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functions of the integument (skin)
- 1. protective, waterproof barrier (keratin, hair, sebum)
- 2. maintains body temperature through capillary dilation/contraction & production of sweat (thermoregulation)
- 3. major sensory input for pain, pressure, heat & cold
- 4. performs important metabolic functions (eg. site of vitamin D synthesis)
- 5. immunological (contains langerhan's cells [DC], wandering lymphocytes)
- (6. absorption: nicotine, steroid hormones)
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the skin consists of two layers:
- 1. epidermis: SSKE (stratified squamous, keratinizing epithelium)
- 2. dermis
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epidermis
- outer layer of the integument firmly attached to the underlying dermis
- made up of distinct layers
- this region is enriched with nerve endings, is AVASCULAR & has a stratified squamous keratinizing epithelium
- epidermis keratinocyte layers exterior --> inward (apical --> basal)
- 1. Stratum Corneum
- 2. Stratum Lucidum
- 3. Stratum Granulosum
- 4. Stratum Spinosum
- 5. Stratum Basale (Germinativum)
- (Come Let's Get Sun Burned)
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Stratum Corneum [come]
- outermost region contains many layers of flattened, cornified, dead cells whose cytoplasms are packed with TONOFIBRILS
- number of cell layers varies with the thickness of the skin (more layers in thick skin like palms & soles of the feet)
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Stratum Lucidum [let's]
thin translucent layer apparent in some sections of thick skin, but not always present (possibly an artifact)
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Stratum Granulosum [get]
- contains 2-3 layers of flattened squamous epithelial cells whose cytoplasms are filled with coarse basophilic (keratohyalin) granules
- granules contain filaggrin (binds tonofilaments into tonofibrils) and loracrin (thickens/stiffens the outer surface of cells)
- last layer before the cells "die"
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Stratum Spinosum [sun]
- THICKEST of the living layers of the epidermis
- cells connected by abundant desomosomes
- have a spiny appearance
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Stratum Basale (Germinativum) [burned]
- innermost layer of cells rest on basement membrane & are single layer of cuboidal cells
- these cells divide regularly meaning a high number of mitotic figures are visible
- they move upward & replenish cells lost at the surface during epidermal turnover
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keratinocytes
cells of the epidermis that contain abundant water insoluble keratin assembled into 7-8 nm tonofilaments inside their cytoplasm
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dermal papilla
- finger like projections of the dermis into the epidermis which enhance the attachment of the epidermis to the underlying dermis
- these structures generate an enormous surface area for strong epidermal-dermal interaction
- stem cells are located near the tips of the dermal papilla
- primary ridge: encompassing single ridge over two dermal papilla
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dermis
connective tissue layer that lies deep to the epidermis and consists of a highly vascularized connective tissue with different types of glands, hairs and sensory structures
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hypodermis (superficial fascia)
- lies deep to the dermis, but is not technically a part of the skin
- several structures associated with the true skin (eg. hair follicles, sensory receptors) may be found in this region
- hypodermis contains adipose tissue & may contain skeletal muscle
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cells of the epidermis
- 1. Keratinocytes
- 2. Melanocytes
- 3. Langerhan's Cells
- 4. Merkel Cells
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Melanocytes
- pigment producing cells derived from the neural crest found in the basal layer
- have long processes containing melanosomes that extend among the keratinocytes
- melanosomes = membrane-bound organelles that make pigments (melanin and phaeomelanin)
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Do keratinocytes synthesize pigment?
- NO, melanocytes do
- keratinocytes phagocytose the pigment-containing cytoplasm of the melanocyte and ACQUIRE pigment
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What might increase melanocyte activity?
exposure to UV light as well as by melanocyte stimulating hormone (MSH)
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Langerhans Cells
- dendritic cells, functionally related to the monocyte-macrophage group (and thus derived from the bone marrow) found particularly in the upper layers of the stratum spinosum
- present antigen to lymphocytes
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Merkel Cells
- cells involved in sensory (mechano) reception & contain granules similar to those found in the cells of adrenal medulla
- occur in small numbers in the basal part of the epidermis
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psoriasis
- caused by pathological changes that lead to over-proliferation of the stratum basale layer
- can lead to basal cell carcinoma (most common form of skin cancer) if the changes are neoplastic
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melanoma
- a much more aggressive form of skin cancer caused by neoplastic changes in the melanocytes
- responsible for the majority of skin cancer deaths
- Initial diagnoses are usually made based upon visual appearance w/ mnemonic ABCDE:
- A - asymmetry
- B - Border (irregular)
- C - Color (usually multiple)
- D - diameter (> 6mm)
- E - enlarging
- good prognosis w/ excision of early identified tumors
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What are the two laybers of the dermis
- 1. papillary
- 2. reticular
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Papillary Dermis
- layer just deep to the epidermis composed of finger-like papilla that project toward the surface, forcing epidermis to acquire an undulating morphology
- this greatly increases the surface area for epidermal-dermal adhesion
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Why do papilla contain sensory nerve endings & capillary beds?
because they bring the dermis as close to the outside world as possible, an ideal location for two such entities
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Reticular Dermis
- constitutes the bulk of the dermis & contains large bundles of type I collagen fibrils [Langer lines] + elastic fibers
- reticular dermis contains sensory receptors, hairs & several types of glands
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What are 3 glands of the skin?
- 1. Eccrine Sweat Glands
- 2. Apocrine Sweat Glands
- 3. Sebaceous Glands
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Eccrine SWEAT Glands (merocrine)
- a basal coil from which a duct leads through the dermis into the epidermis assists in maintaining a normal body temperature
- secretory portions of eccrine sweat glands pump Na+ ions into the lumen to generate an osmotic gradient; water follows by osmosis. In addition to water, several protein components (e.g. dermcidin) and other waste products (urea) are added to the fluid
- ductal cells actively resorb sodium & passively reabsorb chloride; the final product to reach the surface is mostly water & a small amount of protein/sodium
- are found over most of the body but most concentrated in the forehead, axilla & palms of the hand
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What are eccrine sweat gland ducts made of?
stratified cuboidal epithelial cells rich in mitochondria
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cystic fibrosis
- patients have compromised removal of salts by eccrine sweat gland ducts
- --> a high salt content in sweat = marker of CF
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Apocrine Sweat Glands
- large coiled tubular glands that open into hair follicles rather than directly onto the skin surface
- found mainly in the axilla (armpit) & genitals
- less numerous than eccrine glands
- apocrine glands can store their secretion, & various hormonal/neural stimuli will cause them to secrete
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What is the likely primary functions of apocrine glands?
- to produce pheromones
- they become active at puberty
- mode of secretion = eccrine and apocrine (the secretory product as well as a portion of the cells themselves is released)
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Which is apocrine and which is eccrine?
- left: apocrine - larger lumen for sweat storage
- right: eccrine - smaller lumen, no storage
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Sebaceous Glands
- glands found all over the body EXCEPT at palms & soles that secrete sebum, which serves a natural lubricant of the hair and surface of the skin (contains TAGs, FAs, & cholesterol)
- sebaceous glands discharge contents via a SINGLE duct into the lumen of hair follicles
- mode of secretion = holocrine (entire cell & its product is released)
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Where do sebaceous glands secrete sebum directly onto the skin surface?
- fordyce spots (penis bumps)
- lip vermillion border
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Acne
- caused when sebum production (puberty) exceeds the capacity of the short ducts to drain the glands
- can become colonized by surface bacteria
- while bacterium is a normal component found in healthy skin, prolonged colonization of sebaceous glands results in the release of lytic enzymes and damages gland & ducts
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Hairs
dead epithelial fibers composed of cross-linked keratin that is harder than that found on the skin, owing in part to the use of trichohyalin rather than filaggrin to bind tonofilaments into tonofibrils
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What are the three parts of hair?
- 1. shaft: part projecting from the skin surface
- 2. root: the portion embedded in the skin
- 3. follicle: the tubular invagination of the epidermis
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What time of muscle do most hairs have associated with their follicles?
- smooth muscle (arrector pili)
- contraction causes the hairs to become erect (goose bumps)
- arrector pili are absent in axilla, groin, eyelashes & eyebrows
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What are the 3 phases of hair growth?
- 1. anagen (a growth phase that may last for years)
- 2. catagen (a follicular regression stage which lasts weeks)
- 3. telogen, (a resting period lasting a few months).
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What is responsible for the perception of touch, temperature, pressure and pain?
- free nerve endings abundant in the epidermis
- skin at every part of the body functions as a sensory receptor
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Meissners corpuscles
- rapidly adapting sensitive organs found in dermal papillae that confer high discrimination
- they stop firing after continued stimulation --> why we soon become unaware of the clothes touching our skin
- used for tasks such as reading braille
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Pacinian corpuscles
rapidly adapting sensory organs that respond to deep touch pressures (massage) found in the deep dermis and hypodermis
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- stratum spinosum
- rail roads between cells --> desmosome connections between cells
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Langerhan's cells contain Birbeck granules (function unknown)
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- eccrine gland
- dark = ducts (stratified cuboidal)
- light = gland
- thin surrounding gland = myoepithelium
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- clear bubble = sebaceous gland
- thin pink line = smooth muscle (arrector pili)
all next to hair bulb/follicle
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