Skin Physiology and Function PDF

3 Skin Structure: Epidermis Rete Ridges Cells along Rete ridges in basale layer divide and migrate superficially The deeper the ridges and closer the ridges are together the more cells per square inch of skin and the thicker the skin Rete Ridges...

3 Skin Structure: Epidermis Rete Ridges Cells along Rete ridges in basale layer divide and migrate superficially The deeper the ridges and closer the ridges are together the more cells per square inch of skin and the thicker the skin Rete Ridges physically attach epidermis to the dermis below Rete Ridges are like fingers: The more fingers and longer fingers you have, the better you can grip hence greater tissue strength 4  Epidermal / Dermal Junction is a Critical Structure in Skin Physiology and Pathophysiology Provides cells for healing – More cells per area = faster healing Provide mechanical strength – Affected by depth of ridges – Affected by number of ridges per area Clinical correlation = skin tear 5  Stages of Wound Healing Hemostasis Inflammation 0‐3 hours post injury 3 hrs – 3 days Wound Healed Proliferative Remodeling 3‐21 days 21 days – 2 years 6  Molecular Environment of Wounds Platelets aggregate and degranulate PDGF TGF‐B Proliferative EGF Remodeling of collagen IGF ECM (collagen + GAG) Wound increases VEGF Fibroblasts tensile strength Inflammatory cells Vascular endothelial (neutrophils/macrophages) Type III to Type 1 cells collagen TNF, IL‐1, MMP Fibronectin Myofibroblasts Keratinocytes migrating Hemostasis/Inflammation Remodeling 7  Partial versus full thickness wounds Partial thickness Full thickness – Confined to epidermis and dermis skin – Epidermis/dermis damaged to subq layer layers – Tissue loss BELOW the dermis tissue layer – Does NOT penetrate below the dermis – Heals through stages of wound healing – Heal primarily by re‐epithelialization 8  Clinical Correlation Split thickness vs full thickness skin grafts Split thickness grafts – Epidermis + part of the dermis Thin (0005‐.012 in) Intermediate (.012‐.018 in) Thick (.018‐.030 in) – Advantages Better survival rate “take” Cover larger surface area – Disadvantages Contract during healing Hypo or hyperpigmented More function than cosmetic 9 0.012”-0.016” in thickness 10  Full Thickness Skin Graft Epidermis and all the dermis – Higher graft failure – More metabolic needs than STSG Benefits – Superior color match to native skin – Inclusion of addition dermal structures – Less 2° contraction than STSG – Better cosmetic and function graft – Donor site usually primarily closed No secondary wound care Desyndactyly procedure 11  Spongiotic Dermatitis Spongiotic dermatitis – Atopic dermatitis – Nummular eczema – Id reaction (Autosensitization) – Dyshydrotic eczema Mechanism for formation of spongiotic vesicles in epidermis is rupture of desmosome 12 Cell-to-cell adhesion molecule 13  Bullous pemphigoid Clinical correlation – Autoimmune blistering disorder – Autoantibodies cause blistering beneath basal layer of epidermis – Affects hemi‐desmosomes of basal keratinocytes Pemphigus vulgaris – Autoimmune disorder – Desmosomes attacked by antibodies – Flaccid bullae 14  Bullous pemphigoid Clinical correlation – Autoimmune blistering disorder – Autoantibodies cause blistering beneath basal layer of epidermis – Affects hemi‐desmosomes of basal keratinocytes Pemphigus vulgaris – Autoimmune disorder – Desmosomes attacked by antibodies – Flaccid bullae 15  Filaggrin Clinical Correlation – Protein produced in the granular layer of epidermis – Retains water in keratinocytes – Maintains structural integrity of stratum corneum – Mutations can occur  atopic dermatitis – Absent filaggrin in ichthyosis vulgaris *increased allergens entering and water exiting 16  Skin Conditions Related to Dysfunction of Epidermal Layer How quickly does the epidermal skin take to turnover? Psoriasis – Accelerated rate of epidermal turnover – Thickened skin – Doesn’t allow time for differentiation of skin layers 17  Epidermal Barrier ½ liter/day lost by insensible perspiration (skin/lungs) *Not sweat gland Barrier to pathogenic organism Tight junctions 18  Papillary Dermis Located right below the basement membrane Mainly type III collagen in a loose network (loose connective tissue) – Collagen randomly aligned Many fibroblasts Dermal papillae – Interdigitating structures with the rete ridges of epidermis – Contain papillary loops Supply O2 and nutrients to overlying epidermis through the basement membrane Skin tear 19  Vascular Skin Anatomy Subpapillary plexus Cutaneous plexus Epidermis is Subcutaneous plexus supplied Capillary loops ascend from plexus around subpapillary subpapillary dermis to supply dermis is formed each dermal Arterioles ascend papilla from plexus into Plexus of small dermis arteries in subq 20  Arteriovenous Anastomoses ‐ Shunts in Acral Skin Heat Loss Heat Conservation 21 Cold Weather Acral Skin Blood flow AVA 22 Hot Weather Acral Skin Bloodflow 23  ARTERIAL SUPPLY & GLOMUS BODIES Accessory supply which bypasses the pulp Prox. Arcade Functions Distal Arcade Thermoregulation Local blood pressure Interstitial fluid control Digital Artery Glomus body Glomus Tumor 24  Clinical Correlation Charcot Neuroarthropathy -autonomic neuropathy -increased AV shunting -foot becomes red/swollen / dilated dorsal veins 25  Reticular Dermis Base of the dermis below the papillary dermis Thicker collagen fibers in this layer as compared to papillary dermis Primarily type I collagen with fibers arranged in interwoven bundles Relaxed Skin Tension Lines NO clear separation between 2 dermal layers – Collagen gradually changes in size between the 2 layers Collagen orientation  skin lines (Langer’s lines) – Perpendicular to long axis 26  Relaxed Skin Tension Lines Finding RSTL – Relax the skin in an area by passive manipulation or by muscle movement – Pinch test Perpendicular: regularly shaped furrows Parallel: little skin motility Obliquely: an S‐shaped pattern 27  Relaxed Skin Tension Lines Incisions perpendicular to the RSTL will gap open under tension Incisions parallel to the RSTL will close with minimal tension Oblique incision is the next best option Skin plasty placement (skin flap) – LME (line of maximum extensibility) Perpendicular to RSTL 28  Surgical Incision Placement 4 considerations prior to performing an incision 1. An incision should provide adequate exposure to the target tissues 2. Parallel to relaxed skin tension lines (if possible) 3. Spare motor and sensory nerves 4. Allow blood supply from both sides of the incision – angiosomes Agnew SP et al. Angiosomes of the calf, ankle, and foot: anatomy, physiology and implications. Sarrafian’s Anatomy of the Foot and Ankle. 3rd edition. Philadelphia, 2011. 29  Dermal Proteins Collagen – Protein that gives skin tensile strength – 25% of total skin weight – Secreted by dermal fibroblasts Important component for wound healing – Normal human skin is primarily type 1 collagen – Type III: 2nd most abundant type (papillary dermis) – Clinical note: Keloids result from uncontrolled synthesis of collagen at site of dermal injury Elastin – Gives skin its elastic recoil Clinical correlation: skin grafting – Decreases with age Mast cells, macrophages, lymphocytes – Immune system of the skin 30  Clinical correlation: skin tears Physiological changes in aging skin Thinning/flattening of dermal/epidermal junction Atrophy of dermis (decreased collagen) Atrophy of hypodermis (subq tissue) Reduced sweat gland production Degeneration of elastic fibers Slower epithelialization Reduced sensation Reduced arterial perfusion 31  Skin Changes Epidermal Changes Dermal Changes Keratinocytes Fibroblasts – Slower turnover – Decreased number Keratin – Decreased collagen production – Sloughs more slowly Collagen – Thickening of keratin layer – Abnormal / weakened structure Melanocytes Vasculature – Decreased in number – PAD – Produce less melanin – Venous disease – Dilated, thin, weakened vessels 32  Photoaging UVA light  longer wavelength – Deeper skin penetration (dermal tissue) – UVA penetrates window glass – A “aging” UVB  shorter wavelength light – Higher energy photons – DNA damage 33  Payne and Martin Classification System Category I skin tear without loss of tissue epidermal flap completely covers dermis ‐Ia : linear type ‐Ib: flap type Category II: Skin tears with partial tissue loss ‐IIa: scant tissue loss (25% or less) ‐Iib: moderate to large loss of tissue (>25%) Category III: skin tears with complete tissue loss 34  Mast Cells Clinical Correlation – Major cell in the production of urticaria reaction Allergic reaction (hives) – Wheal (hive) – Superficial edema surrounding by erythema Edema in papillary dermis 35  Adipose layer Subcutaneous Tissue Layer Contains subdermal layer of blood vessels and nerves Hair follicles and apocrine/eccrine sweat glands can extend to this layer Full thickness wounds by definition extend to this layer of tissue (ulcerations)  important for documentation Surgical dissection – Nerves / blood vessels Deep to the subq tissue is the deep fascia Role in shock absorption – Fat pad atrophy Energy stored in form of triglycerides – Released as fatty acids when necessary – Clinical correlation  DKA 36  Subcutaneous Tissue layer Clinical Correlation – Erythema nodosum Inflammation of subcutaneous skin layer Acute nodular eruption usually confined to extensor surface of lower legs Hypersensitivity reaction to a variant of antigens Reaction to infection, medications, autoimmune disease (IBD, sarcoidosis) 37 Skin Physiology and Function 38  Skin Function Immunologic Protection Thermoregulation Sensory Organ Vapor Barrier function Barrier Sweat gland Diabetic Fluid volume / Langerhan cells Trauma control peripheral balance intact Radiation Heat loss  neuropathy e.g. burn pts Melanocytes evaporation of Complications sweat due to Infection Vasoconstriction infection / Normal flora Vasodilation dehydration Physical barrier 39  Skin Function: Immunity From Pathogens Primary defense is an intact stratum corneum – Keratin provides a good barrier – Lipids produced in which layer of the epidermis? Stratum granulosum – Helps prevent water loss and protects against mechanical, chemical, and microbial insults from environment Break in the stratum corneum or deeper tissue layers provides entry point for bacterial/viral/fungal/acid fast infections Sebum – Oily substance secreted by sebaceous glands – Provides acidic environment that helps with antimicrobial protection (pH 4 – 6.8) – Sebaceous glands not located on palms or soles 40  Normal Skin Flora Resident (Skin Microbiome) Transient Microorganisms residing Colonizes superficial under the superficial cells layers of skin of the stratum corneum Easier to remove by 2 main protective hygiene (hand washing) functions Microbial antagonism  resistance to microorganisms Competition for nutrients 41  Normal Skin Flora – Depends on Location within the body Skin temperature Humidity Glandular distribution – Occluded areas of the body (skin folds  groin, web space) Gram neg bacilli Corynebacterium spp Staphylococcus (specifically coag neg.) Pseudomonas – Sebaceous glands (face, chest, back) Lipophilic organisms (Propionibacterium spp. / Malassezia spp.) Clinical correlation  seborrheic dermatitis – Arm and leg skin (relative desiccation and fluctuations in skin temperature) Fewer organisms 42  Pre‐op Skin Prep Bibbo et al 9 different surgical skin prep compared Iodine paint vs chlorhexidine / isopropyl ETOH Chlorhexidine / isopropyl ETOH – Prolonged bactericidal – Greater reduction of skin flora Bibbo C, Patel DV, Gehrmann RM, et al. Chlorhexidine provides superior skin decontamination in foot and ankle surgery. Clin Orthop 2005;438:204–8. 43  Isopropyl Alcohol Skin Prep – Works rapidly – Removes skin flora in ≈ 1 minute – 10 second EtOH scrub = 5 min iodine scrub Povidone‐iodine – Slightly more active – Slower onset of action – Stains skin – Contact dermatitis in some patients – Preference with open wounds Chlorhexidine (Hibiclens) – Rapid onset – Sustained level for several hours – Preferred in longer cases or immunocompromised patients – Pts with betadine allergy (shellfish allergy) 44  Skin Immunity Langerhans cells – Antigen presenting cells found in epidermis – 2‐5% of cells in stratum spinosum Macrophages – Directly ingest and kill bacteria – Found in the dermis – Inflammatory phase of wound healing Mast cells – Release histamine (inflammation) – Found in the dermis Dendrocytes – Function similar to macrophages / antigen presenting cells 45  Langerhans Cells – APC Responsible for presenting antigens to T‐lymphocytes Important in induction of delayed type hypersensitivity – Allergic contact dermatitis – Poison ivy Pathogenesis (type IV Hypersensitivity Reaction) – Antigen exposed to Langerhan cells in epidermis – Antigen carried to lymph nodes through lymph capillaries and presented to T cells in lymph nodes – T cells proliferate – 2nd exposure to antigen from Langerhan cells create skin reaction 46 Langerhans Cells – APC 47  Macrophages Derived from monocytes Most important cell of skin’s immune system Many roles – Antimicrobial activity / phagocytose foreign material – Present antigens to lymphocytes – Secrete growth factors / cytokines Important for wound healing purposes Inflammatory phase of healing 48  Mast Cells Located in papillary dermis and subq fat Primary effect is in allergic reactions Also involved in inflammatory diseases – Release histamine causing a vasodilation of blood vessels Increased levels of mast cells in tissues affected by eczema, psoriasis, scleroderma Play a role in immune system – Protect against parasites, stimulate chemotaxis and promote phagocytosis Activate eosinophils – Allergic reactions and parasite infections 49  Dermal Dendrocytes Highly phagocytic cells Bone marrow derived cells Found in papillary and reticular dermis Function similar to macrophages Antigen presenting cell – Distinct from Langerhan cells Give rise to some inflammatory skin manifestations – Release cytokines 50  Skin Function: Thermoregulation Helps maintain constant body temperature Circulation and sweating Insulating properties of fat / hair Accelerated heat loss with sweating ‐AVA – When evaporates, cools the body temperature Clinical note – Raynaud's phenomenon (thermoregulation dysfunction) – Digital ischemia provoked by cold exposure – Autonomic neuropathy 51  Non‐thermoregulatory Sweating Eccrine glands located in palms and soles are non‐thermoregulatory Histology: glabrous skin on palms and soles has a thick epidermis – Needs some moisture – Frictional purchase on the substrate (or on tools) Control = adrenergic SNS and epinephrine – esp. active during stresses Clinical correlation: sudomotor atrophy 52  Skin Function: Nerve Sensation Sensory receptors in skin allows skin to constantly monitor the environment – Pain, sharp, dull, hot, cold Dysfunction can lead to pruritis (itch), dysesthesia (abnormal sensation) and numbness Clinical correlation – Diabetic neuropathy What causes diabetic neuropathy? 53  Autonomic Sensory Neuropathy Motor neuropathy neuropathy Decreased sharp Intrinsic muscle Impaired sweating dull wasting (sudomotor Decreased Imbalance of dysfunction) proprioception long/short flexors Xerosis Decreased Dorsal dislocation Sympathetic failure vibratory sensation of toes (clinical Stocking‐glove correlation: distribution Charcot) Vasodilation A‐V shunting (decreased skin perfusion) 54  Vitamin D Metabolism Sterol (7‐dehydrocholesterol) Calcitriol Cholecalciferol ‐stratum basale / 1,25‐ Vitamin D3 spinosum dihydroxycholecalciferol ‐keratinocytes 55  Terminology Onychauxis – Nail plate thickened – Subungual hyperkeratosis of nail bed Psoriasis, eczema, distal subungual onychomycosis 57  Terminology Onychocryptosis – Ingrowing nail 58  Terminology Onychogryphosis – Hypertrophy of the nail plate – Hooked/incurvated claw‐like deformity Ram’s horn nail – Etiology Pressure from shoegear elderly Inherited autosomal dominant –Keratin produced at uneven rates 59  Terminology Onycholysis – Separation of nail plate from underlying nail bed – Usually begins at free edge  progresses proximally Psoriasis Trauma DSO 60  Terminology Paronychia – Erythema/edema – Purulence – Loss of cuticle Staph aureus Pseudomonas Candida 61  Terminology Onychomycosis – Tinea unguium 62  Terminology Leukonychia – White opaque discoloration of nail plate – Punctate Seen following trauma Psoriasis – Transverse Matrix trauma – Diffuse Nail plate completely opaque / white – Fungal infections (SWO) 63  Terminology Anonychia – Absence of nails – Congenital defect – Self‐inflicted – Lichen planus – Post‐procedure 64  Abnormal Nail Conditions Beau’s Lines – Transverse depression in nail plate – Single nail involvement  traumatic – Multiple nails  systemic Severe, acute illness 65  Abnormal Nail Conditions Green Nail Syndrome – Usually associated with onycholysis – P. aeruginosa Pyocyanin (green pigment produced by bacteria) 66