The skin is the largest and ever-changing organ that contains many specialized cells and structures. The skin functions as a protective barrier that interfaces with a hostile environment. It is also very involved in maintaining the proper temperature for the body to function well. It gathers sensory information from the environment, and plays an active role in the immune system protecting us from disease. Understanding how the skin can function in these many ways starts with understanding the structure of the 3 layers of skin - the epidermis, dermis, and hypodermis.
The visible uppermost layer of the skin. Its average thickness is about 0.2 mm and it contains no blood vessels or nerves. The epidermis is composed of five layers (lowermost to uppermost):
- stratum basale
- stratum spinosum
- stratum granulosum
- stratum licidum
- stratum corneum
Regular (epithelial) cells, Langerhans cells that participate in the immune system, Merkel cells that participate in the sensory and touch process, and melanin cells are found in the basal layer (basale). The melanin cells contain brown pigment that absorbs the ultraviolet radiation and protects the skin against solar radiation.
The cells in the lowermost layer, which have a cone-like appearance, multiply and push the old cells upwards. As they rise higher towards the uppermost skin surface, they die, crystallize into keratin tissue and become completely flat. The keratin layer protects against the penetration of water, light, heat, bacteria, and chemicals when the cells reach the uppermost layer (corneum). The entire process of the cells rising in the epidermis takes between two and four weeks.
The middle, firm layer of the skin, with varying thickness – from 0.3 mm in the eyelid area up to 3 mm in the back area. The dermis is composed of connective tissue,collagen, elastic fibers, nerves and blood vessels. It transmits heat, cold, pain, touch and pressure stimuli through the nerves to the brain.
There are two main layers in the epidermis: The papillary layer extends nipple-like papillae into the epidermis and nourishes it.
The reticular layer is the firm layer built of an elastic network of collagen fibers that is parallel to the uppermost layer and provides it with support and firmness. The hair roots, sebaceous glands, sweat glands, skin muscles and an extensive network of blood vessels are also located in reticular layer.
The subcutaneous layer composed of loose connective tissue, fat tissue and blood vessels. It connects the skin to the tissues beneath it and serves as insulation to maintain the body temperature and protects the internal organs from physical injury. Its thickness varies from a few millimeters in a thin person to many centimeters in a fat person. The hypodermis provides support and flexibility to the skin layers and the shriveling of the fat mass in this layer in old age gives rise to the wrinkled, grooved appearance of the face and loose, sagging skin over the entire body.
The thickness of the subcutaneous fat varies from body area to body area. It tends to be thickest at the waist, and is practically nonexistent at the eyelids.