Defining Skin
Skin is a waterproof, flexible, but tough protective covering for your body. It is the body's largest organ, covering the entire outside of the body. It functions as a protective shield keeping out water, insects, heat and cold, sunlight, dirt, and gases. It also keeps in body fluids such as water and blood, hormones, minerals, vitamins, and heat. Additionally, the skin plays an important role in protecting the body from microorganisms. The skin accomplishes this is several ways; Through the continual shedding of the upper layer of the skin (desquamation), through the existing normal flora of the skin, and through the excretion of sebum and sweat which contain fatty acids and lactic acid respectively all of which assist in protecting against unwanted microbes.
Over different parts of the body, the thickness and color of the skin and the number of sweat glands, sebaceous glands, hair follicles, and nerves vary. The top of the head has many hair follicles; the soles of the feet have none. The soles and the palms have much thicker epidermis and keratin layers. The fingertips and toes contain many nerves and are extremely sensitive to touch.
The skin tends to change throughout a person's lifetime. A baby's skin has a much thicker fat layer (subcutaneous) and a much thinner layer of protective keratin. As people age, they lose much of the underlying fat, the dermis and epidermis become thinner, the elastic fibers in the dermis become fragmented, and the skin becomes more wrinkled. The flow of blood in the skin also decreases with age; so damaged skin heals more slowly in older people. Older skin also makes less protective oil, so the skin dries out more easily.
The Structure of Skin
The skin is composed of three layers, the epidermis, the dermis, and the subcutaneous layer. Each layer performs specific tasks.
EPIDERMIS (protection layer)
The epidermis is the thin translucent outer layer of the skin. The epidermis itself has several layers determined by the cellular content:Stratum Corneum
This is the uppermost layer and contains continually shedding, dead, flattened, non-nucleated, and non dividing corneocytes, which are transformed keratinocytes. The keratin present in this layer is a protein formed from the remains of the dead keratinocytes and protects the skin from harmful substances.
Keratinocytes
This layer contains living keratinocytes (squamous cells), which help provide the skin with what it needs to protect the rest of the body. Keratinocytes make the transformation from being living cells to the dead cornified cells of the stratum corneum.
Basal Layer
The basal layer is the inner layer of the epidermis, containing basal cells. Basal cells continually divide, forming new keratinocytes and replacing the old ones that are shed from the skin's surface. The cycle from cell division of a keratinocyte to exfoliation takes approximately 40 days or roughly 6 weeks. The body is willing to expend the energy necessary to accomplish this process due to the importance of healthy skin as a protector.
The lowest level of the epidermis contains melanocytes, which are cells that produce melanin, the dark colored pigment of skin. The function of melanin is to provide pigmentation and is the first physiochemical defense against the damaging ultraviolet rays of the sun. Melanocytes produce pigment granules, called melanosomes, which are picked up by keratinocytes. One melanocyte provides pigmentation to approximately 36 keratinocytes.
DERMIS (structural layer)
The dermis is the middle layer of the skin and is responsible for water binding, wound repair, and diffusion regulation involving hormones, salts, and other metabolic substances. It contains pain and touch receptors, whose tentacles reach up to the skin surface, and many of the functional glands of the skin: sweat glands, sebaceous glands (which produce the oil sebum to help moisturize the skin), and hair follicles. Also within the dermis lie blood vessels that provide nutrition to the skin and nerves that branch throughout the layers of the skin. Important components of the dermis and their function include:
Proteins
The dermis is held together by a fibrous insoluable protein called collagen which is the most abundant protein in the body. Elastin is a protein substance that forms the principal constituent of yellow elastic tissue and gives the skin it's resiliency and feel. Glycosaminoglycans (GAGs) produce a highly hydrated, gel-like matrix that helps to maintain water balance, and act as a support system for the components of the dermis. Hyaluronic acid and chondroitin sulfate (found in cartilage and bone) are glycosaminoglycans.
Fibroblasts
Fibroblasts are responsible for producing collagen, elastin, and glycosaminoglycans.
Blood Supply
A blood supply is necessary to transport nutrients to the normally functioning skin and to remove waste products generated through cellular metabolism. An adequate blood supply must be present. The blood vessels present are the arterioles which bring oxygenated blood from the heart and lungs; veinules which return oxygen-depleted blood back to the heart and lungs; and capillaries which are the primary sites for exchange of oxygen and nutrients.
SUBCUTANEOUS FAT (cushioning layer)
The subcutaneous fat layer is the deepest layer of skin and insulates the body from heat and cold and serves as an energy storage system. It consists of a network of collagen and fat cells and it helps conserve the body's heat while protecting other organs from injury by acting as a "shock absorber."
Adipose Cells
This layer consists of cells containing fatty deposits, called Adipose cells.
Subcutaneous Fat
The subcutaneous fat lies on the muscles and bones, to which the whole skin structure is attached by connective tissues. The attachment is quite loose, so the skin can move fairly freely. The subcutaneous fat is organized into fat lobules, which are separated by collagen fibers. When these lobules become grossly distended and engorged with too much fat the areas of attachment become more obvious and the skin cannot move as easily and gives rise to cellulite. These characteristic cellulite patterns tend to develop from the teens onwards.
Other
Blood vessels and nerves are present in the dermis and it may also house the hair follicles when they are in the growing phase.
