EPIDERMIS -1MODERATOR DR.JAIDEV.

Dr. ASHWINI.N

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SKIN

• Skin is the largest organ of the body accounting for 16 - 20% of total body weight , with a surface area of 1.8 – 2 m2 .

• Two main kinds of skin - Glabrous Skin ( non-hairy skin).

- Non Glabrous Skin.(hairy skin).

• Skin is divided into 3 layers

a) Epidermis- outermost layer , serves as barrier between exterior & body’s interior environment and is responsible for cornification.

b) Dermis- a deeper layer ,provides structural support to skin.

c) Subcutis- containing fat and connective tissue.

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EPIDERMIS

• Is metabolically active, stratified squamous epithelium.

• Consisting of 4 type of cells.

1.Keratinocytes.

2.Melanocytes.

3.Langerhans cells.

4.Merkel cells.

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Layers Of Epidermis

• Stratum basale.

Stratum malpighian.

• Stratum spinosum.

• Stratum granulosum.

• Stratum lucidium.

• Stratum corneum.

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STRATUM BASALE

• Basal cell layer, consisting of 1-2 layers of columnar/cuboidal keratinocytes ,with large oval nuclei & basophilic cytoplasm.

• The cells in basal layer(keratinocytes) are interconnected by intercellular bridges Desmosomes.

• Basal keratinocytes are attached to subepidermal basement membrane by modified desmosome-Hemidesmosome.

• Basal keratinocytes are interspersed with melanin producing cells, melanocytes at regular interval.

• Stratum basale is the primary site for MITOTIC ACTIVITY

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STRATUM SPINOSUM

• 8-10 layers of cells.

• Keratinocytes in this layer are Polyhedral in shape, with round nucleus, basophilic cytoplasm.

• Spinous processes (abundant desmosomes) hence the name.

• The cells in upper spinous layer are larger & become more flattened and contain organelles “lamellar granules”

• The cells in spinous layer contain large and conspicuous bundle of keratin filaments Tonofibrils, that insert into desmosome peripherally.

• Limited cell division.

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STRATUM GRANULOSUM

• 2-5 Cell layer thick.

• Diamond shaped cells with intracellular basophilic keratohylinegranules, with deeply basophilic cytoplasm.

• Cytoplasm of the keratinocytes in this layer contain lamellatedgranules known as odland bodies.

• Discharge their lipid components into intercellular space, functions asbarrier & intercellular cohesion within the stratum corneum

• Keratohyline granules forms 2 structures.

a)Inter fibrillary matrix or filagrin- that cements keratin filaments

together

b) marginal bond- provides strength and flexibility.

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STRATUM LUCIDIUM

• Is a thin layer of translucent cells seen in thick epidermis of Palms & Soles.

• This layer is present between stratum granulosum and corneum.

• Cells in this layer are still nucleated and referred as “transitional cells”.

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STRATUM CORNEUM

• Outermost layer of epidermis.

• Composed of 20-25 layers of cornified cells.

• Made up of terminally differentiated, dead keratinocytes known as Corneocytes.

• These cells are flat, anuclear & devoid of cytoplasmic organells.

• Corneocytes contain soft keratin

• Cells are arranged together like “ bricks in a wall”

• Fluorescent staining shows cells arranged in orderly vertically stacks.

• The journey of cells from basal layer to surface is called Epidermal turnover or transit time – 52 to 75 days.

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EPIDERMOPOISES

• The epidermal thickness and the number and size of epidermal cells remain constant, with the rate of cell production matching the rate of cell loss.

• 3 populations of cells exist in the basal layer:

- Stem cells.

- Transient amplifying cells

- Post mitotic cells.

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• Keratinocyte stem cells give rise to all the layers of the epidermis, with the majority of these cells committed to terminal differentiation.

• Stem cells have a large capacity for proliferation.

• A transient amplifying cells can undergo a limited number (up to 5 to 6 times) of mitotic divisions.

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CELL CYCLE

• M or Mitotic phase of division

• G1 post mitotic phase or interphase

• S or phase of DNA synthesis

• G2 or Premitotic phase or short resting

• G-o phase or quiescent phase

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• The time taken by keratinocytes to pass from the basal layer to the stratum corneum and the skin surface is called the epidermal turnover time.

• It ranges from 52 to 75 days.

• The approximate transit time from the basal layer to stratum corneum is 12 to 19 days.

• through the stratum corneum is 14 days.

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Regulation of Epidermopoiesis

• A) Stimulatory signals:

1.Human epidermal growth factor (EGF) –

6 kDa polypeptide.

Stimulates cell proliferation and differentiation.

2 Transforming growth factor-α (TGF-α)-

A polypeptide synthesized by epidermal keratinocytes.

Stimulates growth of keratinocytes by an autocrine method .

after binding to human epidermal growth factor receptor (EGFr).

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• 3 Amphiregulin-

Autocrine keratinocyte growth factor regulated by cellular glycosaminoglycans and upregulated by EGF and TGF-α.

• 4. cytokines –

interleukin-1 & 6 can also stimulate the growth of keratinocytes.

others -PDGF, IL-1β and TNF .

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• B) Inhibitory signals:

Epidermal growth is inhibited by a negative feedback mechanism.

- Transforming growth factor-B, inhibits the growth of keratinocytes.

- IFN-α and -γ have cytostatic effects on keratinocytes.

- TNF-α is cytostatic on keratinocytes

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• C) Apoptosis: programmed cell death

A major cellular homeostatic mechanism in the skin.

Terminal differentiation of epidermal keratinocytes occurs by modified apoptotic programs.

• D) Signal transduction pathways:

Growth factors.

Cyclic 3,5-adenosine monophosphate (cAMP).

Protein kinase C, inositol phosphate and protein tyrosine kinase

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• E) Integrins:

play role in bidirectional communication that can result in a change in gene expression, pH and calcium fluxes.

• F) Others:

low calcium level inhibits differentiation of keratinocytes but

stimulates their proliferation.

Vitamin A and retinoic acids are required for normal morphogenesis

and differentiation

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TONOFILAMENTS

• Basal and lower portion of spinous zone of keratinocyte-

• contain abundant Rough Endoplasmic reticulum Ribosomes, Golgi apparatus ,mitochondria.

• Synthesize tonofilaments- 7.0nm in diameter .

• Also known as Keratin intermediate filaments

• Later becomes keratinous protein.

• They are oriented along major axis of cells.

• aggregate into bundles to form fibrous protein termed as Alpha keratin.

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FUNCTIONS OF TONOFILAMENTS

1. Flexibility and elasticity to cornified layer.

2. Cytoskeleton of keratinocytes,

3. Modulate shape of keratinocytes.

4. Promotes centralisation of nucleus.

5. Implement cell-cell adhesion via desmosomes.

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KERATINS

• Filamentous cytoskeleton of all mammalian cells including epidermal keratinocytes.

Contains-

• Actin containing microfilaments 7nm in diameter.

• Tubulin containing microtubules 20-25 nm in diameter.

• Intermediate filament 7-10nm in diameter.

• More than 30 keratins have been noted.

Distinct and separate genes and mol. Wt-40 to 67 KDa

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• Keratin genes -2 groups

Type I (basic)- 1 to 8

Type II (acidic)-9 to 19

• one basic and one acidic forms heterodimers which wrap around by hydrophobic interaction

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KERATOHYALINE GRANULES

• Source of protein (Profillagrin → Fillagrin).

• Electron-dense bodies devoid of internal structure but are biochemically complex.

• They appear first in upper part of spinous zone& become prominent in granular zone.

• Disappear as they enter cornified layer.

• Contains cysteine-rich proteins whose disulphide bonds contribute to chemical inertness & strength of cornifed layer

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FILLAGRIN

• Also known as Filament aggregating protein.

• Histidine rich, cat-ionic protein.

• Functions as an “Interfilamentous glue” to aggregate & align keratin filaments within cornified cells.

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LAMELLAR GRANULES

• Also known as odland bodies.

• Measures about 300 nm in diameter.

• Contain free sterols, polar lipids and hydrolytic enzymes.

• Appears at the top of spinous zone, near Golgi apparatus of keratinocytes & migrate into cytoplasm, fuses with plasma membrane.

• Contents are discharged into intercellular spaces.

• Once contents are discharged, they become organised into lamellae and provide structural basis for the barrier of epidermal permeability.

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CELLS OF EPIDERMIS

MELANOCYTES

Dendritic cells, synthesize & secrete melanin containing organelles-melanosomes.

• Derived from precursors in the neural crest.

• Found during 8th week of fetal life.

light microscope:

-Clear cells in the basal row of epidermis.

H & E

-Small basophilic nucleus & dendritic cytoplasm.

Dendrites of melanocytes may be revealed when melanin is stained blackwith silver salt

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Epidermal melanin unit

36 keratinocytes are associated with each melanocyte.

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• FUNCTIONS

1. Absorbs UV light and protect the skin from solar UV damage.

2.Melanin functions as a scavenger of free oxygen radicals and thus protects cells

3.Imparts various colours to hair.

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Langerhans cells

• Described by Paul Langerhan.

• Derived from mesenchymal precursors in bone marrow.

• Dendritic cells situated in middle of the epidermis.

• They constitute 2% to 8% of total epidermal cell population.

• Enter the epidermis at about 12 weeks

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• Conventional microscope:

clear cells, pale staining and have convoluted nuclei.

• Electron microscopy:

-They demonstrate a lobulated nucleus, clear cytoplasm, well defined

endoplasmic reticulum ,Golgi complexes and lysosomes.

-Langerhans cells have distinctive rod or racquet shaped

granules [BIRBECK Granules].

-They resemble Tennis racquet.

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• FUNCTIONS:

1. Plays role in immune process like ACD, allograft rejection, immune tolerance.

2. Defence against micro organisms.

3. Regulation of epidermal differentiation.

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MERKEL CELLS

• In 1875,FRIEDRICH MERKEL identified at base of rete ridges cells that were in contact with nerve fibrils and named them tastzellen or touch cells.

• They originate in the epidermis itself, presumably from germinative keratocytes.

• Appear in fetal skin by 16th week of gestation.

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• They have pale staining cytoplasm

• Nucleus is lobulated & the margins of cells project cytoplasmic spines towards keratinocytes.

• They have characteristic spherical granules which are membrane limited with a dense central core.

• These cells are embedded in basal layer & form desmosomal connections with the surrounding basal keratinocytes.

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FUNCTIONS

• Slowly adapting type I mechanoreceptor Low threshold touch receptors.

• most sensitive to vibrations at low frequencies, around 5 to 15 Hz.

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INTERCELLULAR JUNCTION

• Links adjacent keratinocytes.

• Responsible for mechanical ,biochemical , signalling interactions between cells.

Components include:

1.Desmosomes

2.Adherens junction.

3.Gap junction.

4.Tight junction.

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DESMOSOMES

-The major adhesion complex in epidermis.

- Anchors keratin intermediate filaments to the cell membrane.

- Bridges adjacent keratinocytes.

- Ultrastructure: cell membrane of two adjacent cells forms a symmetrical junction with a central intercellular space of 30 nm containing a dense line.

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• Components of desmosomes in epidermis:

a) Desmosome cadherin's.

b) armadillo family of nuclear & junctional proteins.

c) Plakins.

Cadherins -1. Desmoglins (Dsg 1-4)

2. Desmocollins (Dsc 1-3)

They are trans membranous calcium rich glycoprotein.

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• The intercellular parts of glycoproteins are attached to keratin filament network via desmoplakin, plakoglobin and other macro molecules.

• Desmosomal proteins acts as autoantigen in various immunobullous blistering disorders.

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ADHERENS JUNCTIONS

• Electron dense , transmembrane structures that engage with actin skeleton.

• They contribute to- epithelial assembly, adhesions, barrier formation, cell motility and changes in cell shapes.

• Characterised by: 2 opposing membranes separated approximately by 20 nm & 0.2 - 0.5 micro metre in diameter.

• Comprise of two basic adhesive units:

1) nectin – afadin complex.

2) cadherin complex.

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• mutations have subsequently been reported in the CDH3 gene, which encodes P-cadherin; these mutations result in autosomal recessive hypotrichosis with juvenile macular dystrophy.

• P-cadherin mutations are also found in a different disorder, ectodermal dysplasia–ectrodactyly–macular dystrophy (EEM) syndrome.

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GAP JUNCTION

• clusters of intercellular channels, known as connexons, that directly form connections between the cytoplasm of adjacent keratinocytes (and other cells).

• Connexons originate following assembly of six connexin subunits within the Golgi network that are then transported to the plasma membrane.

• The connexins are divided into three groups (α, β and γ).

• The formation and stability of gap junctions can be regulated by protein kinase C,calcium concentration, calmodulin, adenosine 3′,5′-cyclic monophosphate (cAMP) and local pH

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• FUNCTION : permits sharing of low-molecular-mass metabolites (<1000 Da) and ion exchange between neighbouring cells, thus allowing intercellular coordination and uniformity to maintain tissue/organ homeostasis in multicellular organisms.

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• Gap junction communication is essential for cell

synchronization

differentiation.

growth and metabolic coordination of avascular organs, including

epidermis.

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TIGHT JUNCTION

• Tight junctions are the major regulators of permeability in simple epithelia, are also present in skin.

• They key role in skin barrier integrity and maintaining cell polarity.

• The principal structural proteins of tight junctions are the claudins, transmembranous proteins - junctional adhesion molecules (JAMs) and the occludin group of proteins.

• The main claudins in the epidermis are claudin 1 and 4.

• Transmembranous proteins do not bind to one another but the claudins and occludins can bind to the intracellular zonula occudens proteins ZO-1, ZO-2, ZO-3.

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• These proteins can also interact with actin thus providing a direct link with the cytoskeleton.

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THANK YOU

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