biology of keratinocytes

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

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Biology of Keratinocytes

Human keratinocytes

Cytoskeleton

DNA

Microtubules

Actin

Functions of the Cytoskeleton

1. Give shape to cell2. Give support to cytoplasm3. Organize organelles4. Transport of vesicles between organelles5. Interact mechanically with environment6. Allow cell movement7. Segregation of chromosomes during mitosis

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)

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

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

IF’s have a rope-like structure

Electron micrograph of intermediate filaments

Formation of intermediate filaments

• Two monomers form dimer• Two dimers form tetramer with staggered anti-parallel ends

Formation of intermediate filaments

• Tetramers pack in helical array containing 8 tetramers• 8 tetramers add to growing filament

4 classes of intermediate filaments (IF)

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

keratinization (Cornification)

• Production of keratin• Terminal differentiation• Loss of nuclei and organelles • Metabolism ceases and the

cells are almost completely filled by keratin

Keratin

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

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.

Keratin

Keratin subfamily pH Members Chromoso

me Keratin Type I acidic (K9–K20) 17Keratin Type II basic (K1–K8) 12

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.

KeratinDifferential expression of keratin proteins

depend on:1. Cell type2. Tissue type3. Stage of embryonic development4. Degree of differentiation5. Disease states6. Drugs

Examples of keratin pairsKERATIN TYPE SITE

K5/K14(less abundantly, K15)

Basal cell layer (proliferation)

K1/K10 Suprabasal compartment (differentiation)

K6/K16(K17) Found normally in:1. Palmoplantar epidermis 2. In keratinocytes of the nail bed3. Hair follicle4. Sebaceous glands5. Sweat glands Rapidly induced by:a) Injury and woundingb) UV radiationc) Hyperproliferative conditions

(psoriasis, LP, DLE, warts)

K4/K13 Non-cornifying cells of the mucosa

KeratinFunctions of keratin in the epidermis:1. Crucial role in keratinization 2. Integral part of the structural network that

make hemidesmosomes, desmosomes, BM (Structural integrity)

3. Maintaining spatial relation between the nucleus and cytoplasmic organelles

4. Transfer of information between the nucleus and cell surface and vice versa i.e. cell signaling.

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

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

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

Granules of suprabasal layers

Keratohyalin granules 1. Profilaggrin filaggrin [filament +

aggregating]2. Loricrin

Lamellar granules (Odland bodies-lamellar bodies) contain:1. Lipids (e.g. ceramides, Phospholipids,), 2. Hydrolytic enzymes (e.g. proteases, acid

phosphatases, glucosidases, lipases) 3. Proteins (e.g. corneodesmosin).

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

NORMAL THICKNESS OF THE EPIDERMIS

DIFFERENTIATION PROLIFERATION

There is a balance between the processes of proliferation and desquamation that results in a complete renewal approximately every 28

days

Disturbed balance

Increased thickness of the epidermis

KERATINIZATION EPIDERMAL DIFFERENTIATION

At the beginning of the granular layer 1. Keratohyalin granules Formation (KHG) (contains

Profilaggrin) 2. Cell envelope proteins cross-linking (Involucrin and Loricrin) 3. Odland bodies get released into the ICS

In the spinous layer: Formation of 1. Lamellar bodies 2. KIF

As the cells enter the spinous layerSwitch of keratin

synthesis from K5/K14 to K1/K10

Initiation of differentiation

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

In the transitional zone1. Filaggrin (keratin bundling protein) acts as a glue matrix that facilitates dense packing of KIF into k. macrofibrils2. activity of keratinocytes,3. Loss of organelles4. Dehydration

5. extracelluar Ca++, 6. activity of transglutaminase, deposition of loricrin, cross linking of involucrin

On entering the transitional zone between the granular cell

layer and the Cornified layerProfilaggrin is transformed into filaggrin

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

Corneocytes are shed into the environment

In upper stratum corneum1. Formation of corneocyte bound lipid envelope2. Plasma membrane and desmosomes become discontinuous, corneodesmosomes are residual intercellular desmosomal interconnections.

In lower stratum corneum1. Dead keratinocytes packed with keratin macrofibrils2. corneocyte bound protein envelope just beneath the plasma membrane

Desquamation of surface keratinocytes from the stratum corneum is regulated by

proteolytic degradation of the cells’ desmosomes.

Any defect along this pathway leads to

DIORDERS OF KERATINIZATION

Genetic defects in the supra-basal keratins results in:

I. Hyperkeratosis e.g.:- In icthyosis- retension hyperkeratosis

II.Barrier function defect

DISORDERS OF KERATINIZATION

1. Icthyosis2. Palmoplantar keratodermas /

Erythrokeratoderma3. Porokeratosis4. Peeling skin syndromes5. Discrete keratotic disorders6. Miscellaneous circumscribed keratotic disorders7. Filiform keratoses8. Confluent and reticulate papillomatosis

Ichthyosis vulgaris

Diffuse palmoplantar keratoderma

Focal palmoplantar keratoderma

Porokeratosis

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 10-20μm 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

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.

Lipid Composition & Role of Lipids in the Stratum CorneumThe major lipid classes in the SC are

I. Ceramides.II. Cholesterol.III. Free fatty acids

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.

CHOLESTEROL In the epidermis is synthesized in situ

from acetate & also basal cells are capable of reabsorbing cholesterol from circulation.

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.

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

Protection against – UV radiation

The skin has 3 barrier to UV radiation I. Melanin barrier II. Protein barrier III. Absorption of radiation by

epidermal lipids may contribute to protection from UV radiation.

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 1. Stevens Johnson syndrome (SJS)2. Toxic epidermal necrolysis (TEN)3. Pustular psoriasis4. Erythroderma of various causes5. Pemphigus vulgaris (PV)6. Graft vs. host disease (GVHD)

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.

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.

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

1. Irritant & allergic contact dermatitis 2. Atopic dermatitis 3. Seborrhoeic Dermatitis 4. Psoriasis 5. Cutaneous T-cell lymphoma .

Most of the blistering diseases especially inflammatory related shows an increase in TEWL especially after loosening of the blister roof .

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.

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:

1. Ceramides.2. Cholesterol.3. Free fatty acids.able to improve skin barrier function & st. corneum hydration in atopic dermatitis.

ReferencesDr Raghavendra K RGMC KOTA INDIAMona R.E. Abdel-Halim (MD) Assistant Professor of Dermatology Faculty of Medicine Cairo UniversityBolognia: Dermatology, 2nd ed.

THANK YOU

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