Skin Anatomy

Skin • Skin is the largest organ in the body. •Surface area : 2 m2 , 10% total body mass• Weight : 5 Kg •Composed of : 1.A stratified cellular epidermis 2.Dermis of connective tissue 3.Subcutaneous fat ( Hypodermis)4.Dermal-epidermal Junction 5.Skin Appendages

2 Types of Skin: 1.Glabrous ( Non Hairy ) 2.Hair Bearing Skin •Glabrous : groved surface with alternating ridges (Dermatoglyphics) •Compact Stratum corneum ( 10 times of flexures) •Encapsulated sense organs with in dermis. •Lack of hair follicles and sebaceous glands. •Hair bearing skin: Lacks encapsulated sense organs .

EMBRYOLOGYEMBRYOLOGY

Origin of SkinOrigin of Skin Prospective epidermisProspective epidermis Prospective mesodermProspective mesoderm Neural Crest (Pigment Cells)Neural Crest (Pigment Cells)

Origin of EpidermisOrigin of Epidermis From the EctodermFrom the Ectoderm Notch and Wnt signalling pathwaysNotch and Wnt signalling pathways

STAGESSTAGES 33rdrd week – Single Layer epidermis week – Single Layer epidermis 4-64-6thth week – 2 layer week – 2 layer

PeridermPeriderm Straitum germinativumStraitum germinativum

1010thth week – Desmosomes and hemidesmosomal week – Desmosomes and hemidesmosomal proteinprotein

8-118-11thth weeks – Middle layer and microvilli appears weeks – Middle layer and microvilli appears 12-1612-16thth weeks – Intermediate layers and weeks – Intermediate layers and

numerous microvillinumerous microvilli 1515thth week week – – Filaggrin Filaggrin protein in granular layerprotein in granular layer 2121st st week – week – Keratohyaline granule Keratohyaline granule increases in increases in

uppermost layeruppermost layer 2424thth week – Periderm shed – vernix caseosa week – Periderm shed – vernix caseosa

Hair follicles- 9 weeksSweat glands-9 weeks:palms and soles15 weeks:other areaSebaceous glands-15 weeksNails-3rd monthLangerhans cells-12 weeksMerkel cells-16 weeksMast cells-6-14 weeksMelanoblast-6-14 weeks

Schematic overview of embryonic development of human skin

TIME

Epidermis •Epidermis is terminally differentiated, stratified squamous epithelium. Renews continuously and forms derivative structure. •Thickness : 0.05-0.1mm•Mainly composed of Keratinocytes : 95% of total cells • other cells: Melanocytes, Langerhans cells and Merkel cells. •Keratinocytes progressively moves outwards from their attachment to the epidermal basement membrane forming several layers. •On morphological grounds: 4 layers 1.Stratum basale 2.Stratum Spinosum3. Stratum Granulosum 4. Stratum Corneum

STRATUM BASALE: STRATUM GERMINATIVUM •Continuous layer,1 cell thick,(2-3 cell thick in glabrous skin)•Cells are small cuboidal with large dark stained nuclei dense cytoplasm, contains many ribosome and dense tonofilamament bundle. •In addition , membrane bound vacuoles containing pigmented melanosomes transferred from melanocytes by phagocytosis.•Is Primary site for mitotically active cells. •Stratum basale attached to BMZ via keratin intermediate filament K5\K14 to hemidesmosomes of BMZ.Applied aspect-k5/k15 mutation causes epidermolysis bullosa simplex

Stratum Spinosum: Prickle cell layer•Spine like appearance of cell margin in the Histologic section. •8-10 cell layers thick.• On Histological section :•Cells are polyhedral with a rounded nucleus. •As these cells differentiated and moves upward through epidermis, become proggressively flatter and develop organelles known as lamellar granule. •Also contain large bundle of Keratin filaments organizes around the nucleus and intersected into desmosomes peripherally.•Spines are abundant desmosomes.

STRATUM GRANULOSUM : •2-5 Cells Layer Thick.•Granular layer due to the presence of intracellular Basophillic Keratohyaline granules which contains Profilaggrin, Keratin filament, Loricrin .KERATOHYALINE GRANULES: particles of irregular shape, and occurring randomly in rowes and lettices.•This layer also contain smaller lamellar granular or Membranes coating granules / odlands bodies .• They discharge their lipid content into the extracellular spaces .•Cornified envelop begins to form here.•APPLIED ASPECT:Upper spinous and granular layer keratinocytes involves in Icthyosis bullosa of Siemens.

STRATUM CORNEUM•20-25 layer thick,outer most layer. •Has cornified cells (corneocytes)•No nucleus and cytoplasmic organelles.•Flattened keratin filament align into macrofibres due to fillagrin.•Fillagrin is protein component in keratohylaine granules. •Profillagrin : in F granules ,minor contribution.•Membrane associated proteins : periplakin , envoplakin, epiplakin ,desmoplakin as well as plectin.•Filaggrin provides interfilamentous material. •Corneocyte has a insoluble CORNIFIED ENVELOPE with in Plasma membrane.Formed by involucrin and loricrin (soluble protien)Others precursors : small protein rich protein (SPR1) = cornifin and pancornifin, SKALP / elafin, keratolinin / cystatin.

STRATUM LUCIDUM

•In palmoplanter skin.•Additional zone.•Electone lucent.•Present between statun granulosum and stratum corneum. •Cells are still nucleated called as “TRANSITIONAL CELLS”

KERATINOCYTES•Ectoderm derived cells. Composes of –• Actin : 7 nm micro filament •Tubulin : 20-25 nm microtubules •Inter mediate filament : 7-10 nm •Keratinocytes contain : keratin intermediate filament – •type I ( Acidic) • type II ( Basic) In Simple epithelium : K8 /K 18In stratified epithelium = K 5/K14 In Suprabasal layers : K 1/K10 for epidermal differentiation.APPLIED ASPECT: 1.SUPRABASAL KERATINOCYTES-BULLOUS CONGENITAL ICTHYOSIFORM ERYTHRODERMA.2.BASAL KERATINOCTES-EPIDERMOLYSIS BULLOSA SIMPLEX.

MELANOCYTES•Melanocytes are dendritic cells derived from Neural crest .Reside in Basal layer primarily. •Pale staining cytoplasm, ovoid nucleus and pigment containing melanosomes.•Melanocytes are in contact with Keratinocytes through their processes but do not form junction with them at any level.•One melanocyte is in contact with 36 basal and suprabasal keratinocytes. This group known as EPIDERMAL MELANIN UNIT.2 forms of melanin pigment: Brown / black eumelanin. Red /yellow phaeomelanin. APPLIED ASPECT:Reduced in Piebaldism and wardenberg syndrome.Defective pigment production or processing:Albinism and Chediak-Higashi syndrome.

MERKEL CELLS.•0.2-5% of total cells•Post mitotic cell ,slow adapting type I mechano receptors (Fine Touch), located amongst basal keratinocytes .•found in hairy skin , tactile areas of glabrous skin, taste buds, anal canal, labial epithelium and eccrine sweat glands.•Oval cell ,Have large bilobed nucleus and clear cytoplasm. • Contain numerous neurosecretory granules. •Forms close connection with sensory nerve endings known as “MERKEL CELLS NEURITE COMPLEX”. In glabrous skin: clustered near the unmyelinated sensory nerve endings and forms “TOUCH SPOTS” at the bottom of rete ridges known as “Hair discs ,Touch domes, Touch corpuscles or Iggo discs.” In hairy skin: also clustered in rete ridges and in the outer root sheath of hair follicle where the arrector pilli muscles attach. APPLIED ASPECT:•absent in vitiligo lesions•Merkel cell carcinoma.

LANGERHAN CELLS They constitute 2-3% of total cells.•Dendritic cells similar to melanocytes but free from pigment and dopa negative and derived from bone marrow. •In combination with macrophages and dermal dendrocytes represent the skin’s mononuclear phagocyte system. •Can also release cytokines as IL-1 to promote lymphocyte chemotaxis and activation .•Are intraepidermal macrophage. •Pale stained cytoplasm with lobulated nucleus, well developed endoplasmic reticulum, golgi complex and lysosomes.•Mostly found in a suprabasal portion but distributed throughout the basal spinous and granular layers. •Possess a characteristic -BIRBECK GRANULES. •Also found in outer root sheath of hair follicle, secretory duct of sebaceous gland and in the epithelium of the crypt of Human tonsils.•APPLIED ASPECT:•CHRONIC DERMATITIS•CHRONIC LEISHMANIASIS•HIV INFECTION

Epidermal attachment complexes.•Desmosomes•Adherens junction •Gap Jnction•Tight junction

DESMOSOMES:Are the major adhesion complex in epidermis, anchors keratin intermediate filaments to the cells membrane and bridging adjacent keratinocytes and allowing cells to with stand trauma. Structure: Cell membrane of 2 adjacent cells form a symmetrical junction with a central intercellular space of 30 nm containing a dense line .•Plaques of electron dense material runs along the cytoplasm, parellel to the junctional region , thus 3 ultra structure bands are formed- •electron dense band next to plasma membrane. •Less dense band •Fibrillar area

Main component of desmosomes in the epidermis :1.Desmosomal cadherins 2.Armadillo family of nuclear and junctional protein 3.Plakin familyDermosmal cadherins - Transmembranous strctures .• comprises mostly heterophillic association of Desmogleins (Dsg 1-3)and Desmocollins(Dsc 1-3) •The intracellular part of these glycoprotein are attached to the keratin filament network via Desmoplakin, Plakoglobin, Plakophillin.•APPLIED ASPECT-DSg 1:staphylococcal skin scalded syndrome and bullous imeptigo.

ADHERENCE JUNCTION •Electron dense transmembranous structure , engage with actin skeleton. •Can associate with tight junction and desmosomes or exist separately. •Characterized by 2 opposing membrane separated by approx. 20nm and are 0.2—0.5 µm in diameter. •Comprises 2 basic adhesion units : 1.Nectin afadin complex2.Classical cadherin complex•Nectin forms a structural link to the actin cytoskeleton via afadin (AF-6).•Cadherins from complex with Catenins (α-,β-,p120-catenin).

GAP JUNCTION Comprises clusters of intercellular channels known as connexons, •Directly form connections between the cytoplams of adjacent keratinocyte and other cells. •13 different connexins are there. •6 Connexins 1 Connexon•At plasma membrane connexons associate with other connexons to form a gap junction.•Homotypic or heterotypic connexins are possible . •FUNCTIONS OF GAP JUNCTION: essential for cell synchronisation, differentiation , cell growth and metabolic coordination of avascular organs including epidermis.

Tight junctionMajor regulators of permeability in simple epithelia.•In skin:role in skin barrier integrity and maintain cell polarity. Principle structural protein of tight junction :• Claudins(1-4)•Other component transmembranous protein : Ig G- like family of junctional adhesion molecules and the occludin group of protein.

DERMAL EPIDERMAL BASEMENT MEMBRANE

Defined as one of the largest epidermomesenchymal junction in body, forms an extensive interface between dermis and epidermis and epidermal appendages. * Dermo- epidermal junction is a contiguous network extending from intra cellular milleu of basal keratinocytes through plasma membrane of basal cells traversing the dermo epidermal basement membrane and extending to the upper papillary dermis.

Basement memb Zone (200 µm across)Many of these components are glycoproteins. • Basement membrane zone is stained by PAS stain.• 2 Distinct layers1. Lamina Lucida : Less electron dense directly

abuts the plasma membrane of the basal keratinocytes.

2. Lamina Densa: Electron dense region at the lower part, interacts with the Mesenchymal matrix of the upper dermis.

(3) Lamina fibroraticularis : Anchoring fibrils are major constituent.

another component of lamina fibroreticularis- elastic microfibrils.(Fibrillin is main elastic micro fibril)

TYPE IV COLLAGEN

•Major component of dermal - epidermal basement membrane. •Each molecule consists of 3 polypeptide subunits known as α –chains. •Hetero polymer (α-chain genetically different but structurally related).•Type IV collagen of alpha -1 and alpha -2 chains with composition of [α1( IV )2 α2( IV )].•The non collagenous globular domains both at the amino and corboxy ends of the individual collagen molecules interact to form dimes and tetramers.•These dimers and tetramers then assemble in a complex hexagonal arrangement .•APPLIED ASPECT:•Mucous membrane pemphigus.•Epidermolysis bullosa aquisita.

LAMININ FAMILY

• 4 out of 16 laminin families are physiologically present in the skin. •Each laminin molecule contain of 3 poly peptide subunit α,β and γ chain which from a cruciform structure with 3 short arms and one long arm.cruciform structure has various functions :•Interaction with other extra cellular matrix molecules such as hemidesmosomal component and type VII collagen, cell attachment and spreading neurite out growth, cellular differentiation.

•Short arm - the N terminal segment of the α, β, and γ chains.•Long arm -rod like structure of the triple standard coiled domain of all three chains. •This domain serves as the site of chain assembly of the 3 subunit polypeptides. • α chain consist of an additional C terminal segment which contain 5 globular segment located at the tip of long arm known as G – domain.•Major Laminin in the cutaneous BM zone is the Laminin 332. Others are 311, 511 .•Cell binding of the laminins is mediated by integrin .•APPLIED ASPECT:•Laminin 332:mucous membrane pemphigus.

OTHER BMZ COMPONENT •Nidogen (previously called entactin) interacts with type IV collagen either alone or as a laminin Nidogen complex. •It is highly conserved sulphated glycoprotein . •Perlecan-main Proteoglycan of basement membrane. •Heparan sulphate proteoglycans Consists of a core protein with covalently associated heparn and sulphate chain highly negatively charge and hydrophillic.

HEMIDESMOSOMAL ANCHORING FILAMENT COMPLEX HEMIDESMOSOMES are electron dense attachment complex extend from the intraculllar compartment of the basal keratincytes to the lamina lucida in upper portion of Dermo Epidermal Basement Membrane .(A) Intra cellular component with in basal keatincytes attaches to the keratin intermediate filament network. (B) Extra cellular component with in lamina lucida they are contiguous with anchoring filaments.

There are 5 major components of hemidesmosomes: HD1 = Corresponds to plectin ( Intra cytoplasmic adhesion molecule)HD 2= correspondens to 230 K Da BPAG 1. HD3= corresponds to β 4 integrins HD 4= Corresponds to 180 K Da BPAG 2HD 5=- α 6 integrins

• Inner plaque : contain the 230 K da BPAG1. non collgenous protein ( Plectin) •Trans membrane region : 180 K da BPAG 2Known as type XVII collagen Interact with α6 β4 integrins. Extends from intracellular compartment to the extra cellular space .• Outer plaque : closely associated with the Basal plasma membrane .• Subbasal dense plate : extra cellular component. •APPLIED ASPECT:Junctional epidermolysis bullosa.

ANCHORING FILAMENT : major component - Laminin 332 , other component – Laudinin.• Very fine structures oriented perpendicularly between the Lamina densa and basal plasma membrane. It tend to concentrate below the hemidesmosomal outer plaque .

ANCHORING FIBRILS.U shaped structures which extend from the lower part of lamina densa to the upper reticular dermis may also insert into the amorphous bodies in the superficial dermis known as anchoring plaques. Component: Type VII collagen is the major component. APPLIED ASPECT:Dystrophic epidermolysis bullosa.

TYPE VII COLLAGEN : •450 nm long central triple helical segment.• Flanked by non helical globular domains at each end of triple helix.• NC1 at amino terminus and NC 2 at carboxy terminus. • There are some imperfections in triple helix. ( Central 39 AA non collagenous segment). •These interruptions are in the glycine X-y sequence. X = Proline Y= Hydroxy proline

• Upon section 2 type VII collagen moleculas align into an antiparallel dimer with overlapping NC 2 domains. •Dimer is stabilized further by intra molecular disulphide bonds. • Thus Type VII collagen aggregate to form anchoring fibrils. •In which NC1 domains at both end attach to basement membrane (inetract with type IV collagen and laminin 332 of Basement Membrane) •U shaped loops then entrap and interact with large interstitial collagen fibres consisting of type I, III and V collagens.

THE DERMIS •Contributes 15-20% of total weight of human body.• About 60 % of total weight of dermis is water .•Dermis is major component of the human skin .•Dermis is largely acellular and consist primarily of the extra cellular matrix of the connective tissue. Thickness : (< 0.5mm - >5mm)

There are 4 major classes of extra cellular matrix 1. Collagen fibres provide tensile strength to the

skin . 2. Elastic fibres provide elasticty and resilience. 3. Non collagenous glycoprotein- fibrillins, fibulins

and integrins, fibronectins. Are organizer of the matrix and facilitated cell

matrix interactions. 4. Proteoglycan/ glycosamino - glycan

maronomecules- provide hydration to the skin Contributes only 0.1-0.3% total dry weight.

(2) Reticualr dermis : forms the bulk of dermal tissue (9/10). composed of primarily of large diameter collagen fibrils organized into large inter woven fiber bundles with branching elastic fibers. •Lowest boundary of reticular dermis is defined by the transition of fibrous connective tissue to adipose connective tissue of hypodermis.•In the region such as nipple , penis ,scrotum or perineum there are also stress oriented smooth muscles in reticular dermis.

Dermis is arranged in 2 major regions (1) upper papillary dermis : abuts the epidermis, molds to its contours and is usually no more than twice its thickness(1/10).

COLLAGEN FAMILY OF PROTEINSMajor extra cellular matrix component in the dermis forms 75% of dry weight of dermis.•Type I = 80% Type III 10% Type V < 5% Structure: is a triple helical conformation of α chain depicting a repeating glycine X –y sequence also have non collagenous flanking segments at the end of the individual molecule. •All collagen molecule consists of 3 subunit polypeptides which can either be homotrimer or can be heterotrimeric molecules.

Type I,II,III, V and IX: align into large fibrils and designated as fibril forming collagen. Type IV : arranged in an interlacing intervals in the basement membrane. Type VI : microfibril forming collagen Type VII : forms anchoring fibrils. Type IX, XII, XIV XIX XX, XXI : FACIT collagen. Type XVII : Trans membranous

ELASTIC FIBRES: •Elastic fibres are a relatively minor component < 2-4 % of total dry weight of dermis.• Extends from lamina densa of the Dermo Epidermal junction throughout the dermis and into the connective tissue of the hypodermis.•In the reticular dermis elastic fiber system consists of horizontally oriented fibers which inter connect to provide a network structure. •consist of 2 types of fibres: •Oxytalan fibres : Bundles of microfibrils. Restricted to papillary dermis . •Elaunin fibres: cross linked elastin

Elastic fibers have 2 principal component (A) Elastin : is a connective tissue protein is forms the core of mature fibers .(B) Ealstin associated micro fibrils consists of family of protein. They are peripheral component. Ultra structure of elastic fibers. An electron lucent core – consists of elastin Electron dense surrounding - microfibrillar structure.APPLIED ASPECT:Cutis laxa

ELASTIN• Major component of elastic fibres. •Initially synthesized as a precursor poly peptide tropoelastin which consists of approx 700 amino acids with a molecular mass of 70 KDa . •1/3 of total amino residues consists of glycine , glycine is not evenly distributed of every 3rd position as in collagen. •The tropo elastin primary sequence shows domains rich in glycine, valine and proline alternatively with lysine and alanine rich sequence. • Lysine residue forms covalent cross links, desmosine and isodesmosine.

ELASTIN ASSOCIATED MICROFIBRILS.

Consists of tubular structures of approx 10-12 nms in diameter. There are many families of them –:1. Fibrillins - fibrillin I fibrillin II2. LTBP Latent transforming growth factor binding family of proteins .3. Fibulins. 4. Other - Microfibril associated glycoproteins or Microfibril associated proteins - Interface protein emilins - Lysyl oxidase

PROTEOGLYCANS / GLYCOSAMINOGLYCAN MACROMOLECULES.

Proteoglycans form a number of subfamilies defined by a core protein to which polymers of unbranched disaccharide units ( glycosamino- glycan) are linked by an ortho- linkage to serine residues.

GLYCOSAMINOGLYCAN•Are highly charged polyanionic molecules which attach to the core protein .• Primary structure of Glycosaminoglycan consists of alternating pairs of different monosaccharides, glucose or galactose joined in 1-3 or 1-4 linkage. •These polymers under go complex post assembly modification catalyses by specific enzymes like sulphatase or epimerase, they include sulphation( the replacement of N – acetyl by N – sulphate) and epimerization of D- glucuronic to L – iduronic acid .•Hyaluronic acid is not sulphated while other are sulphated

EXAMPLE OF GLYCOSAMINOGLYCAN1.Heparin2.Heparan3.Hyaluronic acid4.Keratan sulphate5.Chondroitin sulphate6.Dermatan sulphate

Most Proteoglycans are connected to their protein cores by O- glycosidic bonds of two galactose joined to a xylose and serine residue. PG form a distinctive subsets: (a) small leucine rich PG ( SLRPs) (b) Traditional large molecular PG • They form a distinct subsets (A) Hyalolectins eg. versican and aggrecan (B) Those that do not. The core protein can be :

-Intracellular serglycan-On call surface glypican ,syndecan -Extra cellular – versican

FUNCTION OF PG •Bind extra cellular matrix component . •Bind several growth factors cytokines cell adhesion molecular growth factor binding proteins .•Can serve as antiproteases .•Adhesion of cells to extra cellular matrix. •Syndecan -4 facilitates the adherence of cells in conjunction with other extracellular molecular integrins. •Chondroitin sulphate & dermatan sulphate bind fibro necting and Laminin .•Hyaluronic acid provide physio chemical properties to the skin, Hydrophillicty and viscosity. •Provide hydration to skin. •Expand the matrix and thus aid cell movement.

CELLULAR COMPONENT OF DERMIS Fibroblasts :• Responsible for synthesis of connective tissue in the dermis. •Fully differentiated biosynthetically active cells.•Have an abundant cytoplasm, well developed rough endoplasmic reticulum prominent ribosomes attached to cell membrane surfaces, secretion of extra cellular matrix macromolecules.

MAST CELLS :Ability to stain metachromatically with basic aniline dyes.•Larger then eosinophils and basophills. Distributed close to blood vs nerves and appendages. •Are most numerous in the sub papillary dermis in the region of superficial dermal vascular plexus .•Avoid or spindle shaped, mononuclear or occasionally binuclear and only rarely show signs of mitosis in Skin. • Presence of numerous round cytoplasmic granules. • Mast cells are heterogenous . 2 main types - connective tissue- Mucosal.

Blood Vessels

Skin has a rich vascular network with distributing and collecting channels. 2 Vascular plexus are there – 1. Superficial vascular plexus : Between papillary and reticular dermis. 2. Deep Vascular plexus : at the level of dermis and hypodermis junction.

Structure of Blood Vessels.

1. Inner most layer : endothelium In arterioles : subendothelial elastic tissue In venules : No subendothelial elastic tissue .Capillaries , Small arterioles , venules : Also contain pericytes

surrounding endotheliam . Capillaries have single discontinuous layer of pericytes . Venules have more than 1 layer of pericytes .2. Smooth muscles cells : In ascending arterioles , arterioles of

vascular plexus, collecting venules. 3. Basement membrane : 4. Veil Cells : More closely resemble fibro blast no basement

membrane . Present outside the vessels wall .

Ultra Structure Or Blood Vessels. Arterioles : Contain vimentin Dense bodies Associated with actin like filament .Weibel Palade bodies

Venules:Multi layered basement membrane - Laminin 111 - Type IV collagen , fibronectin , Heparan Sulphate APPLIED ASPECT:Stasis dermatitis•Urticaria•Thrombosis•Thrombophlebitis

Sub cutaneous fat : 80% of fat in subcutis In non obese males 10-12% In females : 15-20% Function : 1. Insulation 2. Mechanical Cushioning 3. Energy Store (40 day’s Reserve) 4. Secretes Leptin – regulate appetite 5. Osteogenesis , angiogenesis 6. PhagocytosisAPPLIED ASPECT:PANNICULITIS

Nerves : Autonomic Nerves :-1. Cholinergic –to Eccrine Sweat glands 2. Adrenergic – to the both eccrine sweat glands and

apocrine glands. - Also in smooth muscles , arterioles , arrector

pilli musclesSansory nerve endings :

1. Free nerve endings - (1) Penicillate nerve endings (2) Papillary nerve endings 2. Corpuscles - (1) Meissner’s Corpuscles (2) Pacinian Corpuscles