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10. Describe the structure and function of hair and nails and explain the process by which these grow. Accessory structures or appendages of integument include hair, nails, and glands; derived from epithelium only;assist in overall function of integumentary system:•Hair (pili) – small filamentous structures that protrude from surface of skin over entire body except in regions with thick skin, lips, and parts of external genitalia •Hair – too sparse in humans to play a significant role in thermoregulation, as it does in other mammals:+Does provide protection by preventing substances and organisms from external environment from entering eyes and nose+On head, protects underlying skin of scalp from UV radiation and mechanical trauma+Hairs are associated with a small sensory neuron; plays a role in detecting changes in environment•Hair – composed of two main parts; shaft and root; both made up of stratified squamous keratinized epithelial cells in various stages of development+Shaft-Portion of hair that projects from skin’s surface-Made up of columns of dead keratinized epithelial cells that have completed keratinization process+Root -Segment of hair embedded in dermis; surrounded by a small sensory neuron-Root is indented at its base by a projection of blood vessels from dermis called a hair papilla-Root and hair papilla are collectively known as hair bulb-Many epithelial cells in root are still alive; have not completed keratinization process+Matrix – small number of keratinocytes found at base of root; actively divide+Root is embedded in hair follicle; an infolding of epidermis called epithelial root sheath; extends deep into dermis or even hypodermis+Epithelial root sheath has an outer component that anchors follicle to dermis and an inner component that is anchored tightly to hair root+Strand of hair has three visible regions in a transverse section:-Inner medulla – soft core only found in thick hair (like on head); composed of a soft keratin
-Middle cortex – highly structured and organized with several layers of keratinocytes containing hard keratin; provides strength to strand-Outermost cuticle – consists of a single layer of overlapping keratinocytes containing hard keratin; provides mechanical strength+Surrounding epithelial root is a dermal root sheath; consists of connective tissue; supports follicle and keeps it separate from dermis-Small bands of smooth muscle called arrector pili muscles attach to dermal root sheath on one end and dermal papillary layer on the other-Contraction of these tiny muscles causes hair to stand up (piloerection); gives skin a dimpled appearance, commonly called “goosebumps”•Hair growth averages between 1-1.5 cm per month; varies between individuals; growth is not continuous but occurs in a cycle with following two main phases:+Growth stage, mitosis occurs in matrix:-Cells divide and push cells above them farther away from blood supply where they keratinize and die