Epidermis 1 Dr.ASHWINI N VIMS & RC

SKIN- Epidermis Structure and Ultrastructure.
PRESENTER Dr ASHWINI.N
MODERATOR Dr. JAIDEV.
VIMS & RC, BANGLORE.

1. EPIDERMIS -1 MODERATOR DR.JAIDEV . Dr. ASHWINI.N 1

2. 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 m 2 . Two main kinds of skin - Glabrous Skin ( non-hairy skin). - Non Glabrous Skin .(hairy skin). Skin is divided into 3 layers Epidermis - outermost layer , serves as barrier between exterior & body’s interior environment and is responsible for cornification. Dermis- a deeper layer ,provides structural support to skin. Subcutis - containing fat and connective tissue. 2

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4. EPIDERMIS Is metabolically active, stratified squamous epithelium. Consisting of 4 type of cells. 1.Keratinocytes. 2.Melanocytes. 3.Langerhans cells. 4.Merkel cells. 4

5. Layers Of Epidermis Stratum basale . Stratum malpighian . Stratum spinosum . Stratum granulosum . Stratum lucidium . Stratum corneum. 5

6. 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 6

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8. STRATUM SPINOSUM 8-10 layers of cells. Keratinocytes in this layer are P olyhedral 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. 8

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10. STRATUM GRANULOSUM 2-5 Cell layer thick. Diamond shaped cells with intracellular basophilic keratohyline granules , with deeply basophilic cytoplasm. Cytoplasm of the keratinocytes in this layer contain lamellated granules known as odland bodies . Discharge their lipid components into intercellular space, functions as barrier & 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. 10

11. STRATUM LUCIDIUM I s 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”. 11

12. 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. 12

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14. 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. 14

15. K eratinocyte 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 . 15

16. 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 16

17. 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. 17

18. 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 ). 18

19. 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-1 β and TNF . 19

20. 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 20

21. 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: G rowth factors. Cyclic 3,5-adenosine monophosphate (cAMP ). Protein kinase C, inositol phosphate and protein tyrosine kinase 21

22. 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 22

23. 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. 23

24. FUNCTIONS OF TONOFILAMENTS Flexibility and elasticity to cornified layer. Cytoskeleton of keratinocytes, Modulate shape of keratinocytes. Promotes centralisation of nucleus. Implement cell-cell adhesion via desmosomes. 24

25. 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 25

26. 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 26

27. KERATOHYALINE GRANULES Source of protein ( P rofillagrin → F illagrin) . 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 27

28. 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. 28

29. 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. 29

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31. CELLS OF EPIDERMIS MELANOCYTES Dendritic cells, synthesize & secrete melanin containing organelles-melanosomes . Derived from precursors in the neural crest. Found during 8 th week of fetal life. light microscope: -C lear cells in the basal row of epidermis. H & E - Small basophilic nucleus & dendritic cytoplasm. Dendrites of melanocytes may be revealed when melanin is stained black with silver salt 31

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33. Epidermal melanin unit 36 keratinocytes are associated with each melanocyte. 33

34. FUNCTIONS 1. A bsorbs 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. 34

35. 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 35

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37. 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. 37

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40. FUNCTIONS: Plays role in immune process like ACD, allograft rejection, immune tolerance. Defence against micro organisms. Regulation of epidermal differentiation. 40

41. 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. 41

42. 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. 42

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44. FUNCTIONS Slowly adapting type I mechanoreceptor Low threshold touch receptors. most sensitive to vibrations at low frequencies, around 5 to 15 Hz. 44

45. 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. 45

46. 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. 46

47. Components of desmosomes in epidermis: Desmosome cadherin's. armadillo family of nuclear & junctional proteins. Plakins. Cadherins -1. Desmoglins (Dsg 1-4) 2. Desmocollins (Dsc 1-3) They are trans membranous calcium rich glycoprotein. 47

48. 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. 48

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50. 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 . 50

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52. 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. 52

53. 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 53

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55. 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. . Gap junction communication is essential for cell synchronization differentiation. growth and metabolic coordination of avascular organs, including epidermis. 55

56. 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. T he 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. 56

57. T hese proteins can also interact with actin thus providing a direct link with the cytoskeleton. 57

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