Biology of keratinocytes

All about biology of Keratinocytes & skin barrier function. This can help postgraduates to understand basics of dermatology.

1. Biology of Keratinocytes

2. Human keratinocytes

3. Cytoskeleton DNA Microtubules Actin

4. Functions of the Cytoskeleton Give shape to cell Give support to cytoplasm Organize organelles Transport of vesicles between organelles Interact mechanically with environment Allow cell movement Segregation of chromosomes during mitosis

5. 3 Major Types of Cytoskeletal Protein Filaments Each has distinct mechanical properties Each formed from a different protein subunit Intermediate filaments (8-16nm) Microtubules (Tubulin) (25 nm ) Microfilaments (Actin filaments) (6nm )

6. Intermediate filaments form a strong network across sheets of cells Distribute mechanical stress forces Immunofluorescence micrograph of sheet of epithelial cells in culture IF’s shown in green Cell boundaries in blue

7. IF’s are connected cell-to-cell via desmosomes

8. IF’s have a rope-like structure Electron micrograph of intermediate filaments

9. Formation of intermediate filaments Two monomers form dimer Two dimers form tetramer with staggered anti-parallel ends

10. Formation of intermediate filaments Tetramers pack in helical array containing 8 tetramers 8 tetramers add to growing filament

11. 4 classes of intermediate filaments (IF)

12. Keratin filaments are most diverse class Keratins have a common domain structure that they share with the other intermediate filament proteins Every epithelial cell type has distinctive set of keratin proteins Specialized keratins in hair, nails

13. keratinization ( Cornification) Production of keratin Terminal differentiation Loss of nuclei and organelles Metabolism ceases and the cells are almost completely filled by keratin

14. Keratin Multigene family of proteins that form the largest component of the intermediate filament cytoskeleton of epithelial cells (epidermis) and related appendages

15. Keratin The epithelial keratins are co-expressed in specific pairings obligatory heteropolymers (heterodimers), each pair consists of one acidic and one basic keratin that become the basic building blocks of epithelial intermediate filaments.

16. Keratin Keratin subfamily pH Members Chromosome Keratin Type I acidic (K9–K20) 17 Keratin Type II basic (K1–K8) 12

17. Keratin Keratin II express in the cells before Keratin I and induce its synthesis There are more than 50 individual members. Based upon their biochemical properties (e.g. isoelectric point, molecular weight), keratins are classified.

18. Keratin Differential expression of keratin proteins depend on : Cell type Tissue type Stage of embryonic development Degree of differentiation Disease states Drugs

19. Examples of keratin pairs KERATIN TYPE SITE K5/K14(less abundantly, K15) Basal cell layer (proliferation) K1/K10 Suprabasal compartment (differentiation) K6/K16(K17) Found normally in: Palmoplantar epidermis In keratinocytes of the nail bed Hair follicle Sebaceous glands Sweat glands Rapidly induced by: Injury and wounding UV radiation Hyperproliferative conditions (psoriasis, LP, DLE, warts) K4/K13 Non- cornifying cells of the mucosa

20. Keratin Functions of keratin in the epidermis: Crucial role in keratinization Integral part of the structural network that make hemidesmosomes, desmosomes, BM ( Structural integrity ) Maintaining spatial relation between the nucleus and cytoplasmic organelles Transfer of information between the nucleus and cell surface and vice versa i.e. cell signaling.

21. A mutant form of keratin makes skin more prone to blistering Disease with mutant keratin: epidermolysis bullosa simplex Cross section of normal skin Cross section of skin with mutant keratin protein disrupting keratin filament network

22. Questions- intermediate filaments T/F: IF’s form a network across sheets of cells to give them strength. True T/F: IF’s are directly connected to IF’s in other cells without intermediary factors. False- connect through desmosomes . Hair and nails contain which IF? Keratin T/F: nuclear lamins are a type of IF? True

23. PROLIFERATION & DIFFERENTIATION Keratinocytes proliferate in the basal layer ( MITOSIS ) As the cells move away from the BM they lose their capacity for cell division and they become committed to terminal differentiation Terminal differentiation (KERATINIZATION) : is a complex process that results in the production of the impermeable stratum corneum

24. Granules of suprabasal layers Keratohyalin granules Profilaggrin  filaggrin [ fil ament + aggr egat in g ] Loricrin Lamellar granules ( Odland bodies-lamellar bodies) contain: Lipids ( e.g. ceramides , Phospholipids ,), Hydrolytic enzymes (e.g. proteases, acid phosphatases, glucosidases , lipases) Proteins ( e.g. corneodesmosin ).

25. Stratum Corneum Continuous sheet of corneocytes (Cornified Cells) Connected by corneodesmosomes Embedded in an intercellular matrix enriched in non-polar lipids and organized as lamellar lipid layers Corneocytes + Intercellular lipid

26. NORMAL THICKNESS OF THE EPIDERMIS There is a balance between the processes of proliferation and desquamation that results in a complete renewal approximately every 2 8 days

27. KERATINIZATION EPIDERMAL DIFFERENTIATION

28. As keratinocytes are transformed from mitotically active cells in the basal layer to fully differentiated, enucleated squames in the cornified layer. Keratohyalin ( profilaggrin - and loricrin -containing) and lamellar (lipid-containing) granules extrude their contents in the granular layer, leading to bundling of keratin filaments and replacement of the plasma membrane with the highly cross-linked, lipid-covered cornified cell envelope

29. In response to certain signals probably an increase in calcium concentration during the transition from the granular layers to the SC the lamellar bodies move to the apex of the upper-most granular cells, fuse with the plasma membrane, and secrete their content into the intercellular spaces through exocytosis . Components of the stratum corneum

30. Desquamation of surface keratinocytes from the stratum corneum is regulated by proteolytic degradation of the cells’ desmosomes.

31. Any defect along this pathway leads to DIORDERS OF KERATINIZATION

32. Genetic defects in the supra-basal keratins results in: Hyperkeratosis e.g.: - In icthyosis - retension hyperkeratosis Barrier function defect

33. DISORDERS OF KERATINIZATION Icthyosis Palmoplantar keratodermas / Erythrokeratoderma Porokeratosis Peeling skin syndromes Discrete keratotic disorders Miscellaneous circumscribed keratotic disorders Filiform keratoses Confluent and reticulate papillomatosis

34. Ichthyosis vulgaris

35. Diffuse palmoplantar keratoderma

36. Focal palmoplantar keratoderma

37. Porokeratosis

38. Barrier functions The skin barrier prevents excessive water loss (TEWL) (inside-outside barrier) and the entry of harmful substances from environment (outside-inside barrier). The physical barrier is predominantly located in the stratum corneum & also by tight junctions of viable keratinocytes. The 10-20 μ m thick stratum corneum forms a continuous sheet of protein enriched cells embedded in an intercellular matrix, enriched in non-polar lipids & organized as lamellar lipid layers . S. corneum proteins , lipids and low-molecular-weight by-products of keratohyalin breakdown, referred to as natural moisturizing factors (NMF) , bind and retain water in the s corneum, thus maintaining its elasticity

39. Natural moisturizing factor (NMF) Collection of water-soluble compounds that are only found in the stratum corneum . These compounds compose approximately 20-30% of the dry weight of the corneocyte . NMF components absorb water from the atmosphere and combine it with their own water content allowing the outermost layers of the stratum corneum to stay hydrated . The lipid layer surrounding the corneocyte helps seal the corneocyte to prevent loss of NMF. Because NMF components are water soluble, they are easily leached from the cells with water contact - which is why repeated contact with water actually makes the skin drier .

40. Lipid Composition & Role of Lipids in the Stratum Corneum The major lipid classes in the SC are Ceramides. Cholesterol. Free fatty acids

41. CERAMIDES Is an amide linked fatty acid containing a long-chain amino alcohol called sphingoid base. Glucosyl-ceramide is enriched in the epidermis and spleen. Ceramide is a major lipid component in the SC, accounting for 30 to 50 % of lipids by weight. Terminal differentiation is a key factor in accumulating ceramides.

42. CHOLESTEROL In the epidermis is synthesized in situ from acetate & also basal cells are capable of reabsorbing cholesterol from circulation .

43. TEWL The normal movement of water from the SC into the atmosphere is known as trans-epidermal water loss (TEWL). TEWL used to assess the barrier function of the st . corneum used to predict irritancy of substances or to contribute to the assessment of clinical methods.

44. Percutaneous Absorption Skin allows some permeation of almost every substances and rates of penetration is different for different materials Percutaneous absorption studied both in vitro and in vivo In vitro – using sheets of epidermis or st. corneum In vivo – corticosteroids – vasoconstriction nicotine – vasodilation histamine –wheal pilocarpine –sweating anesthesia – local anesthetics

45. Protection against – UV radiation The skin has 3 barrier to UV radiation Melanin barrier Protein barrier Absorption of radiation by epidermal lipids may contribute to protection from UV radiation.

46. Skin failure A loss of normal temperature control with inability to maintain the core temperature, failure to prevent percutaneous loss of fluid, electrolytes and protein with resulting imbalance and failure of the mechanical barrier to penetration of foreign materials If generalized It is a dermatological emergency Causes are Stevens Johnson syndrome (SJS) Toxic epidermal necrolysis (TEN) Pustular psoriasis Erythroderma of various causes Pemphigus vulgaris (PV) Graft vs. host disease (GVHD)

47. Regional variation in barrier function Differences esp. in the composition and organization of lipids in the epithelial barriers result in regional variation in permeability The epidermis over scrotum and eyelids are particularly thin Scrotum is particularly permeable to all substances. Face, forehead & dorsa of hands are more permeable to water than the trunk, arms & legs. The palms & soles particularly impermeable to nearly all molecules except water.

48. AGE RELATED VARIATION IN PERMEABILITY The aged skin is more permeable to chemical substances. The aged skin is more dry. Although substances enter aged skin more easily than young skin , they are removed more slowly into the circulation because of changes in the dermal matrix and reduction in vasculature. The water binding capacity is  and the renewal time after damage is  in old age.

49. Dermatoses with skin barrier impairment Mild impairment in skin barrier in skin condition without inflammation 1- Icthyosis vulgaris 2- Darier disease. Pronounced impairment in skin barrier is associated with inflammatory disease Irritant & allergic contact dermatitis Atopic dermatitis Seborrhoeic Dermatitis Psoriasis Cutaneous T-cell lymphoma . Most of the blistering diseases especially inflammatory related shows an increase in TEWL especially after loosening of the blister roof .

50. Therapeutic importance of barrier In inflammatory diseases treatment with corticosteroids, cyclosporine, tacrolimus, pimecrolimus & UV light has been shown to reduce cell inflammation as well as to improve barrier function thus helping to normalize proliferation & differentiation. Because of side effects their treatment should be used for short time only. In contrast application of bland cream & ointments containing lipids & lipid like substances, hydrocarbon, fatty acid , cholesterol esters & triglycerides can be used without side effects for long term treatment of mild to moderate inflammatory disease.

51. Therapeutic importance of barrier Creams and ointments are partially correct or stimulate barrier repair &  St. corneum hydration, thus influences epidermal proliferation & differentiation. It has been proposed that lipid mixture containing the three key lipid groups: Ceramides. Cholesterol. Free fatty acids. able to improve skin barrier function & st. corneum hydration in atopic dermatitis.

52. References Dr Raghavendra K RGMC KOTA INDIA Mona R.E. Abdel- Halim ( MD) Assistant Professor of Dermatology Faculty of Medicine Cairo University Bolognia: Dermatology, 2nd ed.

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