Human skin is made up of 3 main layers:
- Subcutaneous layer
Within these layers of skin exist many types of cells and structures, each playing a vital role in the complex physiology of skin. Some of the vital functions of skin include:
- Protecting the body from the environment, particularly the sun
- Protecting the body from infection
- Protection of underlying organs and tissues from mechanical damage
- Excretion of excess salts, water and urea from the body
- Temperature regulation
- Beauty and attractiveness to opposite sex
- Protection against excessive loss of water from the body by evaporation
- Manufacturing and storage of nutrients
- Detection of stimuli such as temperature, pain and touch and the relay of this information to the nervous system
Now let us examine each part of the human skin in more detail.
The epidermis is the outermost layer of the skin. It is primarily made up of cells called Keratinocytes, which are stacked on top of each other, forming several sub-layers. Other cells found in the epidermis are Melanocytes and Dendritic cells.
The keratinocytes develop from the bottom of the epidermis and rise to the top, where they are shed from the surface as dead cells. The epidermis is therefore constantly renewing itself, the live cells changing into dead, hard, flattened cells.
These cells produce a dark pigment called melanin, which contributes to skin color and provides UV protection. They are located at the bottom of the epidermis. These are the cells than can turn into melanoma.
Dendritic (or Langerhans) cells
These cells are involved in the immune system of the skin. They swallow foreign material that invades the epidermis and migrate out of the skin to stimulate an immune response.
These small cells are found at the bottom of the epidermis. It was believed that basal cell cancer is derived from these cells but basal cell cancers may actually arise from non-differentiated cells from the basal cell layer.
The dermis consists mostly of connective tissue (structural tissue) and is much thicker than the epidermis. It is responsible for the skin's strength, pliability and mechanical resistance and is also involved in the regulation of the body temperature. The dermis also provides the epidermis with nutrients by means of its vascular network. The dermis also contains sense organs for touch, pressure, pain and temperature (Meissner's corpuscles, Pacinian corpuscles, free nerve endings), as well as blood vessels, nerve fibers, sebaceous (oil) glands, sweat glands, and hair follicles. The thickness of this layer can vary significantly in different parts of the body.
The dermis has a network of tiny conduits (also called capillaries) through which blood circulates. The blood vessels supply the skin with fresh blood, which contains nutrients and oxygen, and carry away waste products.
These touch receptors (sensing organs) are especially effective in detecting light touch.
Pacinian corpuscles function as receptors (sensing organs) for deep pressure and vibration.
Free Nerve Endings
Free nerve endings are sensitive to pain, temperature changes and itchiness.
Nerve fibers forward information such as pain and pressure to the spinal cord and the brain.
Sebaceous or oil glands are small organs that secrete sebum. This oily substance is a natural moisturizer that conditions the hair and skin. Sebaceous glands are found all over the body, but they are more numerous in the scalp area and around the forehead, chin, cheeks and nose. These glands play a major role in the development of acne.
These are sweat-producing structures consisting of a coiled body that leads into a duct opening at the skin surface. They are involved in temperature regulation as they help cool the skin by sweating.
Hair follicles are canals of epidermis extending downward into the dermis and specialized to produce hair. Hair follicles are found all over the body except on the palms of the hands and soles of the feet.
Arrector pili muscle
This small muscle is attached to the base of the hair follicle. When it is stimulated by cold or fright, it pulls the hair follicle up, causing it to stand upright, closing up the skin's pores and keeping the warmth in.
The subcutaneous layer below the dermis consists of loose network of connective tissue and fat. It acts as a protective cushion and helps to insulate the body by monitoring heat gain and heat loss. The thickness of this layer can vary significantly in individuals and in different parts of the body.