Bio 102 Integumentary System: Skin Your skin represents the largest (greatest surface area) organ system of the body. Your skin serves to provide a physical barrier to protect against pathogens, helps regulate body temperature and water through perspiration, and in some mammals assists in the production of vitamin D. Skin is composed of the: 1. epidermis: Composed of stratified squamous epithelium. Land vertebrates have several layers of keratinocytes which produce keratin, a water insoluble molecule which aids in water retention. Epidermal cells die (we shed 1.5 lbs a year) and are pushed to the surface forming a dead layer of cells which are then sloughed off. Each month, you replace ALL of your surface skin. Phagocytes assist in defending against pathogens. Specialized keratinocytes evolved into hair/nails/claws/beaks. Hair is nothing more than dead keratinocytes. 2. dermis: Composed of connective tissue (dense) rich in collagen fibers, blood and lymphatic vessels and nervous tissue. The dermis “feeds” the lower epidermis through simple diffusion. The dermis may contain lubricating sebacd eous (oil) glands, sweat glands (2+million), hair follicles (100,000 hairs/human scalp) and/or moistening mucous glands(frogs). Pigmentation occurs due to melanin producing cells which give melanin to the keratinocytes. Lightly pigmented skin displays as pink due to the hemoglobin found in blood. Melanin protects against UV radiation…a “tan” is the bodies production of excess melanin in response to exposure to UV light. Melanocytes also produce the precursor to vitamin D in the presence of sunlight. Vitamin D helps the body control calcium and phosphate levels. If the blood levels of these minerals become too low, the body may produce hormones that cause calcium and phosphate to be released from the bones. This leads to rickets, a condition characterized by weak and soft bones. Overexposure to UV radiation damage collagen fibers and DNA (cancer). Aging results in fewer and less elastic fibers.
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