What is Skin?

The skin is the largest organ in the body, comprising about 15% of the body weight. The total skin surface of an adult ranges from 12 to 20 square feet. In terms of chemical composition, the skin is about 70% water, 25% protein and 2% lipids. The remainder includes trace minerals, nucleic acids, glycosoaminoglycans, proteoglycans and numerous other chemicals.

The skin consists of three main layers: epidermis, dermis and subcaneous tissue.

The epidermis
The epidermis is the topmost layer of the skin. It is the first barrier between you and the outside world. The epidermis consists of three types of cells keratinocytes, melanocytes and Langerhans cells. Keratinocytes, the cells that make the protein keratin, are the predominant type of cells in the epidermis. The total thickness of the epidermis is usually about 0.5 - 1 mm. At the lowermost portion of the epidermis are immature, rapidly dividing keratinocytes. As they mature, keratinocytes lose water, flatten out and move upward. Eventually, at the end of their life cycle, they reach the uppermost layer of the epidermis called stratum corneum. Stratum corneum consists mainly of dead keratinocytes, hardened proteins (keratins) and lipids, forming a protective crust. Dead cells from stratum corneum continuously slough off and are replaced by new ones coming from below. The skin completely renews itself every 3 - 5 weeks. Most mild peels work by partly removing the stratum corneum and thus speeding up skin renewal.

Another significant group of cell in the epidermis are melanocytes, the cells producing melanin, the pigment responsible for skin tone and color. Finally, Langerhans cells are essentially a fore post of the immune system in the epidermis. They prevent unwanted foreign substances from penetrating the skin.

The condition of epidermis determines how "fresh" your skin looks and also how well your skin absorbs and holds moisture. Wrinkles, however, are formed in lower layers.

The dermis
The dermis is the middle layer of the skin located between the epidermis and subcutaneous tissue. It is the thickest of the skin layers and comprises a tight, sturdy mesh of collagen and elastrin fibers. Both collagen and elastin are critically important skin proteins: collagen is responsible for the structural support and elastin for the resilience of the skin. The key type of cells in the dermis is fibroblasts, which synthesize collagen, elastin and other structural molecules. The proper function of fibroblasts is highly important for overall skin health.

The dermis also contains capillaries (tiny blood vessels) and lymph nodes (depots of immune cells). The former are important for oxygenating and nourishing the skin, and the latter -- for protecting it from invading microorganisms.

Finally, the dermis contains sebaceous glands, sweat glands, hair follicles as well as a relatively small number of nerve and muscle sells. Sebaceous glands, located around hair follicles, are of particular importance for skin health as they produce sebum, an oily protective substance that lubricates and waterproofs the skin and hair. When sebaceous gland produce too little sebum, as is common in older people, the skin becomes excessively dry and more prone to wrinkling. Conversely, overproduction or improper composition of sebum, as is common in adolescents, often leads to acne.

The dermis is the layer responsible for the skin's structural integrity, elasticity and resilience. Wrinkles arise and develop in the dermis. Therefore, an anti-wrinkle treatment has a chance to succeed only if it can reach as deep as the dermis. Typical collagen and elastin creams, for example, never reach the dermis because collagen and elastin molecules are too large to penetrate the epidermis. Hence, contrary to what some manufacturers of such creams might imply, these creams have little effect on skin wrinkles.

Subcutaneous tissue
Subcutanous tissue is the innermost layer of the skin located under the dermis and consisting mainly of fat. The predominant type of cells in the subcutaneous tissue is adipocytes or fat cells. Subcutaneous fat acts as a shock absorber and heat insulator, protecting underlying tissues from cold and mechanical trauma. Interestingly, most mammals lack subcutaneous tissue because their fur serves as a shock absorber and heat insulator. Sweat glands and minute muscles attached to hair follicles originate in subcutaneous tissue.

The loss of subcutaneous tissue, often occurring with age, leads to facial sag and accentuates wrinkles. A common procedure performed by dermatologists to counteract this process is to inject fat (collected elsewhere in the body) under the wrinkles on the face.

Important Skin Molecules

Health, resilience and youthful appearance of the skin depends, among other things, on several key classes of biological molecules, just like the quality of a house depends on the quality of bricks, beams and concrete. The most important skin molecules are collagen, elastin, glycosoaminoglycans and proteoglycans.

Collagen is a protein forming the structural grid that holds other skin structures. It plays a role somewhat similar to that of steel rods in a reinforced concrete block. It gives the skin its strength and durability. As any other protein, collagen is composed of amino acids. However, it is unusually rich in a few specific amino acids, proline, hydroxyproline, lysine and glycine. Some experts believe that foods or supplements rich in these amino acids may benefit the skin by stimulating collagen production. There is a number of other ways to stimulate collagen production, including topical vitamin C and copper peptides. Increasing collagen production is important because age-related decline in the collagen synthesis is partly responsible for the signs of skin aging such as thinning, wrinkles and sagging.

Elastin is also a protein. It is more stretchable than collagen and helps maintain skin resilience and elasticity. Elastin contains two special amino acids, desmosine and isodesmonsine. When both elastin and collagen and abundant and undamaged, the skin easily regains its shape after being stretched or folded. Just as collagen, elastin deteriorates with age, leading to wrinkles and facial sag.

Glycosoaminoglycans (GAGs) and proteoglycans are special biological polymers whose key role is to hold moisture in the skin. In essense, they are extremely effective natural moisturizers - far more effective that common cosmetic moisturizers. Hydrated GAGs and proteoglycans help the skin stay plump and fresh and provide mechanical support for skin cells. GAGs are composed of special units (mainly water-holding sugars) such as glucosamine hydrochloride, N-acetyl glucosamine, and glucosamine sulfate. These units combine to form various types of GAGs, such as hyaluronic acid, keratin sulfate, heparin, heparin sulfate, dermatin sulfate, and chondroitin sulfate. Proteoglycans are larger than GAGs and are formed when certain types of GAGs are attached to a protein backbone. Since GAGs and proteoglycans are composed largely of water-holding sugars, supplementing one's diet with these sugars may enhance the skins production of GAGs and proteoglycans. In particular, N-acetyl-D-glucosamine, D-glucosamine hydrochloride, and D-glucosamine sulfate are often used as supplemets to increase skin moisture.

Retinoids

Tretinoin a.k.a. trans-retinoic acid (the active ingredient in such popular products as Retin A and Renova) is one of the very few agents whose skin rejuvenation effects have been documented in many well-designed studies. Tretinoin works by triggering molecular switches in skin cells called retinoid receptors. These switches affect many intracellular processes including cell growth and differentiation, synthesis and breakdown of proteins (including collagen and elastin), sebum production, and possibly even the cell's lifespan clock (telomeres).

However, tretinoin is not the only substance capable of activating retinoid receptors. In fact, a number of related but distinct agents (collectively called retinoids) can treat skin care problems ranging from wrinkles, to acne, to age spots.

Bottom line
Tretinoin (Retin A) remains the primary wrinkle-fighting retinoid. However, due to individual variability some people may have better overall results with other retinoids. It is also worthwhile to keep an eye on the further advances in the retinoid research.

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