An Overview of Cell Structure within the Epidermis

The skin is the largest organ of the body occupying almost 21.5 square feet of surface area. The skin consists of three main divisions; the epidermis (outermost layer), the dermis (vascular connective tissue below the epidermis) and the hypodermis (the deepest portion of the skin). Many topical non-prescription skin care agents may have an indirect effect on the dermis. However, the layer of the skin that is most directly and visibly affected is the epider mis. The epidermis is the nonvascular layer of skin that visibly reflects skin health or disease.

Within the epidermis are found four dif ferent cell types that have different functions, but they collectively govern how the skin functions and responds. The appearance of our skin in terms of texture, evenness of color, hydration and overall youthfulness and health is greatly depen dent on the interaction of these cellular functions.

Currently, there is a great deal of focus on resolving various common skin conditions and bringing about facial skin rejuvenation through the application of topical agents. Most medically based topical therapies are designed to assist in resolving textural changes, lines and wrinkles, pigmentary changes, improving skin barrier function and moisture retention, and enhance the skin's immune function in order to prevent damage at the cellular level. The following briefly details the role of three of these four cell types and their involvement in the appearance of aging or compromised skin.

KERATINOCYTES

Dead cells are constantly sloughed from the upper surface of the epidermis and are replaced by new cells being generated from the deeper layers. This process is often referred to as "cellular renewal". As cells progress through various stages, they finally die and harden or keratinize. This dead cell layer, or keratinocytes, makes up the outer most layer of the epidermis, the stratum corneum. These hardened or cornified cells are designed to act as a protective barrier. Living cells would be unable to provide the protective qualities necessary to insulate the living tissue from exposure to possible damaging environ mental substances and organisms. The living epidermal cells therefore die by an extremely complex process that enables humans to survive as land animals.

As the live cell begins migrating upward, it is surrounded by a membrane that is filled with essential lipid substances. When the cell reaches its destination and hardens and dies, the cell membrane, along with its lipid content, is discharged into spaces between the cells. These lipids then provide the critical barrier that assists the skin in main taining an adequate moisture content and also provide a kind of "waterproofing" that pre vents penetration by unwanted substances.

Cumulative sun exposure slows down cell turnover and also triggers free radical activity that damages the cell membrane. This can result in many of the surface changes that appear in aging skin, espe cially fine lines and wrinkles and coarse thickened texture. Recent medical research has proven that certain topically applied agents can dramatically lessen or reverse abnormal cell functions brought about by sun damage. By normalizing and enhanc ing cellular functions, the appearance of photodamaged aged skin can be greatly im proved.

MELANOCYTES

Melanocytes are cells that are distributed among the keratinocytes. Melanocytes contain an enzyme called tyrosinase that is able to form a pigmented substance called melanin. Melanin is what gives the skin its depth of pigmentation or color. The part of the melanocyte cell that forms melanin or pigment is called the melanosome. As pigment granules form, they are transferred from the melanocytes to the keratinocytes and then are carried to the surface of the epidermis. A major purpose of melanin is to absorb ultraviolet radiation and provide protection to newly dividing cells in the deeper layers. Without protection, the UV radiation can cause the cell's genetic coding or DNA to mutate. This can signifi cantly alter the manner in which the cell functions, resulting in changes associated with aging skin or compromised immune function. In addition, UV exposure can af fect the functioning of the melanocytes, resulting in abnormal increase in the pro duction of melanin or pigment. This may be visually manifested in freckles, uneven patchy coloring or solar lentigos (often referred to as age spots).

Medical research has focused on methods of regulating melanin in order to control abnormal production and resolve the

appearance of hyperpigmented lesions. While there are a number of topical agents that inhibit the synthesis of melanin, researchers are focusing on breakthroughs that directly control the output of the tyro sinase enzyme resulting in greater control over pigmentary changes.

Topical skin care preparations that assist in increasing the rate of cellular turnover and normalization of the epidermis also can lessen the appearance of abnormally pigmented areas because newer cells appear to contain fewer pigment grandules. As the older more damaged cells are replaced by healthier cells, the pigment appears more evenly distributed.

Most recently, technological breakthroughs have focused on protecting the cell membrane and preventing damage from occurring.

LANGERHANS CELLS

Langerhans cells are a primary defense mechanism in protecting the skin from infection and cancerous tumor growth and are part of an overall category referred to as "macrophages". Langerhans cells have "curly-cue" like extensions that reach to the surface of the skin. These receptors have the ability to selectively recognize particles that come in contact with the skin and determine what processes should be initiated in order to protect or repair the skin. Langerhans cells are believed to be crucial to the overall immune functions of the skin and appear to act as a kind of surveillance detector that aids in initiating defensive responses when the skin is exposed to undesirable elements.

The number of skin macrophages, or Langerhans cells, is directly related to skin aging. Decreased activity of the Langerhans cells is believed to be affected greatly by exposure to UV light. UV exposure triggers free radical formation and Langerhans cells are extremely vulnerable targets of free radi cal attack. Also, when Langerhans cells are compromised due to cumulative environ mental damage, it is believed that wound healing is slower and collagen formation is affected.

Currently, research is focusing on topical substances that boost and protect local and systematic immune activities of the Langer hans cells. Some studies indicate that by enhancing this function, the skin is able to increase collagen production and elastic ity in addition to a significant reduction in reactions from environmental irritants.