Skin

The skin is the largest organ in the body.

 

 

The skin along with the hair, nails, sebaceous and sweat glands, and erector muscles, makeup the integument system.

 

 

The skin is a mechanical and waterproof barrier, which protects the body from the environment.

 

The skin protects the body against heat, sunlight, injury, and infection.

 

The skin also helps regulate internal body temperature and water balance.

 

The skin stores water and fat and is responsible for converting vitamin D into its active form.

 

Besides thermo-regulation, the skin has an essential role in sensory perception of the environment.

The skin is made up of several layers or strata with the epidermis or outer layer and dermis or inner layer as the main ones.

 

 

Epidermis

 

The epidermis is a type of stratified squamous epithelium, which contains three kinds of cells 

 

Squamous cells are the thin, flat cells that form the top layer of the epidermis.

 

Basal cells are round cells that lie under the squamous cells.The basal layer, is strongly attached to its underlying dermis and contains mitotically active progenitor cells that divide and give rise to the differentiated suprabasal cells

 

Melanocytes are cells that reside in the lower part of the epidermis.  Melanocytes are the cells that synthesize the pigment melanin. Melanin is the molecule that gives skin its  color. When the skin is exposed to the sun, melanocytes synthesize more pigment. This causes the skin to darken, thereby protecting the body from the suns harmful rays.

 

The skin epidermis continues to renew itself throughout life.  This is due to adult stem cells that provide new cells to replace the damaged or dead ones.

 

 

Dermis

 

The dermis is the layer of skin that lies between the epidermis and the subcutaneous tissue. The dermis is made up of collagen, elastic fibers, and an extra-fibril matrix. 

 

Within the dermis are a series of pilosebaceous units, erector pili muscles, and glands.

 

Within the dermis runs two vascular networks that run parallel to the skin surface; one superficial and one deep. The function of this blood supply is to provide nutrients to the skin, regulate body temperature, participate in wound healing and provide the factors necessary for inflammation. 

 

The dermis is a network of collagen fibers and elastin, surrounded by a gel-like extracellular matrix.

The matrix is what stabilizes skin.

The matrix is made up of glycosaminoglycans.

Glycosaminoglycans promote the ability of collagen fibers to retain water and bind moisture into the stratum corneum of the epidermis.

 

 

Collagen, as the major protein of connective tissues, can be compared to a sponge because of its ability to trap water.

As collagen ages, the fibers aggregate into larger collagen bundles and become less soluble.

The less soluble collagen is, the less water it retains.

This condition, plus the addition of calcium deposits that occur with aging, actually squeeze water out of collagen and cause shrinking of collagen. This results is a loss of skin turgor and the formation of wrinkles.

As collagen forms, glycosaminoglycans become bound to the peptide chains of collagen. This binding promotes the ability of collagen to retain water by keeping the collagen fibers separated.

Subcutaneous Layer

The subcutaneous tissue is a layer of fat between the dermis and the underlying fascia. The main cells in this tissue are the adipocytes or fat cells.

Diseases

The skin is subject to many assaults, some of which result in disease. 

These diseases or dermatoses include cancer, infections, keratosis, a variety of illnesses due to abnormal pigmentation and a host of genetic disorders.