout the skin, but the highest concentrations of the glands are found in the palms, soles, scalp and underarms. The glands produce sweat when a person is hot, stressed or feeling strong emotions. Sweat escapes through pores (tiny holes in the skin). When this sweat evaporates from the surface of the skin, it has a cooling effect on the body.
  
  Apocrine glands are specialized sweat glands that develop when a person reaches puberty. Located in high concentrations under the arms and around the nipples and genitals, apocrine glands secrete a thick, oily sweat when a person is under stress or experiencing strong emotions. When the sweat is digested by skin bacteria in these areas, body odor is produced.
  

  

  Sweat glands control nerve fibers that enable the brain to control sweating and regulate heat. This reaction is activated when a person’s core body temperature reaches or rises above a certain temperature.

Consisting mostly of fat, the bottom layer of the skin is called the subcutaneous tissue. It contains the skin’s nerves and blood vessels. The roots of the oil glands and sweat glands are also located here. This layer is responsible for conserving the body’s heat and insulating the body. It also provides protective padding for the body and acts as an energy storage area. The subcutaneous layer varies in thickness around the body. Though it may measure only a fraction of an inch on the eyelids, it can be several inches thick on the abdomen and buttocks.