Skin: Layers and Functions PDF

SKIN SKIN (INTEGUMENT)  Largest single organ of the body,  Dermal Papillae accounting to 15-20% of total Projections located at the junction body weight between the dermis and epidermis  Layers: Interdigitate with the E...

SKIN SKIN (INTEGUMENT)  Largest single organ of the body,  Dermal Papillae accounting to 15-20% of total Projections located at the junction body weight between the dermis and epidermis  Layers: Interdigitate with the Epidermal Ridges of the epidermis to  Epidermis strengthen the adhesion between  Dermis dermis and epidermis.  Hypodermis PROPERTIES OF THE SKIN  Dermatoglyphs or Fingerprints are dermal-epidermal interdigitations form a pattern that is unique for every individual.  Skin is elastic and can expand rapidly to cover swollen areas  Self-renewing throughout life FUNCTIONS 1. Protection  Physical barrier against thermal and mechanical insults (friction)  Protection against potential pathogens and other materials  Resident macrophages and Antigen-presenting cells  Protection against ultraviolet rays  Melanin (a dark pigment in the epidermis)  Permeability barrier against excessive loss or uptake of water  Allows administration of lipophilic drugs, steroids hormones and medications. FUNCTIONS 2. Sensory  Constantly monitor the surroundings  Mechanoreceptors regulate the body’s interaction with physical objects 3. Thermoregulation  A constant body temperature is maintained due to the skin’s insulating components (fatty layer and hair)  Mechanism for accelerating heat loss (sweat production and dense superficial microvasculature) FUNCTIONS 4. Metabolic  Synthesis of Vit D3 through the local action of UV light on vitamin D precursor.  Vitamin D3 is needed for calcium metabolism and proper bone formation  Excess electrolytes are removed through sweat  Fat cells stores fat 5. Sexual Signaling  Pheromones produced by apocrine sweat glands and other glands LAYERS OF THE SKIN EPIDERMIS  Consist mainly of keratinized stratified squamous epithelium composed of cells called “keratinocytes”.  Epidermal Cell types:  Melanocytes – pigment producing cells  Langerhans Cells – resident macrophages, and antigen-presenting cells  Merkel Cells – tactile epithelial cells LAYERS OF THE EPIDERMIS 1. Basal Layer/ Stratum Basale  A single layer of basophilic cuboidal or columnar cells located on top of the basement membrane at the dermal-epidermal junction.  Hemidesmosomes bind these cells to the basal lamina and desmosome bind the cells at the lateral and apical surfaces  Has a high mitotic activity and contains progenitor cells for all the epidermal layers  Keratinocytes contains cytoskeletal keratins, as they move upward, the amount and type increases. LAYERS OF THE EPIDERMIS 2. Spinous Layer/ Stratum Spinosum ▪ Situated just the basal layer. ▪ Thickest layer, specially in the epidermal ridges ▪ Consist of polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins. ▪ Some cells are still actively dividing → stratum germinativum LAYERS OF THE EPIDERMIS 2. Spinous Layer/ Stratum Spinosum  Keratin filaments or “Tonofibrils” holds the cells together. They are seen as intercellular bridges  “Spines” or prickles at the cell surfaces  Increased in areas subjected to continuous friction and pressure (foot soles). LAYERS OF THE EPIDERMIS 3. Granular Layer/ Stratum Granulosum ▪ Three to five layers of flattened cells, undergoing terminal differentiation of keratinization. ▪ Cytoplasm is filled with intensely basophilic masses: keratohyaline granules ▪ Dense, non-membrane-bound masses of filaggrin and other proteins associated with tonofibrils, linked together to form large cytoplasmic structures. LAYERS OF THE EPIDERMIS 3. Granular Layer/ Stratum Granulosum ▪ Lamellar granules ▪ Golgi-derived small ovoid structures with lamellae containing various lipids and glycolipids ▪ Undergo exocytosis at the terminal stage of keratinization, to produce a lipid-rich impermeable layer around cells. ▪ Formation of lipid-rich membrane and keratinization effectively create a barrier against penetration of most foreign materials. LAYERS OF THE EPIDERMIS 4. Stratum Lucidum  Found only on thick skin (sole of feet, palm of hands)  Composed of thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes  Nuclei and organelles are lost  Cytoplasm consist almost exclusively of packed keratin filaments LAYERS OF THE EPIDERMIS 5. Stratum Corneum  Consists of 15-20 layers of keratinized, squamous cells filled with keratin  Squames  Fully keratinized or cornified cells  Continuously shed at the epidermal surface as the desmosome and lipid-rich envelopes break down. CELLS OF THE EPIDERMIS 1. Melanocytes  Specialized cells in the epidermis found among the cells of the basal layer and in hair follicles  Pigments:  Eumelanin – brown or black pigments  Pheomelanin – red pigment in red hair CELLS OF THE EPIDERMIS 1. Melanocytes  Neural crest derivatives that migrate into the embryonic epidermis’ stratum basale in a 1:6 ratio  Pale-staining round cell bodies attached by hemidesmosomes at the basal lamina, but lacks attachment to the lateral borders CELLS OF THE EPIDERMIS Tyrosine 1. Melanocytes Tyrosinase  Synthesize melanin granules  Melanosomes are then transported L-DOPA to the tips of cytoplasmic extensions  Neighboring keratinocytes Dopamine Melanin phagocytose the tips of the Epinephrine dendrites taking up the Norepinephrine Accumulates in melanosomes melanocyte vesicles Melanosomes CELLS OF THE EPIDERMIS Tyrosine Melanocytes 1.Albinism Albinism Tyrosinase Congenital Synthesizedisorder melanin granules Hypopigmentation Melanosomes aredue totransported then a defect in L-DOPA tyrosinase to the tips of cytoplasmic extensions  Neighboring keratinocytes Vitiligo phagocytose the tips of the Dopamine Melanin Depigmentation of skin in patches Epinephrine dendrites taking up the Norepinephrine Accumulates in Loss or decreased activity of melanosomes melanocyte Melanocytes vesicles Melanosomes CELLS OF THE EPIDERMIS  Melanin  Inside the melanosomes  Transported toward the nucleus, forming a nuclear cap  Protecting the nucleus from harmful Nucleus effects of UV radiation CELLS OF THE EPIDERMIS  Epidermal-Melanin Unit  Melanocyte + Keratinocyte into which it transfers melanosome  Number of unites in skin is the same in all individuals  Rate of production of melanin and rate of accumulation inside the keratinocytes differ between individuals CELLS OF THE EPIDERMIS 2. Langerhans Cells  Antigen-presenting cells, derived from monocytes  2-8% of the cells in the epidermis and most are seen in the spinous layer  Cytoplasmic processes extend in between keratinocytes of all layers  Bind, process, and present antigens to T- lymphocytes for recognition (adaptive immunity) CELLS OF THE EPIDERMIS 3. Merkel Cells  Epithelial tactile cells  Low threshold mechanoreceptors: gentle touch  Abundant in highly sensitive areas (fingertips and base of hair follicles)  Located in the stratum basale  Attached to the keratinocytes by desmosomes CELLS OF THE EPIDERMIS 3. Merkel Cells  Few melanocytes but abundant in Golgi-derived dense-core granules  Synaptic contacts with nerves located at the basal lamina DERMIS DERMIS  Layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis)  Has irregular projections (dermal papillae) at the surface that interdigitate with projections of the epidermis (epidermal ridges or pegs) to form the dermal-epidermal junction  Provide nutrients to the epidermis through its rich vasculature through the basement membrane 2 LAYERS OF THE DERMIS 1. Papillary Dermis  Includes the dermal papillae (dermal projections)  Loose connective tissue: type I and III collagen fibers, fibroblasts and scattered mast cells + Dendritic cells + Leukocytes  Anchoring fibrils of type IV collagen (basement membrane) binds the dermis to the epidermis 2 LAYERS OF THE DERMIS 2. Reticular Dermis  Thicker and composed of dense irregular connective tissue with more fibers than cells compared to the papillary dermis  Network of elastic fibers is also present, providing elasticity to the skin.  Rich in dermatan sulfate (a proteoglycan of connective tissue) NUTRITION SUPPLY OF THE SKIN  Subpapillary Plexus  Located in between the papillary dermis and the reticular dermis  Plexus: network of blood vessels and nerves NUTRITION SUPPLY OF THE SKIN  Larger Plexus  Located in between the reticular dermis and the subcutaneous tissue  Arteriovenous anastomoses/shunts  Thermoregulatory function  Located between the 2 plexuses INNERVATION OF THE SKIN  Sensory Afferent Nerve Fibers  Autonomic Efferent Nerve fibers  Forms a network in the papillary  Sweat glands and smooth muscle dermis and around hair follicles fibers in the skin Afferent Nerve Fibers Brain Skin Efferent Nerve Fibers HYPODERMIS  Subcutaneous Layer, Superficial Fascia  Loose adipose connective tissue that binds the skin loosely to the underlying muscle.  Adipocytes, thin connective tissue fibers  Its extensive vascular supply promotes rapid uptake of injected drugs (insulin, vaccines). CUTANEOUS SENSORY RECEPTORS SENSORY RECEPTORS  The skin functions as an extensive receiver of various stimuli from the environment  Diverse sensory receptors are present  Encapsulated Receptors  Unencapsulated Receptors UNENCAPSULATED RECEPTORS Merkel Cells: Light touch and texture Associated with expanded nerve endings Free Nerve Endings: Temperatures, pain, itch, and tactile Located in the papillary dermis extending into the lower epidermal layers Root hair plexuses: Hair movement A web of sensory fibers surrounding the bases of hair follicles in the reticular dermis ENCAPSULATED RECEPTORS: MECHANORECEPTORS MEISSNER CORPUSCLE  Elliptical structures located at the papillary dermis arranged perpendicular to the epidermis  Consist of sensory axons winding among flattened Schwann cells  Initiate impulses when light touch or low frequency stimuli against skin temporarily deform their shape.  Numerous in fingertips, palms and soles, decline in aging PACINIAN CORPUSCLES  Large oval structures deep in the reticular dermis and hypodermis  Concentric lamellae of flattened Schwann cells and collagen surrounding a highly branched unmyelinated nerve fibers  Specialized in sensing coarse touch, pressure (sustained touch), and vibrations  Also found in the walls of rectum and urinary bladder, where they produce sensation of pressure RUFFINI CORPUSCLES  Have collagenous fusiform capsules anchored firmly to the surrounding connective tissue  Stimulated by stretch (tension) or twisting (torque) in the skin KRAUSE END BULBS  Are simple encapsulated, ovoid structures with extremely thin, collagenous capsules penetrated by a sensory fiber  Found primarily in the skin of penis and clitoris  Sense low frequency vibrations EPIDERMAL APPENDAGES HAIR  Elongated keratinized structures Phases of Hair Growth that form within epidermal 1. Anagen – a generally long period invaginations, hair follicles of mitotic activity and growth  Grows discontinuously, with 2. Catagen – a brief period of periods of growth followed by arrested growth and regression periods of rest. of hair bulb 3. Telogen – a final long period of inactivity, during which hair may be shed Parts:  Hair bulb – the terminal dilatation of hair follicle  Hair root – portion of hair that is embedded in the dermis  Hair shaft – visible extension of hair at the surface of the epidermis HAIR  Dermal hair papilla  Penetrates the base of the hair bulb,  Contains vasculatures supply nutrients and oxygen for proliferating and differentiating cells. HAIR  Medulla  central portion of hair  composed of large, vacuolated and moderately keratinized cells  Cortex  Surrounds the medulla  Composed of heavily keratinized, densely packed cells HAIR  Cuticle  Most peripheral cells of the hair root  Composed of heavily keratinized squamous cells covering the cortex  Epithelial tissue root sheath  Internal root sheath  External root sheath HAIR  Connective Tissue Root Sheath  Surrounds the hair follicle  Separates the hair follicle from the dermis  Glassy Membrane – an cellular hyaline layer composed of thickened basement membrane HAIR  Arrector pili muscle  A small bundle of smooth muscle cells  Extends from the midpoint of the fibrous sheath t the dermal papillary layer  Contractions pulls the hair shafts to a more erect position  Cold weather: contractions will entrap a layer of warm air near the skin  Regions of fine hair: produces tiny bumps on skin surface (“goose bumps”) NAILS  Produced also by keratinization  Hard plates of keratin on the dorsal surface of each distal phalanx NAILS  Nail root - proximal part of the  Nail matrix – formed from the nail nail root, as cells divide and move  Cuticle (Eponychium) distally  Covers the nail root  Nail Plate – hardened nail matrix that is bound to a bed of  An extension of the epidermal stratum corneum epidermis called the nail plate  Hyponychium – distal end of the nail SKIN GLANDS: SEBACEOUS GLANDS  Embedded in the dermis over  Branched acinar glands most of the body converging at a short duct  Except in palms and soles  Except for hairless regions: penis,  Increased number in face and hair clitoris, eyelids, and nipples  Empties into the upper portion of a hair follicle  Sebaceous Gland + Hair follicle → Pilosebaceous unit SKIN GLANDS: SEBACEOUS GLANDS  Have a basal layer of flattened epithelial cells → sebocytes  Sebocytes  Filled with small fat droplets (sebum)  Sebum covers the surfaces of both the epidermis and hair shafts. SKIN GLANDS: SEBACEOUS GLANDS  Sebum  Complex mixture of lipids  Increases greatly at puberty, stimulated primarily by testosterone in men and ovarian and adrenal androgens in women.  Maintains the stratum corneum and hair shaft  Exerts weak antibacterial and antifungal properties SKIN GLANDS: SWEAT GLANDS  Develop as long epidermal invaginations embedded in the dermis  2 types:  Eccrine sweat glands  Apocrine sweat glands ECCRINE SWEAT GLAND  Widely distributed, most numerous on foot soles.  Produces sweat, a physiologic response to increased body temperature  Both secretory and duct are coiled with small lumens ECCRINE SWEAT GLAND  Secretory part consist of unusual stratified cuboidal epithelium with three cell types:  Clear cells – located on basal lamina, have abundant mitochondria and microvilli  Dark cells – filled with strongly eosinophilic granules  Myoepithelial cells – contract to move watery secretion into the duct. ECCRINE SWEAT GLAND  Duct  2 layers of more acidophilic cells with filled with mitochondria and cell membranes rich in Na-K ATPase  Absorb Na+ from the secreted water to prevent excessive loss of electrolyte APOCRINE SWEAT GLAND  Largely confined to the axillary and perianal regions  Development depend on sex hormones and is not complete and functional until after puberty. APOCRINE SWEAT GLAND  Secretory  Duct  Larger lumen than eccrine sweat  Similar to eccrine glands with 2 glands layers of cells  Composed of simple cuboidal,  Open into hair follicles at the eosinophilic cells with numerous epidermis secretory granules APOCRINE SWEAT GLAND  Secretion of apocrine sweat gland  More viscous and initially odorless but may acquire distinctive odor due to bacterial activity  Pheromones are secreted in vestigial capacity (reduced). THE END