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skin_phys - Skin Physiology Irritants Dry Skin and...

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Skin Physiology, Irritants, Dry Skin and Moisturizers Christina Marino, MD, MPH Report Number 56-2-2001a August 2001 (Revised June 2006) Washington State Department of Labor and Industries Safety and Health Assessment and Research for Prevention Program http://www.lni. wa .gov/Safety/Research/Dermatitis/ 1-888-66-SHARP Skin is a physical barrier to the environment. It is the alteration of the barrier properties and actual damage to this barrier that causes dryness and dermatitis when the skin is exposed to water, soaps, gloves, chemicals and harsh weather conditions. The repair of the damage by moisturizers is related to the physical and chemical interactions of the ingredients with the natural skin barrier. Skin Physiology: Structure and Function Basics Skin has two main structural layers—the epidermis and the dermis (Figure 1). The epidermis is the outer layer of skin, which serves as the physical and chemical barrier to the interior body and exterior environment. The dermis is the deeper layer providing the structural support of the skin. Epidermis and Stratum Corneum: The Structure and Growth of Skin The epidermis consists of stacked layers of cells in transition. Protein bridges called desmosomes connect the cells. The bottom layer of cells adjacent to the dermis are the basal cells which reproduce. As the cells mature, they move towards the outer layer of skin leading to terminal differentiation of the cells. During the process of maturation, the physiology, chemical composition, shape and orientation of the cells change. When the cells reach the top layer of skin—the stratum corneum—the cells are called corneocytes and are no longer viable. Corneocytes lack a nucleus and cellular structures. Corneocytes are flat, hexagonal-shaped cells filled with water-retaining keratin proteins surrounded by a protein envelope and lipids (Figure 2). The cellular shape and the orientation of the keratin proteins add strength to the stratum corneum. There are 10-30 layers of stacked corneocytes. The thicker skin on the palms and soles has the most layers of stacked corneocytes. The cells remain connected to each other by protein bridges called desmosomes. Stacked bilayers of lipids surround the cells in the extracellular space. The resulting structure is the natural physical and water-retaining barrier of the skin. Filaggrin: The Breakdown of Skin During the process of maturation, the viable cells moving towards the stratum corneum begin to clump proteins into granules. These granules are present in the granular cell layer of the skin and are filled with a protein called filaggrin. Filaggrin becomes complexed with keratin proteins in the granular cells. This complex protects filaggrin from proteolytic breakdown.
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