Intrinsic and Extrinsic Aging
Some of the skin changes that accompany aging are natural and inevitable, and together make up the process called intrinsic aging or sometimes chronological aging.
More significant for most people are the changes arising from external causes - called extrinsic aging - and in particular the damage caused by ultraviolet radiation from the sun and sun beds, known as photoaging.
These changes affect the dermis in particular and result from changes in the chemical structure of the collagen and elastin, and to the quality and quantity of proteins and natural acids in the skin.
Understanding the changes that occur in the cells and layers of the skin with intrinsic and extrinsic aging will help us to understand why skin looks as it does, and how we can protect or alter this appearance. It also allows us to understand why it is so important to protect the skin of children and to educate them in skin care.
In intrinsic aging, the skin becomes thinner and loses much of its elasticity, while the normal expression lines deepen. The boundary between the epidermis and the dermis
is flattened, and the dermis starts to wither (atrophy). The number of blood vessels in the dermis begins to fall. At the same time the hair often loses its color, and within the skin there are fewer hair follicles and fewer sweat glands. The collagen, elastin and ground substance also decrease in amount, but the proteins remain in a reasonably stable state.
In extrinsic aging the epidermis thickens. The amounts of collagen and elastin increase, but the structures of these proteins become disorganised. Almost all of this is due to effects of radiation from the sun, known as photodamage.
- 'Intrinsic aging' happens inevitably.
- 'Extrinsic aging' is due to outside factors that have affected the skin.
The collagen network from (left) undamaged and (right) sun-damaged skins.
Skin aging in men and women
The processes of aging differ in male and female skin.
In men, there is a gradual thinning of male skin with increasing age of approximately 1% per year. On the other hand the thickness of most women's skins remains surprisingly constant until the menopause, after which there is a significant and sometimes dramatic thinning with increasing age.
There is a relationship between skin thickness and collagen content in men of all ages. A similar relationship exists among women over 60 years of age, but it is less evident in younger women.
In adult skin, the features of aging are closely related to the total collagen content, which in both sexes decreases with increasing age, but at different rates. In later life women may look older than men of the same age and similar experience of sun exposure, partly because their skin has a lower collagen content to start with. Another reason for the gender difference in skin collagen content may be the difference in male hormone production between men and women.
In women, estrogen and androgen output from the ovaries and adrenal glands falls after menopause, resulting in decreased collagen synthesis and repair.
Aging related to the failure of estrogen production at the menopause accentuates intrinsic aging, and together with photoaging may dramatically increase the apparent age of a menopausal woman.
Estrogen deficiency particularly affects the fibroblasts of the dermis, and thinning of the skin is primarily related to a decrease in the production of collagen. This decrease is related to a decline of bone mineral content with age, which can lead to the condition of osteoporosis. The fibroblasts are also responsible for the synthesis of the dermis ground substance, particularly glycoproteins and hyaluronic acid (which is able to bind water). The decrease in fibroblast activity with age accounts for the decreased dermal hydration.
Skin elasticity decreases with age, but the effect is more marked in women than in men.
In the epidermis, a lack of estrogen slows down the activity of the basal keratinocytes, and consequently leads to epidermal atrophy. This atrophic fragile skin is less well protected by the normal surface film of lipids, because of the slow decline in sebum secretion experienced by everyone as they age. The stratum corneum barrier is less effective, and the skin may develop reactions to irritants, particularly if skin care has been inadequate or too aggressive.
Regular daily protection against the sun from an early age, combined with the regular use of well-formulated moisturizers, is of the greatest importance to 'preserving' the skin.
How skin changes with age: a summary
Although the skin is itself a protection against radiation, this protection brings with it consequences of its own, leading to an 'apparent age' that is greater than one's real age.
The appearance of an individual's face and hands are often used as a rapid measure of age. Comparing the appearance of the face and forearms with that of less exposed areas demonstrates the major differences between intrinsic and photoaged skin.
Skin creases and lines
The differences in skin surface markings between young and mature skin that has always been protected are only slight. In the older skin there will have been some natural loss of collagen and elastin, but no other significant damage.
A young baby has the normal skin creases on the hands and face, but very few elsewhere on the body.
As we grow older, fine lines and wrinkles begin to develop. The skin loses its firmness and elasticity. Expression lines form on the face, and patches of discoloration and areas of dilated blood vessels appear. On exposed areas of aged skin, the skin patterns are often markedly changed. Some very old people may find deep furrows develop in their skin. Others suffer from the so-called 'chicken neck'. The reasons for these changes are:
- blood circulation slows down
- metabolism slows down
- chemical changes take place in the tissues
- sebaceous glands diminish in size and number, particularly in women
- collagen production breaks down
- hormone production is altered or reduce.
7 signs of aging
A survey of 6000 women from around the world identified which signs of aging were most relevant across geographical and cultural boundaries. While there were slight variations country by country, women consistently identified seven relevant signs of aging.
Skin cell regeneration
With increasing age, the skin's cell renewal process becomes less efficient. Tissue repair and cell regeneration slow down. The amount of natural moisture present in the skin is reduced. Because collagen production is less, the skin becomes thinner and loses its flexibility.
Changes in the Protective Function
The most obvious sign of intrinsic aging is a decrease in the overall thickness of the epidermis as a whole, with a reduction in the number of cell layers. The number of cells in the stratum corneum does not diminish with age, however; this is important, because of the vital role of this layer as the skin barrier. On the other hand, the numbers of melanocytes and other cells do decrease with age. So do the numbers of the Langerhans cells, which are involved in the body's response to allergens (see Functions of the Dermis). This could be one reason why people tend to experience fewer allergic reactions as they get older.
The rete pegs become less prominent, and the junction of the dermis and epidermis becomes flattened (see The Layers of Skin). This means that the epidermis is not so securely held down, and becomes more vulnerable to damage by friction.
Metabolism in the skin (as everywhere else) slows down. So too does the rate at which epidermal cells are produced, which may interfere with wound healing. The time necessary to repair the stratum corneum barrier increases considerably with age: the replacement of skin cells takes about twice as long for people over 75 as for those around 30.
Although the sebaceous glands themselves do not change much with increasing age, sebum production declines in many older people, especially after the age of 70, though in some the glands on the face actually enlarge in extreme old age.
With age, the number of active sweat glands falls and their output of sweat decreases too. As a result, perspiration is less in elderly skin. This explains why older people often find it hard to adapt to hot weather.
Most older people have a dry skin and therefore have a special need to avoid the over-use of harsh soaps and detergents, in order to prevent problems associated with dryness. Aged skin retains its fundamental ability to control water loss, but may partially lose this ability if the stratum corneum barrier becomes damaged by physical or chemical agents. Many substances will penetrate aged skin more easily than young skin.