Normal Skin Layers
The skin is made up of 3 layers - the epidermis, dermis, and subcutaneous tissue.
The 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.
The 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.
The 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.
The 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 Wrinkles
As 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.