INTEG

• Integumentary system– Consists of the skin and accessory organs; hair, nails, andcutaneous glands
• Skin is the most vulnerable organ– Exposed to radiation, trauma, infection, and injurious chemicals
• Skin is body’s largest and heaviest organ– Covers 1.5 to 2.0 m2 ; composes 15% of body weight
• Layers– Epidermis: stratified squamous epithelium– Dermis: deeper connective tissue layer– Hypodermis—connective tissue layer below dermis (notpart of skin, but associated with it)
• Thick skin covers front of hands, bottoms of feet• Has sweat glands, but no hair follicles or sebaceous (oil) glands• Epidermis 0.5 mm thick
• Thin skin covers rest of the body• Possesses hair follicles, sebaceous glands, and sweat glands• Epidermis about 0.1 mm thick
• Skin thickness ranges from 0.5 to 6 mm
• Function of the skin :

  • synthesizes vitamin D
  • resistance to trauma
  • sensation
  • Thermoregulation
  • Nonverbal communication
• Epidermis —keratinized stratified squamous epithelium– Includes dead cells at skin surface packed with tough keratin protein– Lacks blood vessels• Depends on the diffusion of nutrients from underlying connective tissue– Contains sparse nerve endings for touch and pain
• Stem cells• Undifferentiated cells that give rise to keratinocytes• In deepest layer of epidermis (stratum basale)
• Keratinocytes• Great majority of epidermal cells• Synthesize keratin
• Melanocytes• Synthesize pigment melanin that shields DNA fromultraviolet radiation• Occur only in stratum basale but have branched processesthat spread among keratinocytes and distribute melanin
• Tactile cells• Touch receptor cells associated with dermal nerve fibers• In basal layer of epidermis
• Dendritic cells• Macrophages originating in bone marrow that guardagainst pathogens• Found in stratum spinosum and granulosum
• Stratum basale (deepest epidermal layer)– A single layer of stem cells and keratinocytes resting on the basement membrane• Stem cells divide and give rise to keratinocytes that migrate toward skin surface to replace lost cells– Also contains a few melanocytes and tactile cells
• Stratum spinosum– Several layers of keratinocytes joined together by desmosomes andtight junctions– Named for appearance of cells after histological preparation (spiny)– Also contains some dendritic cells
• Stratum granulosum– Three to five layers of flat keratinocytes– Cells contain dark-staining keratohyalin granules
• Stratum lucidum– Thin, pale layer found only in thick skin– Keratinocytes packed with clear protein eleidin
• Keratinocytes are produced by mitosis of stem cells instratum basale or mitosis of keratinocytes in deepest part ofstratum spinosum
• Calluses or corns—thick accumulations of deadkeratinocytes on hands or feet
• Dermis —connective tissue layer beneath epidermis– Ranges from 0.2 mm (eyelids) to 4 mm (palms, soles)– Composed mainly of collagen– Well supplied with blood vessels, sweat glands, sebaceous glands, and nerve endings– Houses hair follicles and nail roots– Is the tissue of the facial skin to which skeletal muscles attach and cause facial expressions of emotion– Has a wavy, conspicuous boundary with the superficial epidermis
• Dermal papillae are upward, finger-like extensions of dermis
• Epidermal ridges are downward waves of epidermis
• Prominent waves on fingers produce friction ridges of fingerprints
• Papillary layer—superficial zone of dermis– Thin zone of areolar tissue in and near the dermal papilla– Allows for mobility of leukocytes and other defense cells– Rich in small blood vessels
• Reticular layer—deeper and thicker layer of dermis– Consists of dense, irregular connective tissue– Stretch marks (striae): tears in the collagen fiberscaused by stretching of the skin due to pregnancy orobesity
• Hypodermis– Subcutaneous tissue– Has more areolar andadipose than dermis has– Pads body and binds skin tounderlying tissues– Common site of druginjection since it has manyblood vessels
• Subcutaneous fat– Energy reservoir– Thermal insulation– Thicker in women– Thinner in infants, elderly
• Melanin—most significant factor in skin color– Produced by melanocytes, accumulates in keratinocytes
• Two forms of the pigment:• Eumelanin —brownish black• Pheomelanin —reddish yellow (sulfur-containing)
• Exposure to UV light stimulates melanin secretion and darkens skin– This color fades as melanin is degraded and old cells are exfoliated
• Hemoglobin —pigment in red blood cells• Adds reddish to pinkish hue to skin
• Carotene —yellow pigment acquired from egg yolks and yellow/orange vegetables• Concentrates in stratum corneum and subcutaneous fat
• Cyanosis —blueness due to oxygen deficiency
• Erythema —redness due to increased blood flow to skin
• Pallor —paleness due to decreased blood flow to skin
• Albinism —milky white skin and blue-gray eyes due to genetic lack of melanin synthesizing enzyme
• Jaundice —yellowing due to bilirubin in blood (can becaused by compromised liver function)
• Hematoma —bruising (clotted blood under skin)
• Friction ridges —markings on the fingertips that leave oily fingerprints on surfaces we touch– Everyone has a unique pattern formed during fetal development that remains unchanged throughout life– Not even identical twins have identical fingerprints– Allow manipulation of small objects