Anatomy Of Skin

ANATOMY OF SKIN INTRODUCTION:  The skin has a surface area of 2 sq.meter.

 

   

It accounts for 16-20% of the total body weight. It is the largest organ of the body It is composed of epidermis, dermis and the subcutaneous tissue (the hypodermis). Full thickness skin is 1.5- 4mm. Human skin is of 2 types --------- nonhairy (glabrous skin) and hairy skin. The glabrous skin is marked by a series of ridges ad grooves with a configuration unique to each individual ------- known as dermatoglyphics.

THE EPIDERMIS:

  

  

It is made of stratified squamous epithelium. It gives rise to derivative structures ( pilosebaceous units, nails and sweat glands) called appendages. It is approximately 0.4-1.5 mm in thickness. The keratinocytes and are organized into 4 layers IMMIGRANT CELLS OF THE EPIDERMIS: They are melanocytes, langerhans cells, and merkel cells. Other cells like the lymphocytes are extremely rare in normal skin. The epidermis rests on and is attached to a basal lamina that saperates the epidermis and dermis



KERATINOCYTE: It is an ectodermally derived cell that constitutes 80% of the epidermis. All keratinocytes have cytoplasmic keratin intermediate filaments and desmosomes.



Keratin filaments: They are a hallmark of keratinocytes and other epithelial cells. They are the cytoskeleton. More than 30 different keratins are present ------- 20 epithelial and 10 hair keratins, all within the range of 4070kDa. The keratins are saperated into acidic (type I; cytokeratins K10 to K20) and basic to neutral (type II; cytokeratins K1 to K9). Keratins are “obligate hetero polymers” meaning that a member of each family (acidic and basic) must be coexpressed for a filament structure.



KERATINIZATION is a genetically programmed, carefully regulated, complex series of morphologic and metabolic events that occur in postmitotic keratinocytes and involve -------• Increase in cell size and flattening of the cell shape • Appearance of new cellular organelles and the structural reorganization of those present

• •

• •

A change from a generalized cellular metabolism to a more “focused” metabolism associated with the synthesis and modification of molecules (structural proteins and lipids) related to keratinization. Alteration of the properties of the plasma membrane, cell surface antigens and receptors Eventual degradation of cellular organelles with chromatin fragmentation ------- characteristic of apoptosis. Dehydration.

The end point of keratinization is a terminally differentiated, dead keratinocyte (corneocyte) that contains keratin filaments, matrix proteins and protein reinforced plasma membrane with surface associated lipids.  BASAL LAYER • Consists of 1-2 layers of cells. • Contains mitotically active, columnar shaped keratinocytes.

•

It has a large nucleus and organelles The K5 and K14 pair of keratins are expressed in the basal layer of the epidermis and other stratifying epithelia.

•

Other keratins like K15 and K19 are present in putative stem cells. Microfilaments and microtubules are other cytoskeletal elements present in basal cells.

•

Some microfilaments maintain important links with the external environment via their association with integrin receptors of the cell surface. Integrins are a large family of cell surface molecules involved in cell- cell and cell – matrix interactions and INITIATION OF TERMINAL DIFFERENTIATION.

•

Not all basal cells display equal proliferative properties. Based on cell kinetics, 3 populations coexist in this layer ---- stem cells, transient amplifying cells and postmitotic cells. Stem cells are present in the bulge region of the hair follicle and also in the basal layer. These cells happen to be clonogenic, progress rapidly through the S phase of the cell cycle and divide infrequently during stable self renewal. However, under conditions requiring rapid proliferative activity ( wound healing) or after exposure to exogenous growth factors ------- THE STEM CELLS UNDERGO RAPID MULTIPLE CELL DIVISIONS.

•

•

•

•

Transient amplifying cells are a subset of daughter cells produced by the stem cells. These transient amplifying cells are NEEDED FOR STABLE SELF RENEWAL AND ARE THE MOST COMMON CELL TYPE OF THE BASAL LAYER.

• •

The postmitotic cells arise from the TAC after they undergo several cell divisions. It is the post mitotic cells that undergo terminal differentiation, detaching from the basal lamina and migrating superficially. Cell divisions in stratum corneum occur every 18-19 days.

•



The normal transit time of a basal cell, from the time it detaches from the basal lamina to the time it reaches the stratum corneum is at least 14 days. Transit through the stratum corneum and desquamation require another 14 days. SPINOUS LAYER • It contains 8-10 layers of cells

•

They are named for the spine like appearance of the cell margins in histologic sections. The spines are the abundant desmosomes, calcium dependant cell surface modifications that promote adhesion of epidermal cells and resistance to mechanical stresses.

•

CELLS OF STRATUM SPINOSUM:

  

Polyhedral with a Rounded nucleus Cells of the upper spinous layers are larger; more flattened and contain organelles called lamellar granules.

•

KERATIN PAIRS IN STRATUM SPINOSUM: Keratin filaments are present as usual. K5, K14 of the basal cells are present but in addition a new pair K1/K10 also occurs in the spinous layers. This new pair is characteristic of epidermal pattern of differentiation and thus referred to as differentiation or keratinization specific keratins.

•

In hyperproliferative states ( psoriasis, actinic keratoses), the sprabasal keratiocytes switch to an alternative pathway of differentiation in which the synthesis of K1/K10 is down regulated and K6/K16 is favored. Correlated with this change in keratin expression is a loss of normal phenotypic differentiation in granular and cornified cell layer.

•

DESMOSOMES: Within each cell is a desmosomal plaque associated with the internal surface of the plasma membrane and is composed of 6 polypeptides ------- PLAKOGLOBIN, DESMOPLAKIN I & II, KERATOCALMIN, DESMOYOKIN AND BAND 6 PROTEIN (plakophillin). Transmembrane glycoprotein of the cadherin family provide the adhesive properties on the external surface or core of the desmosome. These glycoproteins are ----- desmogleins 1 &3 and desmocollin I & II. The extracellular domains of these proteins form part of the core. The intracellular domains insert into the plaque, linking then to the KIF.

•

GAP JUNCTIONS: gap junctions between keratinocytes are the sites of physiologic communication. Gap junctions are more abundant in more differentiated keratinocytes. Communications with these junctions are important in the regulation of cell metabolism, growth and differentiation.

•

LAMELLAR GRANULES:

  

 

Deliver precursors of stratum corneum lipids into the intercellular space. The granules are first evident in the cytoplasm of the upper statum spinosum. Their primary site of action is the granular – cornified layer interface. They are 0.2-0.3 micrometer in diameter, membrane bound, secretory organelles that contain a series of alternating thick and thin lamellae. They contain GLYCOPROTEINS, GLYCOLIPIDS, PHOSPHOLIPIDS, FREE STEROLS AND A NUMBER OF ACID HYDROLASES INCLUDING LIPASES, PROTEASES, ACID PHOSPHATASE AND GLYCOSIDASES. GLUCOSYLCERAMIDES, the precursors to ceramide and the dominant component of the stratum corneum lipids, are found in the lamellar granules.

•



Roles of lamellar granule in providing the epidermal lipids responsible for the barrier properties of the stratum corneum, synthesis and storage of cholesterol and adhesion/ desquamation of cornified cells have been hypothesized. GRANULAR LAYER • This layer consists of 2-5 cell layers

•

KERATOHYALIN GRANULES: This is so called because of the presence of intracellular BASOPHILIC keratohyaline granules. KERATOHYALIN GRANULES are composed primarily of an electron dense protein, profillagrin. Loricin, a protein of the cornified cell envelope is also found in the granules.

•

Profillagrin is a high molecular mass histidine rich phosphorylated intermediate filament associated protein . it is made up of tandem repeats of filaggrin monomers joined by small linker peptides.

•

Conversion of profilaggrin precursor to filaggrin subunits occurs stepwise during the transition of granular cell to cornified cell by proteolysis and by dephosphorylation. Final proteolysis to free amino acids occurs in the outer layers of the stratum corneum. This is important for the regulation of epidermal osmolarity and flexibility.

•

The function of filaggrin: In the stratum corneum it is thought to serve as a matrix protein and promotes the aggregation and disulfide bonding of the KIF.

•

Proteins of the CORNIFIED ENVELOPE constitute a significant fraction of protein in the granular cells and are rendered insoluble by cross linking via disulfide bonds formed by TRANSGLUTAMINASES. The components of CE are -------

 



Loricin ---- it is a highly insoluble, sulfer- and glycine/serine rich protein . it is a later differentiation product and is the major CE protein comparising 75% of the total CE protein mass. The cytoplasmic surface of the CE is composed primarily of loricin.

  

Small proline rich proteins ( cornifin, SPR 1 & SPR-2) Serine proteinase inhibitor elafin Filaggrin linker segment peptide.



Envoplakin --- links CE to the desmosomes and KF.

•

Although the envelope precursors are first synthesized in the spinous and/or granular layers where they can be recognized biochemically and immunohistochemically, the CE is evident morphologically only in cornified cells.

•

The calcium requiring transglutaminases are present in all stratified squamous epithelia and in hair follicles. There are 3 transglutaminases in the epidermis.



 



Involucrin ------- it is an insoluble, cysteine rich protein, first synthesized in the cytoplasm of spinous cells. It becomes cross linked by transglutaminases in the granular layer into an insoluble cell boundary that is resistant to denaturing and reducing chemicals. Keratolinin

TGase 1 (keratinocyte TGase) : Exists as membrane bound activated forms responsible for most of the activity in differentiating keratinocytes. Some activity is present in the basal cells but the enzyme activity is especially prevalent in the granular layer of the epidermis. Mutations occur in lamellar ichthyosis. TGase 2 is present in the fetal epidermis and in the basal layer of the adult. It appears to play a role in apoptosis. It is similar to tissue transglutaminase. It is an autoantigen reacting in DH. TGase 3 ( epidermal TGase) is expressed after early differentiation marker ( K1 & K10 ) and is coincident with loricin and filaggrin.

•

ACTION OF LAMELLAR GRANULES STARTS IN THE GRANULAR CORNEAL INTERFACE: The lamellar granules act in the interface between the granular and cornified cell layers. In this position they aggregate into clusters, fuse with the plasma membrane and release their contents into the intercellular space. The extruded material is stacked into discs, similar to the internal organization of the granule, and is rearranged in various stages, leading to the formation of sheets. The hydrolytic enzymes are released with the lipids and are involved in the reorganization and the subsequent assembly into intercellular lamellae. Thus the probarrier lipids (glycolipids, free sterols and phospholipids) are converted to barrier lipids ( eg. sphingolipids that create a seal at the interface of the stratum granulosum and stratum corneum.

•

SELF DESTRUCTION OF CELLULAR ORGANELLES IN THIS LAYER: The granular layer also plays a role in its own programmed destruction . the change involves the loss of nucleus and virtually all cell components with the exception of the KERATIN FILAMENTS AND FILAGGRIN MATRIX. DNAase, RNAase, acid hydrolases, esterases, phosphatases, proteases and plasminogen activator are implicated in the destruction .

STRATUM CORNEUM • This layer is composed of 20-25 layers of cells • The stratum corneum barrier is formed by a two compartment system

• • •

The flattened polyhedral horny cell is the largest cell of the epidermis. They contain keratins and fillagrin. High molecular mass keratins stabilized by intermolecular disulfide bonds account for upto 80% of the cornified cell. The remainder of the cell contains electron dense matrix material, probably filaggrin surrounding the filaments.

•

INTERCELLULAR LIPIDS: The lipid enrichment results from the deposition of lamellar body contents. Three key lipid types ----- cholesterol, ceramides and FFA ---- form the lipid bilayers.

•

Changes in the structure, composition and function of cornified cells accompany their movement towards the outer surface of the skin. Cells of the lower SC , sometimes called the stratum compactum, are thicker and have more densely packed organized parallel arrays of KF, a more fragile cornified CE and modified desmosomes. They have less capacity for water binding than the mid and upper regions. The outer SC cells called the stratum disjunctum are more prone to desquamation. Cells in the mid SC have the highest concentration of free AA and therefore are able to bind water with greater efficacy. The CE becomes rigid. The desmosomes undergo proteolytic degradation -----shedding.

•

LOST BARRIER: Lipid extraction and metabolic imbalances such as FA deficiency perturb the barrier function as well as resulting in epidermal hyperproliferation, scaling and inflammation.

NONKERATINOCYTES OF THE EPIDERMIS (A) MELANOCYTES -------

•

It is a DENDRITIC, pigment synthesizing cell NEURAL CREST DERIVED CELL

•

CONFINED TO BASAL LAYER. Melanocyte processes contact keratinocytes in basal and the more superficial layers BUT DO NOT FORM JUNCTIONS WITH THEM AT ANY LEVEL. Melanocytes are recognized light microscopically by their pale staining cytoplasm, ovoid nucleus and the intrinsic color of the pigment containing melanosomes.

• • •

•

Differentiation of the melanocyte correlates with the acquisition of its primary melanogenesis, arborization and transfer of the pigment to keratinocytes.

•

The melanosome is a distinct organelle of the melanocytes. It is an ovoid membrane bound structure where melanin is produced

• •

4 different stages (I to IV) of the melanosome.

•

PHEOMELANOSOMES: Melanosomes that synthesize red or yellow pheomelanin pigments have a spheroidal shape and a microvesicular internal structure.

•

EPIDERMAL MELANIN UNIT: Approximately 36 basal and suprabasal keratinocytes are thought to coexist functionally with each melanocyte in an epidermal melanin unit.

•

Within the unit, the melanocytes transfer pigment to associated keratinocytes. As a result, pigment is distributed throughout the basal layers and to a lesser extent, the more superficial layers where it protects the skin by absorbing and scattering potentially harmful radiation. Once within the keratinocytes, melanosomes exist either individually or in membrane bound aggregates (melanosome complex). Melanocytes within the keratinocytes are degraded by the lysosomal enzymes as the cells differentiate and move upwards. .

•

CONTROL OF MELANIN PRODUCTION:  Keratinocytes produce growth factors that are mitogenic for the melanocytes ( eg. bFGF & TGF – alpha), but also produce growth inhibitory factors ( IL-1, IL-6, TGF- beta).

functions:

EUMELANOSOMES: Melanosomes that are involved in the synthesis of brown or black eumelanin are ELLIPTICAL AND HAVE AN INTERNAL ORGANISATION OF LONGITUDINALLY ORIENTED CONCENTRIC LAMELLAE.



Proliferation of the melanocytes and their dendrites, melanogenesis as well as transfer of pigment also relies on hormonal control (melanocyte stimulating hormone, sex hormone), inflammatory mediators and vitamin D3 synthesized within the epidermis. (B) MERKEL CELLS---------

• •

MECHANORECEPTORS: They are slowly adapting type I mechanoreceptors.

•

DISTRIBUTION IN THE SKIN: They are present among basal keratinocytes and extend cytoplasmic spines towards the keratinocytes.

• • •

Merkel cells receive stimuli as the keratinocytes are deformed.

•

•

LOCATION: They are located in sites of high tactile sensitivity like the digits, lips, regions of the oral cavity and the ORS of the hair follicle

They have a pale staining cytoplasm and lobulated nucleus. MARKER: Immunohistochemical markers of the Merkel cells include -certain keratin peptides. KERATIN 20 is restricted to Merkel cells in the skin and thus may be the most reliable marker. Merkel cells make synaptic contacts with nerve endings to form the Merkel cell- neurite complex. In the ultra structural levels, membrane bounded dense core granules that contain neurotransmitter like substances ----- metenkephalin, VIP, neuron specific enolase and synaptophysin.

(C) LANGERHANS CELL-------

 

The Langerhans cells are dendritic cells. They account for 2-8% of the total epidermal cell population.

•

MORPHOLOGY: Langerhans cells are pale staining with a convoluted nucleus. The cytoplasm of the Langerhans cells as seen by EM contains dispersed VIMENTIN INTERMEDIATE FILAMENTS AND SMALL ROD OR RACKET SHAPED STRUCTURES CALLED LANGERHANS CELL GRANULES OR BIRBECK GRANULES. It is a cup shaped disc which forms when membrane bound antigen is internalized by endocytosis. Phagolysosomes are common in the cytoplasm of the Langerhans cells

•

DISTRIBUTION IN THE SKIN: They are distributed in the basal, spinous and granular layers showing preference for suprabasal position

•

BONE MARROW DERIVED: They migrate from the bone marrow to the circulation into the epidermis early in embryonic development and continue to repopulate the epidermis throughout life.

•

ANTIGEN PRESENTING CELLS OF THE SKIN: They are antigen processing and presenting cells that are involved in a variety of T cell responses.

•

They are implicated in the pathologic mechanisms underlying ALLERGIC CONTACT DERMATITIS, CUTANEOUS LEISHMANIASIS AND HIV INFECTION.

•

They are reduced in the epidermis of patients with certain skin diseases like psoriasis, sarcoidosis.

•

OTHER AREAS WHERE LANGERHANS CELLS ARE FOUND: It is found in the other squamous epithelia including the oral cavity, esophagus, and vagina and also in lymphoid organs such as the spleen, thymus and lymph nodes. It is also present in the dermis.

•

LIFE CYCLE OF LANGERHANS CELLS: It is characterized by 2 distinct stages----The Langerhans cells in the epidermis can ingest and process antigens efficiently but are weak stimulators of unprimed T cells. Activated Langerhans cells that have been induced to migrate after contact with antigen are not phagocytic but are potent stimulators of naïve T cells.

•

LANGERHANS CELL MARKERS: They are characteristic of other cells of the monocyte- macrophage lineage.

•

RELATION WITH UVR: They are impaired functionally by the UVR. Following UVR, there is a decrease in the ability of Langerhans cells to present antigens, a decreased production of cytokines by keratinocytes and depletion in Langerhans cell numbers. Thus UVB irradiation results in overall diminished capacity for immune surveillance.

•

FUTURE PROSPECTS: Because of their effectiveness in antigen presentation and lymphocyte stimulation, dendritic cells and Langerhans cells have become prospective vehicles for tumor therapy and tumor vaccines.

THE DERMOEPIDERMAL JUNCTION ----- It forms an extensive interface between the epidermis and the dermis and also at the junctions between epidermal appendages and the dermis.  It is not visible in H&E stain.

 



On staining with PAS, it is seen as 0.5-1.0 micrometer thick homogenous band. It can be divided into 3 supramolecular network ----- The hemidesmosome-anchoring filament complex  The basement membrane itself  Anchoring fibrils The hemidesmosome – achoring filament complex binds the basal keratinocytes to the basement membrane.  The hemidesmosome has cytoplasmic plaque, transmembranous and extracellular components.

 

KIF insert into the cytoplasmic plaque portion which consists of BP230 antigen and plectin



The extracellular matrix components of the hemidesmosome are the subbasal dense plate and the anchoring filaments.

The transmembrane component consists of BP180 ( collagen XVII) and a6b4 integrin . The extracellular domain of BP180 has been localised to the extracellular space beneath the hemidesmosome ---- the lamina lucida. The intracellular domain of BP180 is localised in the hemidesmosomal plaque.



Anchoring filaments originate from the hemidesmosome and insert into the lamina densa. The major component of the anchoring filaments is LAMININ 5 which is localized mainly in the lamina densa and the lower lamina lucida. It is associated with a6b4 integrin of the hemidesmosome.



The lamina lucida is the primary location of several noncollagenous glycoproteins ------- laminins, entactin/nidogen and fibronectin. Because these molecules self aggregate, bind to other matrix molecules and to the cells, they are all important in promoting the adhesion between the epidermis and the lamina densa. THE LAMINA LUCIDA APPEARS TO BE THE WEAKEST ZONE OF THE DEJ. Type IV collagen is the primary component of the LAMINA DENSA. It is a nonbanded network forming collagen synthesized by the keratinocytes that provides structural support and flexibility to this layer. Type V collagen is codistributed with type IV collagen. Sulfated proteoglycans in this layer probably assist in regulating permeability by restricting the passage of cationic molecules. LAMININ is also present in this layer.





ANCHORING FIBRILS are broad (20-60nm), elongated (200-800 nm) flexible, banded, fibrillar structures that originate at the lamina densa and extend into the dermis. The fibrillar portions of anchoring fibrils have the morphologic appearance of collagen fibrils and have been shown to be composed of parallel bundles of type VII collagen molecules. These are synthesized by the epidermal keratinocytes. They arise from the subbasal dense plaque and pass through the lamina lucida to the lamina densa where they loop around and merge to the lamina densa or terminate in the papillary dermis (where they interweave with type III collagen fibres).



The anchoring fibrils penetrate the deepest zone of the DEJ, the sublamina densa region or the reticular lamina. Interstitial collagens and procollagens are present in this zone. In addition the first fibres of the elastic fibre system are organized and probably form part of the complex that anchors the epidermis and the dermis through the basement membrane zone. The oxytalan fibres are bundles of the microfibrillary components of the elastic fibres. They originate in the lamina densa and insert into the planar network of elastic fibres at the junction of the papillary and reticular dermis. The oxytalan fibres are coated with soluble elastin: they are flexible integrating elements that accommodate deformation of the skin without compromise to structural integrity. These fibres exert tension on the epidermis.

THE DERMIS

 

The dermis varies in thickness from 1mm in the eyelids to 5 mm on the back. The dermis forms the main bulk of the skin. It consists of

• • • • • • 



A supporting matrix, Several protein fibres like the collagen, elastin and A number of cells like the fibroblasts, macrophages and mast cells. Vessels, Lymphatics and Sensory and motor nerve endings.

The superficial one tenth part of the dermis is called the papillary dermis and the lower nine tenth part is called the reticular dermis. The papillary dermis and the periadnexal dermis together are also called the adventitial dermis. Unlike the epidermis no differentiation occurs in the dermis but the matrix and the connective tissue undergo restructuring and remodeling in response to external stimuli. The dermis

•

Provides pliability, elasticity and tensile strength. It protects the body from mechanical injury,

• • • •

Binds water,

•

Aids in thermal regulation and Includes receptors of sensory stimuli. The dermis interacts with the epidermis in maintaining the properties of both the tissue and collaborates during morphogenesis of the DEJ and epidermal appendages (teeth, pilosebaceous units and sweat glands) and interacts in the repairing and remodeling the skin as wounds are healed.

CONNECTIVE TISSUE MATRIX OF DERMIS ------1. Fibrous Connective tissue: Collagen and Elastic connective tissue.

2. 

Non fibrous Connective tissue: Filamentous glycoproteins and the proteoglycans and the glycosaminoglycans (GAGs).

Collagen tissue -------

• •

Collagen fibres are soft and flexible but strong and inelastic. They provide tensile strength.

•

Approximately 80-90% collagen fibres in the dermis are type I, 8-10% are type III and less than 5% type V.

•

Type I fibres are larger in diameter forming large and coarser bundles present mainly in the reticular dermis.

•

Type III fibrils are smaller and form small and fine network which are particularly concentrated in the papillary dermis and around blood vessels and adnexa. Type V is also present in the papillary dermis. Type VI is abundant throughout the dermis and is associated with the interfibrillar space.

•

•

They are the major components of the dermis accounting for 75% of the dry weight of the skin. They are of different types.

On light microscopy they are seen as 12-15 micrometer unbranched fibres either as finely woven network or as thick bundles.

•



On electron microscopy, they are seen as regular cross striated fibrils with a 60-70nm periodicity WHICH IS THE CHARCATERISTIC BANDING PATTERN OF COLLAGEN FIBRILS. EM also reveals triple helical structure of the collagen fibres. The triple helix formation is initiated by the spontaneous association of 3 individual C- terminal peptides (procollagens) in the fibroblasts. This is followed by the ordered helical binding from the C- to the N- terminus of the 3 alpha chains. Elastic tissue ----

•

Where present?It forms a continuous network from the lamina densa of DEJ to the CT of the subcutis. Elastic fibres are also present in the walls of the blood vessels and lymphatics.

•

Function and special properties: Because of their rubbery recoil, they maintain the normal configuration (elasticity) of the skin.

•

The sequence of elastogenesis: is initiated with the synthesis and deposition of microfibrils . Elastin is then deposited in variable amounts on the microfibrillar network. It is resistant to chemical cleavage and extremely insoluble. This insoluble elastin is formed from a soluble precursor, tropoelastin, by the action of lysyl- 6- oxidase which forms cross linkages. Mature elastic fibres contain as much as 90% elastin; microfibrils are embedded within and collected on the surface of the elastin matrix. Elaunin fibres have an intermediate amount of insoluble cross linked elastin.

•

Type of elastic fibres: Oxytalan, elaunin and mature elastic fibres occur in order progressively beginning at the DEJ.

•

The Oxytalan fibres: extend perpendicularly from the DEJ to the junction between the papillary and reticular dermis where they merge with the horizontal network of Elaunin fibres.

•

Elaunin fibres: are flexible and in turn evolve into the network of mature elastic fibres that extend throughout the reticular dermis. Elastic fibres are positioned between the collagen fibres.

•



•

Fibrillin: Elastic fibres have microfibrillar and amorphous matrix components. Several glycoproteins have been identified as constituents of the microfibrils. Among the most characterized of these molecules is FIBRILLIN, a 350 kDa molecule.

•

Appearance on light microscopy: elastic fibres are seen as darkly stained, unbranched fibrils that occur both separately and intermingled with collagen fibrils and matrix.

•

Appearance on transmission electron microscopy: they show distinctive sheet like agglomerates. The central speckled amorphous element of the elastic fibrils contains elastin whereas the peripheral part contains fibrillin and a heterogenous collection of other glycoproteins.

The diffuse and filamentous dermal matrix ---• They are present between the fibrous structures and make up the ground substance. • The main components are the PROTEOGLYCANS ( PGs) and GLYCOSAMINOGLYCANS( GAGs). • The PGs and GAGs can bind upto 1000 times their own volume and thus regulate the water binding capabilities of the dermis and influence the dermal volume and compressibility. • They also bind growth factors ( eg bFGF) and link cells with the fibrillar and filamentous matrix, thereby influencing proliferation, differentiation , tissue repair and morphogenesis. • Other matrix glycoproteins are ---------

    

Fibronectin --- insoluble filamentous glycoprotein synthesized in the skin by both epithelial and mesenchymal cells; it ensheathes collagen and elastin fibres, the surfaces of cells ------mediate cell matrix adhesion. Laminin Thrombospondin Vitronectin Tenascin

ORGANIZATION OF THE DERMIS: MAJOR REGIONS OF DERMIS The dermis is organized into papillary and reticular dermis; the distinction of the two zones is based largely on their difference in connective tissue organization, cell density, and nerve and vascular patterns. (A) PAPILLARY DERMIS :

• •

•

• • •

It is characterized by small bundles of small diameter collagen fibrils and oxytalan elastic fibres. Mature elastic fibres are usually not found in the normal papillary dermis but they are common in the skin of patients with certain inherited connective tissue diseases (e.g. dominantly inherited forms of EHLER DANLOS SYNDROME), in aging skin, and in actinically damaged skin. LARGE DENSE ELASTIC FIBRES WITH ABNORMAL STRUCTURE ARE THE HALLMARK OF SUN DAMAGED SKIN. The structural characteristics of the matrix in the papillary dermis permit the skin to accommodate to impact. The papillary dermis also has a high density of fibroblastic cells that proliferate more rapidly, have a higher rate of metabolic activity and SYNTHESIZE DIFFERENT SPECIES OF PROSTAGLANDINS as compared to those of reticular dermis. Capillaries from the subpapillary plexus project into the epidermis within the dermal papillae. The epidermis and the proximal dermis exchange a number of cytokines and growth factors, and matrix components of the dermis are linked to the cytoskeleton of the epidermis through transmembrane receptors. It is possible therefore that the organization and composition of the papillary dermis reflect the zone of influence of epidermis through its soluble and diffusible factors.

•

The sublamina densa region or the reticular lamina can be identified as a subdivision of the papillary dermis. It stains selectively for type I procollagen and is characterized by fine but dense organization of fibrils. Because of this, the structure is sometimes called a COMPACT ZONE. It is a region that is rich in cells that bear receptors for growth factors produced by the epidermis (e.g. PDGF) and has abundance of molecules that have adhesive properties (e.g. tenascin). This region is more prominent and thickened in pathologic skin. (B) RETICULAR DERMIS :

• •

It is composed primarily of large diameter collagen fibrils organized into large, interwoven bundles. Mature elastic fibres form a superstructure around the collagen fibre bundles. This gives a strong and resilient mechanical property. In normal individuals, the elastic fibres and collagen bundles of the reticular dermis increase in size progressively toward the hypodermis.

CELLS OF THE DERMIS



Fibroblasts, macrophages and mast cells are the regular residents of the dermis.



They are found in greatest density in normal skin in the papillary region and surrounding vessels of the subpapillary plexus, but they also occur in the reticular dermis where they are found in the interstices between the collagen fibre bundles.



Small numbers of lymphocytes collect around the blood vessels in normal skin and at the site of inflammation, the lymphocytes and leucocytes from the blood are prominent.

 

Pericytes and veil cells ensheath the walls of blood vessels. Schwann cells encompass nerve fibres. The fibroblast:

 





It is a mesenchymally derived cell. They appear as bipolar cells with ovoid nucleus.



It is responsible for the synthesis and degradation of FIBROUS AND NONFIBROUS CONNECTIVE TISSUE MATRIX PROTEINS AND A NUMBER OF SOLUBLE FACTORS. Thus they provide the ECM framework as well as interact with the epidermis and dermis via soluble mediators.



The cytoplasm of the fibroblast contains RER and Golgi complexes ----- suggesting active synthetic activity.



Human fibroblast cell lines are not identical and are a highly diverse population. Resting and proliferating fibroblasts respond to immune mediators including IL-1α and ß. Fibroblasts from hypertrophic scars appear abnormally sensitive to TGF-ß whereas interferon 1 alpha can decrease their proliferation and collagen production.

The macrophages, monocytes and dermal dendrocytes are a heterogenous collection of cells that constitute the MONONUCLEAR PHAGOCYTIC SYSTEM OF THE SKIN.

 

Macrophages are bone marrow derived.



Several antigenic markers characterize the macrophages and distinguish them from fibroblasts ---- MAC387, RFD7, KiM8, RFDR.



They have an extensive list of functions. They are phagocytic, they process and present antigens to immunocompetant lymphoid cells. They are microbicidal, tumoricidal, secretory ( growth factors, cytokines and other immunomodulatory molecules) and hematopoetic.

  

They are special secretory cells distributed in the connective tissue throughout the body,

They are difficult to distinguish from fibroblasts except that macrophages have lysosomes and phagocytic vacuoles.

Mast cells:



They occur typically at sites adjacent to the interface of an organ and the environment. In the skin, the mast cells are present in greatest density in the papillary dermis, near the DEJ, in the sheaths of epidermal appendages and around blood vessels and nerves of subpapillary plexus. They are also common in the SC tissue.



They are easily identified histologically as oval or spindle shaped cells with round or oval nucleus and abundant cytoplasmic granules that do not stain with H&E but stain metachromatically with methylene blue.



At the ultrastructural level the granules are membrane bound can be separated into Secretory and lysosomal granules.



Mast cells originate in the bone marrow from CD34+ stem cells. Differentiation towards mast cells occurs in the tissues under the influence of other cells and ECM matrix.

 

Like basophils they contain metachromatic granules and have IgE antibodies on their surface. Differentiation towards mast cells occurs in the tissues under the influence of other cells and ECM matrix.



Mast cells produce many mediators. Some of them are preformed and stored ----

• • • • • • •

Histamine,

• • •

Growth factors,

Heparin, Tryptase, Chymase, Carboxypeptidase, Neutrophillic cationic factor and Eosinophilic cationic factor. The mast cells synthesize and release other substance without storage ------Cytokines ( IL-1,3,4,5,GM-CSF,TNF-α), Leukotreins and



•

PAFs.

•

Lysosomal granules ----- Acid hydrolases that degrade GAGs, PGs, and complex glycolipids intracellularly. These take part in the repair process and degradation of foreign material.

The Dermal Dendrocyte:

    



It is a stellate, dendritic or sometimes spindle shaped It is a highly phagocytic cell in the dermis of the normal skin. They are APCs. They originate in the bone marrow. They are particularly abundant in the papillary dermis and upper reticular dermis frequently in the proximity of vessels of the subpapillary plexus. Dermal dendrocytes are also present around the vessels in the reticular dermis and in the subcutaneous tissue.

THE CUTANEOUS VASCULATURE:  The skin is richly supplied with a vascular network consisting of distributing and collecting channels and horizontal plexuses located at boundaries within the dermis and supplying the epidermal appendages.  The microcirculatory beds in the skin contain arterioles, precapillary sphincters, arterial and venous capillaries, postcapillary venules and collecting venules.   By comparison with the vasculature of other organs, the vessels of the skin have a thick wall supported with connective tissue and smooth muscle cells. This structure is advantageous to an organ that is regularly subjected to shearing forces.





The vessels that supply the dermis are small branches from the musculo cutaneous arteries that penetrate the subcutaneous tissue and enter the deep reticular dermis where they are organized into a deep horizontal arteriolar plexus. Ascending arterioles extend vertically from the plexus toward the epidermis. At the junction of the papillary and reticular dermis, these terminal arterioles form the superficial or subpapillary plexus. From here the capillary loops ascend into the papillary dermis, one capillary for each dermal papilla. The postcapillary descending limb of the loop is the venous capillary which drains into the subpapillary venous plexus ------- desending venous channels to the deep horizontal plexus ---- drains into larger deep veins. Structure and ultrastructure -----

• •



The ascending arterioles: They have * Two layers of smooth muscle cells, * A discontinous internal elastic lamina, * Pericytes and veil cells outside the vessel wall.



The anastomosing arterioles have

Single layer of smooth muscle and Lack elastic fibres.

  





The ascending limb of the capillary loop has a continuous endothelium, basal lamina and few pericytes. At the apex of the loop, both the endothelium and the basal lamina are attenuated to allow transport of the material out of the capillary. The descending post capillary limb and the descending venule have multiple layers of basal lamina and a loose sheath of pericytes and veil cells. These descending venules develop gaps between the adjacent endothelial cells allowing extravasation of fluid and escape of cells from the lumen. The endothelial cells on an ultrastructural level demonstrate many common cytoplasmic organelles like the Golgi apparatus, RER, SER, mitochondria and lysosomes. In addition they have many pinocytic vesicles, intermediate filaments containing vimentin and dense rod shaped bodies, the Weibel- palade bodies. These WP bodies are associated with actin like filaments which are 5-6 micrometer in diameter. Laminin and type IV collagen is present in the vascular basal lamina.

GLOMUS BODIES: Direct connections exist between arterial and venous circulation in certain regions of the skin (e.g. fingerpads, nail beds, palms and soles) without the interposition of capillaries. They serve as alternative bypass routes that shunt blood around congested capillary beds. These sites consist of an ascending arteriole (called a glomus body) which is modified by 3-6 layers of smooth muscle cells and has associated sympathetic nerve fibres and venules. The glomus can close completely when the blood pressure is below a critical level. Glomus bodies are associated with temperature regulation.

THE CUTANEOUS LYMPHATICS ------The cutaneous lymphatics regulate the interstitial fluid by resorption of leaked fluid. They also clear the tissue of unwanted proteins, lipids, organisms, cells and degraded material. Arrangement is like the blood vessels. Bicuspid valves prevent backflow and stasis of lymphatic fluid. Lymphatic vessels have a larger lumen as compared to blood vessels but a thinner vessel wall consisting of flattened endothelial cells, discontinous basal lamina and elastic fibres. Smooth muscle fibres in the lymphatic vessel wall are present only at the level of SC tissue. On electon microscopy, lymphatic endothelial cells contain cytoplasmic filaments that represent vimentin filaments.

THE NERVES AND SENSORY RECEPTORS OF THE SKIN ----- The nerve networks of the skin contain







Somatic sensory: The sensory fibres alone (free nerve endings) or in conjunction with specialized structures (corpuscular receptors) function at every point of the body as receptors of TOUCH, PAIN, TEMPERATURE, ITCH AND MECHANICAL STIMULI. The density and types of receptors are regionally variable and specific. Receptors are particularly dense in the hairless areas such as the AREOLA, LABIA AND GLANS PENIS



Sympathetic autonomic fibres: Sympathetic fibres are codistributed with the sensory nerves in the dermis until they branch to innervate the sweat glands, vascular smooth muscle, the arrector pili muscle of the hair follicles and the sebaceous glands.

The skin is innervated by the large myelinated cutaneous branches of the musculocutaneous nerves that arise segmentally from the spinal nerves. Small branches that enter the deep dermis are surrounded by the epineural sheath. Perineural sheath cover the fibre bundles and endoneural sheath cover individual fibres respectively. The pattern of nerve fibres of the skin is similar to the vascular pattern.



The sensory nerves, in general, supply the skin segmentally (dermatomes), but the boundaries are imprecise and there is overlapping innervation to any given area. Autonomic innervation does not follow exactly the same pattern because the post ganglionic fibres distributed in the skin originate in the sympathetic chain ganglia where pre ganglionic fibres of several spinal nerves synapse.



FREE NERVE ENDINGS ------ RAPIDLY ADAPTING

• • •

They are the most widespread and the most important sensory receptors of the body. They are always unsheathed by Schwann cells and a basal lamina. They are particularly common in the papillary dermis just beneath the epidermis.

•

Penicillate fibres: They are the primary nerve fibres found subepidermally in the hairy skin. Separate, unmyelinated branches from one or more myelinated stem axons are unsheathed collectively by the processes of a Schwann cell. They are further surrounded by the basal lamina and collagen fibrils. THEY ARE RAPIDLY ADAPTING RECEPTORS AND FUNCTION IN THE PERCETION OF TOUCH, TEMP, PAIN AND ITCH. In areas such as the palms and soles, they have a precise distribution and project into the dermal papilla individually and vertically. In other areas they are not so precisely distributed.

•



Papillary nerve endings: They are found at the follicular orifice. They are branches of nerves that innervate the deeper portion of the follicle. They have more mitochondria and vesicles and are PARTICULARLY RECEPTIVE TO COLD SENSATION. • Other free nerve endings: They are associated with specific structures ---- hair follicles (slow adapting that respond to bending or movement of hair), eccrine sweat gland (cholinergic sympathetic), arrector pili muscle (adrenergic and cholinergic fibres) • Free nerve endings are associated with individual Merkel cells. CORPUSCULAR RECEPTORS ------ Meissner’s corpuscles  Pacician’s corpuscles  Mucocutaneous end organs.

• •

They have a capsule and an inner core and contain both neural and nonneural components. The capsule is the continuation of the perineurium and the core contains both preterminal and terminal portions of the fibre surrounded by the lamellated wrappings of Schwann cells. Remodeling of the corpuscular receptors occur throughout life.

•

They express neuron specific enolase, neurofilaments, calbindin D28K and other calcium binding proteins, S-100 and p75 neurotrophin receptor.

•

THE MEISNNER’S CORPUSCLES: It is an elongated or ovoid mechanoreceptor located in the dermal papillae of digital skin and oriented vertically towards the epidermal surface. 1-6 myelinated axons enter the base of the corpuscle, lose their myelin coverings, ramify extensively and terminate in bulboid endings that are surrounded by lamellae.

•

PACINIAN CORPUSCLES ----RAPIDLY ADAPTING MECHANORECEPTORS:



They lie in the deep dermis and SC tissue of the skin that covers the weight bearing areas of the body.



The perineurium (capsule) is organized into 30 or more concentric layers of cells and fibrous connective tissue. There is a subcapsular zone of collagen fibres and fibroblasts and an inner core that consists of Schwann cell derived hemilamellae around the nerve fibre.

 

Usually one axon from a large nerve extends to the corpuscle without branching. They respond to vibrational stimuli.

•

Mucocutaneous end organs have the same structure as Meissner’s corpuscles and are located at the junctions of skin and mucous membrane like the lips, eyelids, perianal region, glans, prepuce, clitoris and labia minora.

THE MUSCLES OF THE SKIN:



SMOOTH MUSCLES: The smooth or involuntary muscles of the skin occur as ARRECTOR PILI, TUNICA DARTOS AND IN THE AREOLA OF NIPPLES. Smooth muscles lack striations and contain a central nucleus with a cigar shaped nucleolus. The muscle fibres of the arrector pili arise in the connective tissue and get attached to the hair follicle at an obtuse angle. When they contract, the hair follicle is pulled into a vertical position. EM demonstrates myofilaments in the smooth muscle cells and the narrow spaces between the cells are occupied by collagen fibres and Schwann cells. They contain vimentin and desmin intermediate filaments.



STRIATED MUSCLE: The striate or voluntary muscles are present in the skin of the neck as PLATYSMA and the MUSCLES OF FACIAL EXPRESSION. These muscles originate in the fascia or periosteum and extend to the lower dermis through the SC tissue.

THE HYPODERMIS OR THE SUBCUTANEOUS TISSUE: There is an abrupt transition from dermis to the SC tissue. Both are functionally integrated via vessels, nerves and epidermal appendages. Actively growing hair follicles extend into the SC fat, and the apocrine and eccrine sweat glands are normally confined to this depth of the skin



The SC fat consists of lobules composed of adipocytes separated by tin fibrous septa through which nerves, vessels and lymphatics pass.

 

Adipocytes are the primary cells of the SC tissue.

  



It is a mesenchymally derived cell. It has a cytoplasm with membrane bound lipids that displace the nucleus eccentrically. Lipids dissolve in routinely processed specimens giving the appearance of empty spaces within the cell. But lipids are visible in glutaraldehyde fixed plastic embedded specimens. SC fat deposits begin to form in the midtrimester fetus and are already well developed in the newborn infant. The synthesis and storage of fat continues throughout life by enhanced accumulation of lipid within fat cells, proliferation of adipocytes and by recruitment of new cells from undifferentiated mesenchyme. The hormone LEPTIN secreted by adipocytes appears to provide feedback control regulating fat mass.

ORIGIN AND DEVELOPMENT OF THE SKIN:  The juxtaposition of two major embryological elements, the prospective epidermis originating from the gastrula and the prospective mesoderm, leads to the development of the skin at about the third week of fetal life.

   

3rd week = epidermis is a single layer of glycogen filled cells



16th- 26th week = the intermediate layers increase in number with keratohyalin granules being present in the upper layers by the 21st week.

 

24th week = cells are shed which along with lanugo and sebum constitute vernix caseosa.



6th week = the epidermis consists of 2 layers, the epithelial layer and the germinative layer. 8th-11th week = a middle layer forms and a few microvlli appear on the surface of the epidermis 12th-16th week = more intermediate layers and many microvilli are formed. The cells of the intermediate layers contain mitochondria, golgi complexes and tonofilaments within and around them.

Most of the dermis develops from migrating mesenchymal cells and a small part from the ventrolateral part of the somite (dermatome). The dermis develops at about 4 weeks and is at first very cellular with collagen fibres detectable at about 4 weeks and elastic fibres at 22 weeks. Between 8-14 weeks, cells in the dermis also include stellate cells, phagocytes, melanoblasts and mast cells. From 14-22 weeks, fibroblasts, perineural cells, pericytes, merkel cells and mast cells can be demonstrated. SC tissue is primarily developed from mesenchymal cells starting at 14 weeks with lipoblasts appearing close to the developing blood vessels. These lipoblasts then develop into lipocytes.