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The Integumentary SystemBy Nigel Henderson

The Skin

• Largest Organ of the body• Complex and has essential function for survival

Structure• Two layers• Epidermis • Dermis

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Epidermis• The most superficial layer of the skin• Stratified keratinised squamous epithelial cells• Varies in thickness• Consider palms and feet• More production of keratinised layers• Consider calluses• No Blood vessels or nerve endings present• Deeper layers bathed in interstitial fluid from dermis• Layers or strata evident and number from 4-5

Epidermis Layers

• Stratum Basale (germinative layer)• Stratum Spinosum• Stratum Granulosum (keratin)• Stratum lucidum (only in palm and soles)• Stratum corneum (most upper layer)

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Epidermis• The cells on the surface are flat thin and none nucleated called

squames or dead cells• The cytoplasm has been replaced with keratin• Constantly rubbed off• Cells in lower layers slowly migrate to upper layer• Process of keratinisation takes place

The maintenance of a healthy epidermis relies on synchronisationof:

• Desquamation (shedding)• Effective keratinisation• Continual cell division in deeper layers

Epidermis• Whole process about two weeks long• Complete replacement of epidermis occurs in about 1 month• Hairs, secretions from sebaceous glands and sweat ducts pass

through epidermis to surface of skin• Surface of skin ridged due to papillae in dermis• Caused by projections of dermis• Form fingerprints since patterns different to each individual• Downward projections believed to aid nutrition of epidermal cells• Also thought to stabilise two layers by preventing damage due to

shearing• Consider blister formation

Epidermis• Blister formation due to separation of dermis from epidermis and leaking of

interstitial fluid in the space between layersSkin colour affected by:

• MelaninSecreted by melanocytes in germative layerManufactured from tyrosine absorbed by surrounding epithelial cellsGenetically determined and protects from harmful effects of sunSunlight promote synthesis of melanin

• Saturation of blood with haemoglobin and amount of blood circulation in skin

• Excessive levels of bile pigments and carotenes in subcutaneous fat (Consider Tattoos!)

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Dermis• Composed of connective tissue both collagenous and

elastic fibres.• Interlaced structure• Rupture of elastic fibres caused by over stretching

causes striae to develop• Consider pregnancy and Obesity• Collagen fibres bind water and give the skin its tensile

strength• Declines with age hence wrinkle development

DermisThree cells type mainly present:

• Fibroblasts (produce the collagen and elastic fibres) • Important in wound healing• Macrophages (phagocytic) important defence

mechanism• Mast cells similar to basophils except they are within

tissues, contain histamine• Deeper layers contain adipose tissue

DermisStructures within the dermis:

• Blood Vessels provides nutrients to dermis cells and interstitial fluid for deeper layers of epidermis

• Lymph Vessels throughout the dermis• Sensory Nerve Endings sensitive to touch, temperature

pressure and noxious stimuli (nociceptors) utilise somatic cutaneous pathways

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DermisThree receptors identified

• Meissner’s Corpuscle (Light Pressure)• Pacinian Corpuscle (Deep Pressure)• Free Nerve endings (Pain)• Sweat Glands (sudoriferous ):

Subdivided into Apocrine (Puberty onwards) Eccrine (most common)

• Sweat glands opening into hair follicle function from puberty• These produce the body odour smelt when left to accumulate

Structures in the Dermis

• Hair Roots, Follicles and HairsHairs formed from the bulb at the bottom of the follicleCollection of cells which proliferate to produce the hairCells become keratinised as they are pushed upwardsHair above the skin called the shaft and below called the root

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Structures in The DermisSebaceous Glands

• Secretory epithelial cells same origin as hair follicles• Produce sebum• Secreted into hair follicles• Present all over body apart from palms and soles• Most numerous on scalp, face axillae and groins• In regions of transition of superficial epithelial tissue from one type to

another i.e. eyelids, nipple, labia minora and glans penis sebaceous gland not related to hair follicles

Structures in the DermisSebum

• Hair softness and pliability• Gives shiny appearance to hair• Waterproofing for skin• Bactericidal• Fungicidal• Moisturising preventing drying and cracking• Glands more active in puberty less so at extremes of age

Arrectores Pilorum• Smooth muscle fibres attached to hair follicle• Contraction evident by “goose bumps”• Stimulated by sympathetic NS• Act to provide insulation• Consider more primitive uses!

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Physiology

Body Temperature regulation• Most heat lost through skin• Other routes available for heat loss but less controllable• Heat loss directly related to clothing and ambient temperature• Air poor conductor of heat • Layers of clothing trap heat• Important in elderly!!!

PhysiologyHeat loss via:

• Evaporation due to sweatWhen production outstrips ability to evaporate as in high humidity or heat environments, beads of sweat seen

• ConductionClothing and touching objects have heat transferred from body

• RadiationExposed parts of body radiate heat

• ConvectionAir passing over body is heated and rises pulling cool air to replace the air that is rising

Physiology• The amount of heat directly related to amount of blood

flow to skin• Controlled by Vasomotor Centre in brain stem Medulla

oblongata• Temperature sensed by hypothalamus in its

temperature sensing centre• Utilises Autonomic Nervous System• Vasoconstriction and Vasodilatation instrumental

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Physiology• Sensory ( See earlier)• Excretion

Minor excretory roleSodium Chloride in sweatExcessive sweating may lead to hyponatraemiaUrea especially when renal function impairedAromatic substances

• Vitamin D Synthesis7 Dehydrocholesterol , lipid based substance within skinConverted to Vitamin D when exposed to ultraviolet light Used with calcium and phosphate in maintenance of bone

Physiology• Protection

Prevents invasion of microbesBarrier against chemicalsProtects against mild trauma and Ultraviolet lightPrevents Dehydration

• Immunity : Cells within the epidermis stimulate T cells when antigens invade the epidermisSuch specialised cells called Langerhans cellsTravel to lymph tissue to present antigenAlso phagocytic

Physiology

Absorption• Skin absorption of some materials has been discovered

and exploited• Consider Transdermal administration of drugs• Also danger of toxic chemical absorption such as

mercury