Many people may not realise it, but the skin is actually the largest organ in the body, measuring 18 sq. feet in area. Apart from its cosmetic significance, the skin also performs many other important functions:-

• It is the outermost covering of the body and the first line of defence against pollution, micro-organisms, radiation and physical trauma. Even when this defensive barrier is breached, the skin has specialised cells which recognise foreign invaders and stimulate the immune system to counter the invasion.
• It protects the body against dehydration.
• The skin also helps to regulate body temperature, cooling the body down when it is hot and conserving heat when it is cold.
• It also manufactures vitamin D from sunlight and helps to prevent children from developing rickets - a condition in which bones soften and bow.
• The skin is also a sensory organ. It contains many nerve endings which enable us to sense heat, pressure, pain, touch and pleasure.
  
  Before you can understand what happens when the skin goes wrong and learn how to look after your skin better, it is necessary to take a close look at the skin.

The skin (see figure above) is composed of three layers: -

• Epidermis (epi- meaning on top of) which is the part that is directly visible,
• Dermis (dermis meaning skin) and
• Subcutis (sub- meaning below and cutis is just another term for skin).

The epidermis

The epidermis is the uppermost layer of the skin. It usually measures less than 1mm in thickness and is thickest on the palms and soles and thinnest on the eyelids. The epidermis itself is comprised of 5 layers - the top layer of the epidermis is known as the stratum corneum and the bottom layer is known as the basal layer. In between these, there are three transitional layers known as the stratum lucidum, stratum granulosum and stratum spinosum. The cells of the stratum corneum are dead and composed primarily of keratin, a hard protein also found in hair, nails, horns (hence, the alternative term for stratum corneum - horny layer) and hooves.

Cell division occurs in the basal layer of the epidermis which is why this layer is also called the stratum germinativum or germinative layer). This very important part of the epidermis is composed of two main cell types - the basal keratinocytes which form nine-tenths and the melanocytes which form about one-tenth of the total.

The basal keratinocytes are one of the most actively dividing cells in the body. They produce new keratinocytes which move upwards pushing the older cells above towards the surface of the skin. In the next layer, the stratum spinosum, the cells begin to flatten and as they enter the stratum granulosum, karatohylin granules accumulate around the nuclei. As the cells move further up, granules (known as keratohylin granules) accumulate around the nuclei. These granules are converted into keratin by an enzyme, the nuclei disappear and the cells die and become hard (a process known as keratinisation). These dead, hardened cells make the stratum corneum quite impermeable and this is why large molecules such as collagen applied to the skin cannot penetrate and cannot reach the dermis where wrinkles originate. The stratum corneum layer cells are constantly exposed to and damaged by the elements and need to be shed and replaced by newer cells from below. It takes approximately 28 days for the new cells to reach the surface of the skin and to be shed as scales. This is known as the cell renewal time. The efficiency of the cell renewal process (and consequently, skin healing), is dependent on a good circulation to supply nutrients and oxygen and remove waste products such as carbon dioxide. If the blood flow is sluggish, less oxygen and nourishment reaches the epidermis and the cells of the basal layer do not grow and divide as rapidly as they should. This may occur as part of ageing and in smokers.

The melanocytes contain an enzyme, tyrosinase which produce the melanin pigment that protects the body against the sun's ultraviolet (UV) rays and gives the skin its natural colour. The melanocytes do not function as efficiently with age so blotchy pigmentation and age spots (solar lentigines) may develop.

The condition of the stratum corneum affects the overall appearance of the skin. The cells on the surface of the stratum corneum are the oldest and are meant to be shed. Washing and scrubbing helps to remove these cells, unveiling the younger healthier cells below.

The amount of moisture in the stratum corneum is also important. Normally, the stratum corneum cells contain 40% water, 40% keratin and 20% lipids and a well-hydrated stratum corneum appears smooth and transparent so that the pinkness of the dermis underneath shows through. If it is dehydrated, the cells crumble and the skin takes on a rough, dry and dull character. Other substances also help the stratum corneum retain moisture. These include natural moisturising factors (NMFs) produced by the keratinocytes below and sebum or skin oil produced by the sebaceous glands. Sebum coats the stratum corneum and prevents the skin from drying out. Excessive removal of sebum by soaps or a reduction in the production due to age exacerbates the dryness. These secretions give the skin a slightly acidic pH of between 5.6 - 6.8.

Also found within the epidermis are Langerhan cells. These are specialised cells that capture foreign proteins and present them to the immune system. The Langerhan cells play a role in protecting the skin against cancers and infection and are also involved in skin allergies. The number of Langerhan cells decreases with age and especially with sun-damage and this may be one reason why skin cancers develop in sun-damaged skin and and become more common with age.

The Dermis

The dermis is comprised of connective tissue which itself is comprised of collagen, elastin, reticulin, fibres and ground substance.

Collagen fibres are wavy structures which interlock with each other, providing firmness (in the same way that the springs in a mattress do) and also allows the skin to be stretched without tearing. In the upper part of the dermis known as the papillary dermis, the collagen fibres are thinner, more loosely arranged and wavy. In the deeper part of the dermis known as the reticular dermis, the collagen fibres are thicker, denser and more horizontal. Damage to the papillary dermis is easier to repair than damage to the reticular dermis and this is why in resurfacing treatments (chemical peels, laser resurfacing and dermabrasion), the doctor tries not to go too deep and risk damging the reticular dermis (especially the deeper reticular dermis).

Elastin fibres are loosely interwoven like a latticework and can be stretched and yet return to its original shape and length. They provide elasticity and resilience to the skin. Reticulin fibres run in between and through the collagen and elastin fibres and help to support and keep these fibres in place.

Degeneration of collagen and elastin fibres with age leads to wrinkling, deep expression lines and sagging.

Ground substance is composed of complexes of proteins and sugars called mucopolysaccharides. The mucopolysaccharides provide support for the connective tissue and for the other structures (about which you will learn shortly) and have extremely good water-binding abilities.

The dermis also contains hair follicles, sebaceous (oil) glands, sweat glands, nerve endings, lymphatic vessels and blood vessels. The hair follicle is an indentation of the epidermis and contains the same cell layers as the epidermis except that its base has been modified to form the hair bulb, a specialised structure that forms hair. Each follicle has a sebaceous gland attached to it. Sebum (skin oil) produced by the sebaceous gland flows through a tube into the follicle and emerges from the same opening or pore as the hair. These pores may get clogged up, causing acne vulgaris or they may enlarge, making the skin appear like an orange peel). Excessive sebum production results in oily, shiny skin while too little oil, leads to dry, flaky, rough skin with many fine lines.

The sweat glands are coiled structures with separate, tiny openings that are invisible to the naked eye. They produce sweat which help to cool the body down through evaporation.

Lymphatic vessels are an important component of the body's immune system. They carry lymphocytes (a variety of white blood cells produced in the lymph nodes) throughout the body and help defend the body against infection and other foreign invaders.

Blood vessels provide the skin with nutrients and oxygen and remove waste products such as carbon dioxide. The blood vessels dilate (open up) when the body is hot so that more blood flows to the surface of the skin where it can be cooled and constricts when it is cold so as to reduce blood flow to the surface and hence, conserve heat.

The nerve endings allow us to feel heat, pressure, pain, touch and pleasure and keeps the body informed of changes in the environment and impending dangers.

The subcutis

The subcutis consists of mainly fat. It acts as a shock absorber, protecting deeper structures, insulates against heat loss and also acts as an energy reserve.