Skin lecture slides - full slide deck with notes.pptx

Skin Dr B. Skosana Division Medical Physiology Room 3024 BMRI Building e-mail: [email protected] Kopiereg Kennisgewing/Copyright Notice Materiaal wat by wyse van elektroniese databasis aan u beskikbaar gestel is, mag onderhewig wees aan bepaalde lisensievoorwaardes...

Skin Dr B. Skosana Division Medical Physiology Room 3024 BMRI Building e-mail: [email protected] Kopiereg Kennisgewing/Copyright Notice Materiaal wat by wyse van elektroniese databasis aan u beskikbaar gestel is, mag onderhewig wees aan bepaalde lisensievoorwaardes. Sulke voorwaardes behels normaalweg dat u die materiaal slegs vir u eie private studie en navorsing aan die Universiteit kan gebruik en vir geen ander doeleindes hoegenaamd nie. Daar word van u verwag om te alle tye aan hierdie voorwaardes gehoor te gee. Gedrukte materiaal word eweneens slegs vir klasbespreking, private studie en navorsingsdoeleindes beskikbaar gestel. Die verdere reproduksie daarvan is ontoelaatbaar. Deur om op die Potgooi skakel te druk om die Potgooie te besigtig/aflaai, stem u in om te voldoen aan hierdie voorwaardes. Material which is made available by way of an electronic database may be subject to certain licensing conditions. 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By clicking on the Podcast link and viewing/downloading the Podcasts, you are agreeing to adhere to these conditions. Objectives List the functions of the skin List the different layers of the skin List the most important cellular components of the different skin layers and specify the compartment where each is located Explain the functions of the touch receptors Describe how the skin manages its defense barrier List the hormones produced by the skin Explain the thermoregulatory role of the skin Introduction Skin – Integumentary system (integument – “covering”) Largest organ in the body – Mechanical barrier – Protective Covers the entire surface of the body (1.2 – 2.2m2) 7% of the total body weight of an adult Prevents exposure to pathogens, heat and water loss Varies in thickness in different parts of the body (1.5 – 4 mm) Functions of the skin Protection (keep the outside out) – multi-tier barrier: Physical barrier, Microbiome barrier, Chemical barrier, Immunological barrier Body temperature regulation Immunological organ – part of the Immunological barrier Electrolyte regulation (sweating) Vitamin D synthesis Social interactions – Touch receptors The skin is an organ system No single cell can carry out all the functions of the body Cells assemble into large units called tissues – Can be the same cell type – simple tissues – Can be different cell types – complex tissues Connected to one another by cell junctions Work together to achieve a common purpose  Skin is an organ system What tissues are found within skin? Epithelia – One or more layer of cells connected to one another – Covers exposed surfaces, e.g. skin, internal passageways – Protects the internal environment – Regulates exchange of materials – 2 types Sheets of tissue – lie on surface of the body Secretory – synthesize and secrete substances Connective tissue – Extracellular matrix with cells and proteins scattered within it – Provide structural support and sometimes act as a physical barrier that helps defend the body from foreign invaders such as bacteria – Consistency of extracellular matrix is highly variable Watery matrix of blood, hardened matrix of bone Skin has loose connective tissue – more matrix than cells and fibers Neural (Nervous) – Neurons carry information in the form of chemical and electrical signals Muscle – Can contract and produce movement  Skin is an organ system Epithelia – a sheet of cells that covers a body surface or lines a body cavity Cell shape Layering –Simple – one cell layer thick –Stratified – multiple layers Function –Different types of epithelia http://www.proteinatlas.org/images_dictionary/cell_junctions__4__35121__1_blue_red_green.jpg Exchange epithelia –Regulate the exchange of material between the internal and external environment (e.g., O 2 and CO2) –Cover exposed surfaces, e.g., skin –Line internal passageways, e.g., blood vessels Transporting epithelia –Selectively and actively regulate the exchange of ions and nutrients between the internal and external environment –Secretion = movement of material from internal to external environment; absorption = external to internal environment Protecting epithelia –Protect areas exposed to mechanical and chemical stress –Stratified epithelia –Toughened by the secretion of keratin –Cells have a short life span and are constantly replaced Secretory epithelia –Manufacture and secrete substances and chemicals into nearby tissues/surfaces –E.g., sweat is secreted onto the skin and saliva is secreted into the mouth; hormones are secreted into blood Ciliated epithelia –Not found in skin Skin is an organ system Layers of the skin Epidermis (epi – “on top” or “upon”; cutis – skin) Outermost protective shield Dermis Makes up the bulk of the skin Mostly composed of dense connective tissue Vascularised – Nutrients reach the epidermis by diffusing through blood vessels in the dermis Hypodermis (hypo – “underneath”) Also known as the subcutaneous tissue (sub – “below”; cutaneous – “skin”) Not part of the skin Protective Anchors skin to the underlying structures (e.g. muscle) Stores fat – Insulation – Shock absorption Layers of the skin Layers of the skin Epidermis Numerous layers of epithelia Types of epithelia (shape) – Cuboidal (cube-like) – Squamous (squama – “flattened”) – Inner epidermis – cuboidal epithelia New cells constantly being produced Newly forming cells push the older cells closer to the surface Connected by cell junctions (desmosomes) – Outer epidermis – squamous epithelia Older cells have filled with keratin and died, become flattened Cells tightly connected to one another – Exposed to “wear and tear” – No direct blood supply to epidermis Cells nourished by diffusion of nutrients from blood vessels in the dermis Layers of the skin Epidermis – Melanocytes Spider-shaped epithelial cells Synthesize the pigment melanin Found in the deepest layer of the epidermis Melanin – Made in membrane-bound granules (melanosomes) within the melanocytes – Forms a pigment shield that protects the cell nucleus from UV radiation in sunlight Epidermis varies in thickness – Thick and thin skin Based on location of the skin that it covers Refers to the epidermis only – Thick skin Covers areas subject to abrasion E.g., palms, fingertips, soles of the feet – Thin skin Covers the rest of the body Layers of the skin Layers of the skin Dermis – Made of connective tissue with several components embedded in it – Has 2 layers Papillary, reticular Components – Cutaneous glands Sweat glands – apocrine, eccrine Sebaceous glands – Hair cells – Muscle – Blood vessels – Receptors  Layers of the skin Dermis Papillary dermis – Top, “wavy” part of the dermis – Connective tissue with interlacing collagen and elastic fibers form a loosely woven “mat” with many small blood vessels – Looseness of connective tissue allows phagocytes and other defensive cells to wander freely – Peg-like projections (dermal papilla) indent the overlying epidermis. Many contain capillary loops Others have free nerve endings (pain receptors) & touch receptors In thick skin, these papillae lie on top of dermal ridges, causing the overlying epidermis to form epidermal ridges Many epidermal ridges are collectively called friction ridges (e.g., fingerprints) Layers of the skin Dermis Reticular dermis – The deeper layer of the dermis – Accounts for 80% of the dermis – Nourished by a network of blood vessels that lies between this layer & subcutaneous tissue – Has numerous tissue components (cells, glands, proteins and nerves) embedded in it Each have a unique role in maintaining homeostasis – Skin appedanges Sweat glands, sebaceous glands Hair and hair follicles Nails Layers of the skin Dermis Components – Cutaneous glands – one or more cells that make and secrete a specific product (product = watery secretion, usually contains proteins or lipids) Sweat glands – Apocrine – Sweat is produced and accumulates just beneath the overlying surface until released into hair follicle – Eccrine – Coiled tubes. Secretory part lies in the coiled in the dermis, the duct extends to an opening on the epidermis Sebaceous glands – Oil is produced and accumulates within the gland until released – Sebum = accumulated lipids and cell fragments – Hair and hair follicles Distributed all over the skin surface (except palms, soles of feet, lips, etc.) Flexible strands made primarily of dead, keratinized cells Cells at the base of the follicle rapidly divide, allowing hair growth (rate changes with age) Layers of the skin 2 main regions of hair – root, shaft Hair texture determined by the shape of the hair shaft – Flat = kinky, ribbon-like hair – Oval = wavy, silky hair – Round = straight, tends to be course – Muscles Arrector pili muscle (pili = “hair”) Associated with each hair follicle Bundle of smooth muscle cells (unconsciously controlled) Contraction pulls the hair shaft upwards – Temperature regulation – Emotional response – Forces sebum out of hair follicles (lubrication) – Blood vessels Capillary loops in the papillary dermis Network of blood vessels lies between reticular dermis & subcutaneous tissue – Receptors Sense and respond to stimuli Generate and conduct nerve signals Transmit these signals over long distances across the body Layers of the skin Vellus hair (vell = “wool” or “fleece Terminal hair Touch Receptors in the Skin The skin is richly supplied with cutaneous sensory receptors – Part of the nervous system Respond to stimuli from outside of the body Touch Receptors in the Skin Merkel receptors Meissner’s corpuscle sense steady pressure responds to flutter and and texture. stroking movements. Hair Free nerve ending Free nerve ending of Free nerve ending nociceptor responds of hair root senses to noxious stimuli. Hair root hair movement. Sensory nerves Pacinian corpuscle carry signals to senses vibration. spinal cord. Ruffini corpuscle responds to skin stretch. Touch Receptors in the Skin Temperature Receptors Free nerve endings Terminate in subcutaneous layers Small receptive field (1mm diameter) Slowly adapt between 20°C - 40°C Do not adapt outside 20°C - 40°C Cold receptors –Lower than body temperature (37°C) Warm receptors –Above body temperature to about 45°C –Pain receptors activated above 45°C Body temperature The body uses 4 mechanisms of heat transfer – Radiation – Conduction – Convection – Evaporation Radiation – Thermal energy – Loss/gain of heat in the form of infrared waves – Heat always flows down its concentration gradient from a warmer region to a cooler region – ± 50% of body heat lost in this way Body temperature Conduction – The transfer of heat from a warmer object to a cooler object when the two are in direct contact with one another – E.g., Warm water from a hot tub transfers heat to the skin Convection – Warm air expands and rises, cool air falls (is denser) – Warm air enveloping the body is continually replaced by cooler air molecules – Convection enhances heat transfer from the body surface to the air Cooler air absorbs heat by conduction more rapidly than the already warmed air – Convection and conduction account for 15 to 20% of heat loss to the environment. – Enhanced by anything that moves more rapidly across the body surface such E.g., wind, a fan (forced convection) Evaporation – Water evaporates because its molecules absorb heat from the environment and become energetic enough to escape as a gas (water vapor) – The evaporation of water from body surfaces removes a large amount of body heat Body temperature  Body temperature The body works best when its temperature remains within homeostatic limits – As long as the external temperature is lower than body temperature, the skin surface loses heat to the air and to cooler objects in its environment. The skin uses 3 main ways to control body temperature – Sweating (evaporation) – Physical barrier – Blood vessel dilation/constriction (radiation, conduction, convection) Sweating – Sweat’s major function is to prevent the body from overheating – Evaporation of sweat from the skin surface dissipates body heat and efficiently cools the body preventing overheating – Insensible perspiration Under normal resting conditions (and environmental temperature is < 31 to 32°C), sweat glands secrete about 500ml of sweat per day Routine and unnoticeable sweating Constant – dissipates 10% of basal heat production by the body – Sensible perspiration On a hot day, sweat becomes noticeable and can account for the loss of up to 12 litres of body water in one day. When body temperature rises the nervous system stimulates dermal blood vessels to dilate, and sweat glands into vigorous secretory activity Visible output of sweat Vigorous exercise can increase body temp. by 2-3°C – causes 1-2L/h of sweat loss Body temperature – Sweat glands Contract when stimulated by the nervous system Contraction forces sweat through the duct system onto the skin surface Eccrine sweat glands more numerous – Most abundant on forehead, palms, soles of feet – Secrete hypotonic fluid filtered from blood plasma (water, salts, metabolic waste) Apocrine sweat glands play very little role Body temperature Physical barrier – Sebaceous glands Sebum lubricates the hair and skin Prevents water loss from the skin Blood vessels – Peripheral thermoreceptors constantly send signals to central thermoreceptors in the brain Controls heat-promoting and heat-loss activities Body temperature – Cold temperatures Heat-promoting center is activated Dermal blood vessels constrict (sympathetic NS) Causes warm blood to bypass the skin temporarily and allows skin temperature to drop to that of the external environment. Slows passive heat loss from the body, conserving body heat – Hot temperatures Activate heat-loss mechanisms (inhibits heat-promotion) Dermal blood vessels dilate (parasympathetic NS) Immunological function Skin is our most vulnerable organ system – Exposed to microbes – Needs to be protected Chemical barrier – Acid mantle = created by low pH of skin secretions Blocks bacteria from multiplying – Eccrine sweat glands Sweat contains a dermicidin = a microbe-killing peptide Sweat is acidic = pH 4 – 6 – Sebaceous glands Sebum is bacteriocidal (cide = “killer”) Immunological function – Skin secretes natural antibiotics Defensins = punch holes in bacteria, making them leaky (lose viability) Cathelicidins = released by wounded skin to prevent group A strep infections Physical barrier – Top layer of epidermis (stratum corneum) contact glycolipids (carbohydrate-lipid secretions) – Prevent entry into the body Biological barrier – Dendritic cells in the epidermis Patrol beneath the skin's surface, engulfing any foreign invaders Leave the skin and migrate to the nearest lymph node to display their prey to other immune cells. This triggers and immune response – Macrophages in the dermis Second line of defense. Dispose of viruses and bacteria that managed to penetrate the epidermis. Can initiate an immune response Immunological function Immunological function Blood flow to the skin Skin blood flow – Provides nutrition – Regulates body heat Forms part of the circulatory system – Major systemic vessels have branches going to and from the skin Blood flow to the skin Skin blood flow Heart ↓ Systemic arteries ↓ Arterioles ↓ Capillaries ↓ Venules ↓ Systemic veins ↓ Heart Blood flow to the skin Microcirculation – Arterioles, capillaries, venules – Precapillary sphincters control blood flow into the capillaries If they constrict, capillary circulation is reduced If they dilate, capillary circulation is increased – Controlled by sympathetic and parasympathetic nervous system Control contraction and relaxation of precapillary sphincters Blood flow to the skin Major arteries supply blood to underlying muscles Arteries divide into arterioles and capillaries in the hypodermis and dermis – Plexuses – Supply hypodermis and dermis Capillaries further divide to form capillary loops – Papillary dermis – Drain into venules Venules form plexus which exits the skin  Blood flow to the skin Capillary loops in the papillary dermis – Capillaries are exchange vessels Smallest blood vessels Exchange gases, nutrients, hormones, etc. to the epidermis – Capillary beds Allow flow of blood from arterioles to venules Nutrients flow to the tissues, waste products flow away from tissues Vascular plexus in the reticular dermis – The dermal vascular supply is extensive and can hold about 5% of the body's entire blood volume – Provides nutrients to (and removes wastes from) the dermis and epidermis – Vasodilation and vasoconstriction occurs in response to temperature changes – When other body organs (e.g., vigorously working muscles) need greater blood supply, the nervous system constricts the dermal blood vessels. This construction shunts more blood into the general circulation, making it available to the muscles and other body organs Hormones Hormone – a chemical secreted by a cell (or group of cells) into the blood for transport to a distant target – Act by binding receptors Hormones Vitamin D – The skin is a chemical factory fueled in part by the sun's rays – UV rays from sunlight cause modified cholesterol molecules to be converted to a vitamin D precursor Precursor is transported via the blood to other body areas to be converted to vitamin D Plays various roles in calcium metabolism. – E.g., Calcium cannot be absorbed from the digestive tract without vitamin D Hormones Sweat – Apocrine sweat glands = normal components of sweat, plus fatty substances and protein Consequently, it is viscous and sometimes has a low key or yellowish colour The secretion is odorless – when bacteria on the skin decompose it's organic molecules , it takes on a musky and generally unpleasant odor Begin functioning at puberty (influenced by sex hormones (androgens) Activity has some variability – Activity increased by sexual foreplay – Enlarge and recede with the phases of a woman menstrual cycle – Secretions may act as pheromones (chemical messengers released by one individual that trigger response in other members of the same species, same or opposite gender) Hormones Among other metabolic functions the epidermis makes chemical conversions that supplement those of the liver – Keratinocytes can activate some steroid Can transform cortisone applied on to irritated skin into hydrocortisone (a potent anti-inflammatory drug)