Among the most important of these cells are the melanocytes, which are found in the basal layer of the epidermis. These manufacture a special pigment called melanin, which helps to determine the hair and skin pigmentation. The pigment is made on tiny structures called melanosomes, which aggregate as granules and are delivered in small ‘packages’ to each basal cell by slender filaments called dendrites. One melanocyte supplies about 36 keratinocytes with melanin granules. These tiny packages of pigment sit over the nucleus - the vital centre of the cell - in every cell in the epidermis, and protect it from the harmful rays of the sun.

Melanin
There are two forms of the pigment melanin: eumelanin granules, which tend to be round and smooth and produce black and brown skin pigmentation, and phaeomelanin granules, which are more irregular in shape and which are more prominent in lighter skins, particularly in association with red hair and freckles. These two forms of melanin are often both present together, and occur in varying proportions.

Differences in skin color
Eumelanin is the commoner and more dominant pigment of the two, particularly in hair. Most of the world’s people have black hair, but skins that range from very fair to black. Skin color and hair color tend to go together and may reflect our ancestors’ adaptation to their environment. Scientists believe that the earliest humans originated hundreds of thousands of years ago in an areanow found in the African continent. The gradual evolution of the human race continued along different lines, until there were essentially three different ancestral racial groups:
* Asian - Oriental peoples
* African - people of direct African descent
* Caucasian - including the people of north-western Europe and also very dark-skinned Indians

Melanin production in skin varies in the three racial groups. ‘Black’ skins do not contain any more melanocytes than white ones do. But there are differences in the melanin granules in the differently colored skins. In black skins the granules are larger, whereas in white skins they are less obvious.

* In Asian people, the melanosomes are relatively large in size, and are distributed within the skin cells as a mixture of single and complex forms.
* In African skin the melanosomes are even larger; they are heavily pigmented and scattered singly throughout the keratinocytes.
* In white Caucasian skin the melanosomes are smaller and have less melanin; they are distributed as clumps in keratinocytes.

In equatorial regions of Africa, Latin America and India, where there is a high degree of sun exposure, many of the indigenous people have highly pigmented and thick skins that protect them from the harmful rays of the sun - very dark skin offers about 30 times more protection against the sun than pale skin does. There is not, however, a definite relationship between skin pigmentation and the degree of exposure to sunlight. There are people with unexpected skin colors for the area in which they live. For example, the Tasmanian Australoids are dark-skinned although they live in a temperate latitude; also the pigmentation of American Indians, who are descendants of Asian peoples, is similar across the whole continent of North America. These examples are probably the result of migrations forty or fifty thousand years ago. A few thousand years ago, unknown factors triggered a great migration of people from east to west. The native peoples of central and western Europe were pushed westwards. Among these were the original Celts (people with blue eyes and very pale skins easily burnt by the sun), who eventually populated parts of Scotland and Ireland; their descendants can still be identified in those countries.

Similarly, in the last few hundred years peoples with white skins have migrated to Australia and South Africa - areas of high sunshine to which their skins are not well adapted, and among them sun damage and skin cancer rates are high. Some skin types appear to show specific and curious adaptations to their climate. Many Scandinavian people have pale skins and light hair in winter. In the short but sunny summer, many of them tan quite markedly and quickly while their hair bleaches to almost white. In the last few centuries, increasing ease of travel and the creation of multinational countries such as the USA have led to a wide range of different shades of skin and hair types and colors among the world’s population.

Skin appearance and skin color
When we look at skin many factors affect what we actually see, including the brightness and color of the light, the state of the skin and the basic skin color. These all combine to produce an effect that can alter dramatically. In normal daylight, what we see is partly light reflected from the surface of the stratum corneum and partly light reflected back from the dermis through the translucent epidermis. If the stratum corneum contains adequate moisture and the dead cells (squames) have been removed, it is more translucent and reflects light more evenly, giving the skin a ’shine’. The dermis and the state of the dermis itself play a greater part in the ‘complexion’. What we see as the actual skin color, as distinct from the condition, depends on light that is reflected by four different colored components of the skin, which are found at different levels throughout the epidermis and the dermis. These reflections combine to give us our unique skin color. They are:
* melanin in the epidermis
* red blood cells containing oxygen in the small blood vessels of the dermis
* red blood cells without oxygen in the same blood vessels
* orange-yellow chemicals called carotenoids
in the stratum corneum and the subcutaneous fat layer; these are principally responsible for the yellow tones of skin color, and are more abundant in men’s skin than in women’s.

Carotenoids are found in carrots
Eating too many carrots can turn you orange!
Of these four factors, melanin is the most important in deciding skin color. The contribution of blood to the complexion color is most obvious in the cheeks, where capillaries are most numerous and closest to the surface. Apparent skin color can change if the combination of its colored components changes. Changes like these are more obvious if very little melanin is present, since melanin can hide most of the other colors. This is why people with very pale skins - ‘porcelain’ skins -can look blue if they get cold: blood that moves sluggishly carries less oxygen, and so looks bluish rather than having the bright red color that is given by full oxygenation.

The dermis Beneath the epidermis lies a much thicker skin layer, the dermis. The dermis can be as much as 3000 micrometres thick. The dermis is composed largely of the protein collagen. Most of the collagen is organised in bundles running horizontally through the dermis, which are buried in a jelly-like material called the ground substance. Collagen accounts for up to 75% of the weight of the dermis, and is responsible for the resilience and elasticity of the skin. Both collagen and elastin fibres are made by cells called fibroblasts, which are scattered through the dermis.

hyaluronic acid has been experimentally injected into skin, in an attempt to reduce wrinkles.
Another reason for skin aging is that collagen and elastin production declines as the years go by, particularly after the menopause, so that some of the skin’s natural properties are lost.

Injuries to the dermis
The deeper part of the dermis contains fewer blood vessels than the upper layers do, and many thick collagen bundles. These bundles lie parallel to each other along recognisable lines which are important to understand in wound healing. If a cut is made across these lines the skin gapes, and when the cut is healed there is more scarring than with wounds made along the lines of the bundles. Surgeons follow these lines when making their incisions, to ensure the best possible healing, which is why everyone’s appendix scars are practically identical. If the skin is seriously over-stretched, whether by too much fat or by pregnancy, the deep collagen fibres may actually rupture. This results in deep scars, which are seen through the intact epidermis as ’stretch marks’. Taking high doses of steroids for too long may have a similar result, as the collagen withers away (atrophies) under the influence of these drugs. In a graze only the epidermis is sheared off. New epidermal cells very rapidly cover over the area with unscarred skin.

Interestingly, these new cells are provided by cells from the hair follicles. Where a wound has damaged both the epidermis and the dermis, both the basal cells in the epidermis and the fibroblasts in the dermis go into intense production to seal the gap. In a very narrow wound repair is relatively simple. But in large wounds the resulting repair is never perfect. Granulation tissue forms: this is a mixture of tiny blood vessels and fibroblasts frantically making collagen. This tissue eventually forms the scar: the greater the area to be covered, the larger will be the scar. Sometimes the skin cells go on working at the repair process for much longer than necessary, so that far too much scar tissue is formed. This special substances in the ground substance, called glycoproteins, can hold large amounts of water, and are responsible for maintaining a mass of water in the dermis.

Hyaluronic acid is another important substance that forms part of the tissue that surrounds the collagen and elastin fibres. It has the ability to attract and bind hundreds of times its weight in water. In this way it acts as a natural moisturising ingredient responsible for the skin’s plumpness and moisture reserve. As we get older the amount of hyaluronic acid produced in the skin naturally gets less. This is one reason why aging skin becomes less resilient and supple (pliable).

Recently Sometimes the skin cells go on working at the repair process for much longer than necessary, so that far too much scar tissue is formed. This produces a permanent raised scar called a keloid. Keloids are common with certain types of skin, particularly in young people and those from an African background. They can be injected with steroids by a doctor, which sometimes helps. Cutting them out is seldom effective, and usually makes them worse. Eventually they decrease in size.