Skin and derivatives of skin
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Skin and Derivatives of Skin. Main Layers of Skin. Epidermis (outer layer) Dermis (inner layer). Epidermis. Functions Protection against physical, chemical, and radiation injury.

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Main layers of skin l.jpg
Main Layers of Skin

  • Epidermis (outer layer)

  • Dermis (inner layer)


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Epidermis

  • Functions

    • Protection against physical, chemical, and radiation injury.

    • Protection against microorganisms (by barrier, by bacterial binding to cells which will be released and by some phagocytosis)

    • Reduction of fluid loss


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Sub-layers of Epidermis

  • Basal lamina – separates epidermis from dermis

  • Stratum basale

    • Deep layer contains living cells that divide to produce new cells

  • Stratum spinosum

  • Stratum granulosum

  • Stratum corneum

    • Surface layer of dead cells


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Stages of Epidermal Cell Development

  • Synthetic Stage

    • Most cells found in lower part of spinosum layer

    • Synthesis of specific proteins and lipids (protein filaments, a polar lipid material that will be released from the cell, and keratohylain granules which will form the protein keratin)


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Stages (Continued)

  • Transformation Stage

    • Occurs in layers below Stratum corneum

    • Intracellular enzymes are released that attack and lyse all organelles

    • Keratohyalin material fills cell


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Stages (Continued)

  • Terminal Stage

    • Excessive loss of fluid from cells

    • The narrow intracellular space becomes filled with lipid material

    • Cells at the end of this stage are found in upper Stratum corneum layer


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Dermis of Skin

  • Thick layer of:

    • Fibrous connective tissue (anchoring fibers, collagen fibers, elastic fibers)

    • Muscle tissue

    • Adipose tissue

    • Blood vessels

    • Nerves including sensory receptors


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Dermal Structures of Epidermal Origin

  • Sebaceous Glands

    • usually associated with hair follicles – serve primarily to lubricate hair

    • These glands are important in waterproofing the pelage

    • Secretions are mainly cellular debris and lipids (oils) from cells that breakdown

    • Androgens stimulated these glands


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Structures (Continued)

  • Sweat Glands

    • Hypotonic secretions that contain a lot of water

      • Apocrine Type – open into a hair follicle

      • Eccrine Type – open onto the surface of the skin independent of a hair follicle

        • Responsible for most of the sweat that is produced


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Structures (Continued)

  • Sweat functions in

    • Evaporative Cooling

    • Improving tactile sensitivity

  • No sweat glands in – Cetaceans, Sirenians, Elephants, Pangolins, Echidnas, Moles, Rodents


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Structures (Continued)

  • Only apocrine sweat glands in – Bats, some primitive primate species

  • More eccrine than apocrine sweat glands in – higher primates and humans


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Structures (Continued)

  • Scent glands

    • May be modified sebaceous glands or modified sweat glands

    • Used for defense, territory marking, orientation, social interactions


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Structures (Continued)

  • Hair

    • Hair is a keratinized derivative of the epidermis which is found in all mammals at some point in life cycle

    • Structure is

      • Central medulla (continuous or with air spaces)

      • Cortex which constitutes bulk of hair follicle

      • Cuticle (layer of outer scales that vary in shape)


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Types of Hair

  • Vibrissae – Long, stiff hairs well well innervated bases, tactile receptors, definitive growth

  • Guard Hairs – Serve primarily for protection

    • Spines – enlarged, stiff guard hairs, ex. quills of porcupines, definitive growth, shed and replaced periodically


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Types of Hair (Continued)

  • Guard Hairs (Continued)

    • Bristles – long, firm hairs with angora growth, ex. horse and lion manes

    • Awns – firm, expanded distal portion with smaller base, definitive growth


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Types of Hair (Continued)

  • Underhair – primarily for insulation

    • Wool – long, angora growth

    • Fur – short, definitive growth

    • Velli – down or fuzz, first hair on an animal


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Color of Hair

  • Pigment molecules

    • Eumelanin – provides shades of black and brown

    • Pheomelanin – produces shades of red and yellow


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Pigment in Cells

  • Contained in melanosomes within melanocytes

  • Melanosomes are transferred from melanocytes to epithelial cells that become keratinocytes

  • Pigment in melanocytes and keratinocytes shield stratum basale from UV radiation

  • Pigment also acts as free radical scavenger, preventing cellular damage


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Pigment in Hair

  • Melanocytes transfer melanosomes to epithelial cells at base of hair shaft

  • Melanosomes are phagocytized by cells that will form the cortex of the hair shaft


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Head Ornaments

  • Occur only in Orders Artiodactyla and Perissodactyla


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True Horns

  • Found only in Family Bovidae of O. Artiodactyla

    • An inner core of bone from the frontal bone and a sheath of keratinized epidermis

    • Grow continuously throughout life of animal

    • Unbranched

    • Never shed

    • Usually in males but can be in females too


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Pronghorns

  • Found only in Pronghorn antelope (F. Antilocapridae, O. Artiodactyla)

    • Similar in structure to true horns except

      • Sheath is shed annually

      • Sheath is branched but bone is not

      • Occur in males and some females, horn on female may be smaller than on male and could be unbranched


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Giraffes Horns

  • F. Giraffidae of O. Artiodactyla

    • Derive from separate bony ossifications that fuse to the skull near the junction of the parietal and frontal bones (are not projections from frontal bone!)

    • Permanently covered by skin and hair

    • Occur in both sexes


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Rhinoceros Horns

  • O. Perissodactyla

  • Horn is composed of large mass of elongated, dermal papillae that fuse to form the horn

  • No core of bone

  • Horn is really keratinized skin


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Antlers

  • F. Cervidae of O. Artiodactyla

    • Made entirely of bone

    • Branched

    • Outgrowths of frontal bone

    • Shed annually

    • Covered by “velvet” while growing

    • Size cannot be used to directly age and animal, size depends upon nutritional state


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