Layers of the skin
The epidermis is the most superficial layer of skin that you can see, feel and touch, and is further subdivided into several layers. The epidermis only 0.1mm thick, and acts as a barrier preventing water loss, UV damage, toxin, antigen and pathogen invasion. Most of this barrier function is provided only by the uppermost layer, the stratum corneum and production of a healthy stratum corneum is the real function of the epidermis. The stratum corneum is also covered with a thin film of sebum, a substance which is secreted by the sebaceous glands, keeping the skin lubricated and healthy.The cells in the basal layer also contain melanocytes which produce the melanin that provides skin with it’s colour.
The dermis lies beneath the epidermis, connected by a continuous membrane, a complex structure called the basement membrane zone. The dermis is the fibrous part of the skin consisting chiefly of collagen produced by fibroblasts (cells that build). Variations in skin thickness with body site (e.g., thick on the back and thin around the eyes) are due to differences in the dermal thickness. The dermis acts as a barrier against physical trauma and supports the blood vessels, nerves and lymphatics, thus ensuring the integrity of the overlying epidermis.
Your skin is constantly renewing itself. Skin cell renewal occurs every 2-4 weeks. Fresh cells develop in the bottom layer of the epidermis, then make their way up to the surface (stratum corneum). Eventually these cells dehydrate and flatten until they slough off and are replaced by new cells. As we age, this renewal process slows down.
Ageing of the Skin
There are two distinct types of skin ageing, intrinsic and extrinsic:
Intrinsic Skin Ageing
The mechanisms behind intrinsic ageing remain a mystery, though the accumulation of "free radical damage" or an inbuilt property of DNA to wind down have both been much mooted. Whatever the causes, the alterations are not difficult to see. The epidermis becomes thinner and flatter, but interestingly the stratum corneum stays the same thickness. The epidermal cells show reduced division activity and are smaller in older age. The dermis becomes thinner as well and, in fact, at the age of 80 both the epidermis and the dermis are on average about one third as thick as they were at the age of 18. This leads to lines and wrinkles, thin and transparent skin, loss of underlying fat leading to hollow cheeks, and eye sockets.
The sun-exposed skin of the face, neck and extensor aspects of the forearms looks much older than the usually covered skin of the breasts in women, or the upper inner arms of men. The exposed skin looks old because it is damaged by the sun's ultraviolet radiation (UVR). This damage (which is known as photo damage) is cumulative so that the more solar irradiation one has soaked up over the years, the older one looks. As you probably realize, continual exposure to solar ultraviolet radiation is carcinogenic and causes both melanoma and non-melanoma skin cancer.
This 'pseudo-ageing' is known as photo-ageing and is quite different from the ordinary changes due to chronological or intrinsic ageing. It is thought that both long-wave UVR (known as UVA) and medium-wave UVR (known as UVB) are involved in causing the damage, but UVA has a particularly important role.
The major alteration in sun-exposed skin, accounting for the bulk of the aged appearance, is in the dermal connective tissue and is known as solar elastotic degeneration, or more simply as solar elastosis. The greater the solar exposure the more extensive the solar elastosis. The result of this is to substantially reduce the "springiness" and the support function of the dermal connective tissue. This can be seen when a fold of skin is picked up from the back of the hand, which then falls back much more slowly in a photo-damaged subject compared with a non-sun-damaged person of the same age. The changed mechanical function also accounts for appearance of the wrinkling and fine lines that develop - especially at the sides of the eyes ("crow’s feet") and around the mouth (“Marionette lines”).
In addition to dermal fillers and botox, Retinoids (analogues of vitamin A) have several uses in dermatology. Tretinoin ( "all trans-retinoic acid") has been used topically for the past 30-40 years in the treatment of acne. In the late 1980s it was noticed that mature patients with acne treated with topical tretinoin for extended periods showed a concomitant improvement in any accompanying signs of photo-damage. A few years later, extensive clinical studies confirmed the clinical findings and now the agent (tretinoin 0.05% cream) has been licensed for the treatment of photo-damage.
Numerous other substances have been promoted for their supposed value in photo-damage but none seems to be as effective as the topical retinoids. Alpha hydroxy acids - and in particular lactic, glycolic acid and pyruvic acids have been used at concentrations of 2-6%. They certainly enhance desquamation but there is no solid evidence that they have any lasting effects on significant photo-damage.
Controlled ablation of the superficial layers improves the dyspigmentation and fine lines seen in photo-damage and removes many of the changes on sun-damaged skin that are so unpopular. Various methods have been used to remove the skin surface including physical dermabrasion with a rotating abrasive disc, microdermabrasion with a high-powered stream of silica particles and chemical peeling. The last mentioned is quite popular and several different corrosive agents have been employed. The most often used is 70% glycolic acid. Many patients want a quick result with minimal downtime and opt for regular superficial peels. The result is fresh, clear youthful skin.