The skin and its associated structures (adnexa) make up the largest and most complex organ of the human body. Named the Integumentary System, the skin acts to insulate and regulate the internal environment of the body. It also serves as the first barrier of the innate immune system. It protects what lies internally. It is important to understand the structure, adnexa, healing response, and normal aging changes seen in the skin.

Normal Structures

The skin is made up of several layers:

• The EPIDERMIS, a layer of dead cells that protect the deeper structures--the first barrier to the outside world.

• The DERMIS, the "living" layer that contains the functional components of the skin.
• Beneath this is the SUBCUTANEOUS layer, which contains the connective and adipose tissue, functions as a mode of fat storage, and provides insulation.

Advanced Integumentary System

EPIDERMIS

The epidermis has several essential elements:

1. The stratum corneum is the outermost layer of the epidermis made of dead flattened cells filled with keratin.

2. The stratum germinativum is the deepest layer of the epidermis made up of a continuous living layer of cells that are derived from the stem cells in the dermis. As the deeper cells of the epidermis age and die, they are moved up by the cells that replace them and are increasingly "keratinized," that is, saturated with keratin for its hard consistency. 
3. Melanocytes are specialized cells found in the dermal-epidermal junction that produce and store melanin--the pigmented polymer responsible for skin and hair color.

DERMIS

The dermis is most notable for the stem cells that replete the constantly shedding epidermis. It also contains the vessels that regulate body temperature, the arterioles. These small vessels respond to heat by dilating, increasing blood flow through the skin, resulting in heat leaving the body. The same arterioles respond to cold by constricting, preventing heat loss.

SUBCUTANEOUS TISSUE

Subcutaneous tissue is found between the dermis and the muscles, mainly being used for fat storage, maintaining body temperature, and protecting the nerves and blood vessels contained within.

SKIN ADNEXA

The remaining structures of the integumentary system are known as the adnexa. The vital ones are the

• HAIR: acts to protect the body, regulate temperature, and facilitate evaporation of perspiration

• NAILS: protect the fingertip and enhance the sensation of touch and the accuracy of fine motor movements. From an evolutionary (survival) standpoint, important in defense.

• SEBACEOUS GLANDS: secrete sebum, an oily wax that waterproofs and lubricates the skin and hair.

• CERUMINOUS GLANDS: produce and secrete wax which is a barrier to insects in the ear while remaining conducive to sound.  

• ECCRINE (SWEAT) GLANDS: a primary mechanism for temperature control, activated by the sympathetic nervous system to secrete water, which cools the body via evaporation.

 

Injury and Healing

Whether dramatic or trivial, deep or superficial, injury to the dermis heals in essentially the same way:

1. The clotting proteins, platelets, and white blood cells form a matrix into which the cells of the stratum basale divide, congregating around the edges of the scab.
2. The edges of the wound are then pulled together as the underlying contractile connective tissue shrinks, bringing the edges of the wound closer together.
3. Stem cells then create collagen fibers into which blood vessels grow and new tissue is gradually built.

If the wound is relatively small, all damage is eventually repaired by epithelial cells. If major injury has occurred, the normal repair mechanisms are unable to restore the skin to its original condition, resulting in a fibrous scar devoid of glands, hair follicles, muscle cells, and nerves, and with little regained blood supply.

 

Aging and Normal Changes Over the Span of Life

The effects of sunlight and time on the skin lead to the loss of elastic tissue (elastin), causing the skin to hang loosely and become translucent due to thinning of the epidermis. Skin also becomes fragile in response to the thinning and flattening of the epidermal-dermal junction. Thinner blood vessel walls also allow for easier bruising.