Normal Skin LayersThe skin is made up of 3 layers - the epidermis, dermis, and subcutaneous tissue.
EpidermisThe epidermis is the outer layer and functions as a barrier to the external environment. The cells of the epidermis, keratinocytes, move from the bottom layer of the epidermis to the top layer building up a large amount of keratin and developing a tough outer shell. Once these cells reach the top layer, they flake off. If this process becomes abnormal the skin can look scaly.
DermisThe second layer of skin is the dermis, which contains the structural elements of the skin, the connective tissue. There are various types of connective tissue with different functions. For example, collagen gives the skin its strength, proteins called glycosaminoglycans give the skin its turgor, and elastin fibers give the skin its elasticity or spring.
Dermal-Epidermal JunctionThe junction between the dermis and the epidermis is an important structure. The dermal-epidermal junction interlocks forming fingerlike projections called rete ridges. The cells of the epidermis receive their nutrients from the blood vessels in the dermis. The rete ridges increase the surface area of the epidermis that is exposed to these blood vessels and the needed nutrients.
Subcutaneous TissueThe bottom layer of skin is the subcutaneous tissue containing fat cells. These fat cells provide insulation to the body and make the skin look plump or full.
Chronological Aging and WrinklesAs a person ages the epidermal cells become thinner and less sticky. The thinner cells make the skin look noticeably thinner. The decreased stickiness of the cells decreases the effectiveness of the barrier function allowing moisture to be released instead of being kept in the skin. This causes dryness. The number of epidermal cells decreases by 10% per decade and they divide more slowly as we age making the skin less able to repair itself quickly.
The effects of aging on the dermal layer are significant. Not only does the dermal layer thin, but also less collagen is produced, and the elastin fibers that provide elasticity wear out. These changes in the scaffolding of the skin cause the skin to wrinkle and sag. Also, sebaceous glands get bigger but produce less sebum, and the number of sweat glands decreases. Both of these changes lead to skin dryness.
The rete-ridges of the dermal-epidermal junction flatten out, making the skin more fragile and making it easier for the skin to shear. This process also decreases the amount of nutrients available to the epidermis by decreasing the surface area in contact with the dermis, also interfering with the skin's normal repair process.
In the subcutaneous layer the fat cells get smaller with age. This leads to more noticeable wrinkles and sagging, as the fat cells cannot "fill in" the damage from the other layers.
Aging Effects of the Sun and WrinklesExposure to ultraviolet light, UVA or UVB, from sunlight accounts for 90% of the symptoms of premature skin aging. Most of the photoaging effects occur by age 20. The amount of damage to the skin caused by the sun is determined by the total lifetime amount of radiation exposure and the person's pigment protection.
Sunlight Effects on the EpidermisChanges in the epidermis caused by the sun include thinning of the epidermis and the growth of skin lesions such as actinic keratoses, basal cell carcinomas, and squamous cell carcinomas.
Sunlight Effects on the DermisIn the dermis, sun effects cause collagen to break down at a higher rate than with just chronologic aging. Sunlight damages collagen fibers and causes the accumulation of abnormal elastin. When this sun-induced elastin accumulates, enzymes called metalloproteinases are produced in large quantities. Normally, metalloproteinases remodel sun-injured skin by manufacturing and reforming collagen. However, this process does not always work well and some of the metalloproteinases actually break down collagen. This results in the formation of disorganized collagen fibers known as solar scars. When the skin repeats this imperfect rebuilding process over and over wrinkles develop.
Free Radicals and WrinklesFree radicals are unstable oxygen molecules that have only one electron instead of two. Because electrons are found in pairs the molecule must scavenge other molecules for another electron. When the second molecule looses its electron to the first molecule, it must then find another electron repeating the process. This process can damage cell function and alter genetic material. Free radical damage causes wrinkles by activating the metalloproteinases that break down collagen. There are several factors that start this cascading process including exposure to even small amounts of UV radiation in sunlight, smoking, and exposure to air pollution.
Hormone Effects and WrinklesIt is likely that there are skin changes as a result of the hormonal effects of menopause or decreased estrogen production. However, studies in humans have not documented which skin changes are specific to decreased estrogen and which skin changes are a result of sun exposure or just normal chronological aging. In animal experiments lack of estrogen can cause a decrease in collagen levels of 2% per year and a decrease in skin thickness of 1% per year.
Muscle Use and WrinklesHabitual facial expressions cause the skin to wrinkle as it looses elasticity. Frown lines between the eyebrows and crows feet radiating from the corners of the eyes develop as the tiny muscles in those areas permanently contract.
Gravity and WrinklesThe effects of gravity make the loosening of the skin more apparent as skin sags more. This causes jowls and drooping eyelids.
