Skin Structure. Functions of the Skin.

Skin forms the largest organ of the body, accounting for about 16% of a person’s weight. It performs many vital roles as both a barrier and a regulating influence between the outside world and the controlled environment within our bodies.

Your skin is constantly renewing itself. Skin cell renewal occurs every 15 to 30 days. Here's how it works: Plump, fresh cells develop in the bottom or basal layer of the epidermis, then make their way up to the surface or stratum corneum. Eventually these cells dehydrate and flatten until they slough off and are replaced by new cells. As we age, this renewal process tends to slow. It can also be affected by certain skin disorders, including dermatitis and psoriasis.

The Three Layers of Skin

Skin consists of three individual layers:

• Epidermis
• Dermis
• Hypodermis

Epidermis

Epidermis is the surface of your skin. It's the area that you touch, feel and see. The thickness of your epidermis depends on a number of factors, including your age, your sex and the location on your body. In most parts of the body the epidermis is about 0.1 mm thick but on the soles of the feet and the palms of the hands it can be 1mm thick or more.

The epidermis forms the external surface of the skin and is mainly composed of keratinocytes which differentiate to form 4 layers, the Stratum Basale (basal layer), Stratum Spinosum (spinous or prickle cell layer), Stratum Granulosum (granular layer), and Stratum Corneum (surface layer). Basal cells are responsible for maintaining the epidermis by continually renewing the cell population. Above the basal layer is the spinous or prickle cell layer so named because of the prickly appearance at high magnification due to the fine cell processes containing desmosomes attaching one polyhedral-shaped cell to another. Above the spinous layer is the granular layer, in which each keratinocyte contains basophilic keratohyalin granules. Stratum corneum is only about as thick as a sheet of fine paper, but it is what provides your skin with all of its barrier properties.

The main skin cell that makes up the epidermis is called the keratinocyte, thus named because it produces a tough protein called keratin. Keratin is also the protein from which nails and hair are formed. It gives skin much of its resistance to physical wear and tear and makes skin waterproof. Keratinocytes change in the their size and shape from square cells at the base of the epidermis, gradually becoming flatter towards the surface, by which time they have also lost their internal structure. In healthy skin these surface cells lie closely together in overlapping fashion, which adds to their protective function.

Epidermis functions:

• Deflects damage from sun, dirt and bacteria
• Helps regulate body temperature
• Prevents moisture loss

Dermis

The dermis lies beneath the epidermis, connected by a continuous membrane. The dermis is the thickest section of your skin. Here you will find blood vessels, white blood cells, nerve endings, hair follicles, sweat glands and sebaceous glands. Fibroblasts are the main cell type in the dermis. They manufacture collagen and elastin, the fibrous proteins which are the primary structural components of your skin. These proteins give your skin its resilience and elasticity. A reduction in these proteins with age is normal and contributes to the more fragile skin of elderly people. Medications, in particular steroid drugs, also weaken the collagen fibres, causing thinning of the skin in the long term and an increased tendency to bruising.

The dermis is separated from the epidermis by the basement membrane and this junction consists of the interlocking rete ridges and dermal papillae. The dermis is divided into 2 layers, the papillary dermis and the reticular dermis.

Dermis functions:

• Provides moisture to the epidermis
• Produces the collagen which keeps skin firm
• Produces the elastin which keeps skin flexible
• Generates sebum which keeps skin supple and moist

Hypodermis

Beneath the dermis is the hypodermis or subdermis. It's composed of an extensive network of connective tissue, laced with fat cells. This layer contains adipose tissue and serves to attach the dermis to its underlying tissues.

Hypodermis functions:

• Gives skin its shape and contour
• Cushions the outer layers
• Connects skin to underlying tissues and muscles

Functions of the Skin

Protection
The skin acts as a barrier against such things as water loss/entry, chemicals, bacteria and fungic as well as against minor trauma. This function is served by the lipids and proteins of the stratum corneum, the outermost epidermal layer, which is continually sloughed off and regenerated.

Sensation
Elaborate neural receptors and small nerve endings mediate touch, position, pressure, temperature, and pain. There are several types of nerve ‘sensors’ in the skin which have different functions. The simplest are free nerve endings, and are similar to bare wires in an electrical circuit. These detect pain, temperature and itch. Other more complex structures detect pressure or vibration.

Thermoregulation via the eccrine sweat gland apparatus
Internal body temperature (core temperature) is controlled through several processes, including the combined actions of sweat production and the rate of blood flowing through the network of blood vessels within the skin. In very cold conditions skin blood flow drops very low, retaining heat in the centre of the body and preserving the flow of warm blood to the vital organs within the chest and abdomen and to the brain. When in a hot environment or when generating heat from muscle activity skin vessels open up, potentially increasing skin blood flow to as much as a third of the total output of blood from the heart. Thus the skin turns into a heat radiator. The evaporation of sweat from the skin greatly increases the rate at which heat can be lost from the body.

Immunological defense via the Langerhans cells
Apart from the physical barrier protection of the skin there are specialised cells of the immune system throughout its layers. Some of these cells detect invasion by foreign proteins such as bacteria or viruses and other cells have the function of destroying and removing such material.

Vitamin-D synthesis in response to sun exposure
Skin exposed to sunlight is the main site of manufacture of vitamin D, which is essential for the growth and maintenance of our bones.