Dr. Madhulika MhatreJR-3 Dept of Dermatology

Process of keratinization

Introduction

• Skin consists of a tough connective tissue, the dermis, overlaid by a multilayered waterproof epithelium, the epidermis.

• The epidermis is continually renewed from stem cells,with a turnover time of 59-75 days• 26-42 days :transit through str.malpigii• 14days: transit through str. corneum

Functions Of The Epidermis• Form a protective barrier from physical insults - Chemical - Biological - Temperature - Mechanical

• Protect body homeostasis - Temperature regulation - Prevent fluid loss

• Immune surveillance

• Sensory organ

Characteristics Of The Epidermis • TISSUE RENEWAL : Continuous self-renewal of

keratinocytes

• STRENGTH : Both intracellular and intercellular strength

• CORNIFICATION : Process that creates a water impermeable barrier

Self-renewing tissue requires

•A highly regulated process that balances cellular proliferation and cell death

• The epidermis is continuously renewed from stem cells which are present at the base of rete ridges of interfollicular epidermis

REGULATION OF EPIDERMOPOESIS

• STIMULATORY FACTORS HUMAN EPIDERMAL GROWTH FACTOR TRANSFORMING GROWTH FACTOR ALPHA AMPHIREGULIN INTERLEUKIN 1 AND 6 GM- CSFKGF (dermal fibroblasts) VITAMIN A AND RETINOIC ACIDS

• INHIBITORY FACTORS INTERFERON ALPHA AND GAMMA TUMOR NECROSIS FACTOR

Stem cells

• Stem cells are not terminally differentiated and have the ability to divide throughout the organism’s lifetime, yielding progeny that differentiate and others that remain stem cells.

• The epidermal stem cells lie in the basal layer, attached to the basal lamina.

• They progressively differentiate, switching from expression of one set of keratins to expression of another until, eventually, their nuclei degenerate, producing an outer layer of dead keratinized cells that are continually shed from the surface.

• The stem cells seem to be clustered near the tips of the dermal papillae and base of rete ridges.

• They divide giving rise (through a sideways movement) to transit amplifying cells.

• The transit amplifying cells divide frequently, and begin to differentiate and slip out of the basal layer

Keratinocyte proliferation• most of the proliferation

is by transient amplifying cells (TA cells)

• in normal epidermis, all TA cells remain attached to basement membrane

• transition from stem cell to TA cell is the first step in keratinocytes differentiation

Epidermal proliferation• TA cells migrate

laterally along the basement membrane

Epidermal proliferation• Once TA cells stop

proliferating, they lose their attachment to the basement membrane

• Proceed towards terminal differentiation

LAYERS OF EPIDERMIS• STRATUM CORNEUM• STRATUM LUCIDUM• STRATUM GRANULOSUM• STRATUM SPINOSUM• STRATUM BASALE

Stratum basale • Single layer of cuboidal or columnar cells, on basal lamina.

• These cell divide by mitosis to renew the entire population of keratinocytes, replacing the dead cells that are lost from the skin surface.

• The nucleus is large and mitotic figures are numerous. The cytoplasm is basophilic due to ribosomes and tonofilaments occuring singly or in bundles.

• Desmosomes attach cells to each other and hemidesmosomes attach the basal surface of the cell to the basal lamina.

Stratum spinosum• A layer of polyhedral shaped cells, flattened parallel to the surface, with central

nuclei that are slightly flattened. The adjacent cells extend numerous processes toward each other that attach by desmosomes. Thus, each cell appears "spiny", giving rise to the name spinosum.

• The cytoplasm contains the same organelles as Basale cells, but with distinctive lamellated granules or membrane-coating granules (ODLAND BODIES)

• Ovoid, membrane bound granule, 0.1-0.5um in diameter, containing parallel lamellae orientated transverse to the long axis of the granule.

• rich in glycolipids and sterols (they contain acylglucosylceramide molecules) and hydroytic enzymes

• Secretion into the intercellular space between spinosum cells provides a water proof barrier to render the skin impermeable to water.

• The accumulation of keratin filaments starts in these cells and ultimately results in the death of the cells in the statum corneum

Stratum granulosum:• Three to 5 layers of flattened cells, with irregular, large keratohyalin granules.

• Granules, 1-5um in diameter, have no membrane and consist of closely packed 2nm subunits. The granules are associated with tonofilaments, which may pass through or be partially embedded.• Two kinds of granules, small dense homogeneous deposits, and large irregular

keratohyalin granules containing histidine-rich protein, RNA, proteoglycans and lipid.

Keratohyalin forms dense cytoplasmic granules that promote dehydration of the cell as well as aggregation and cross-linking of the keratin fibers. The nuclei and other organelles then disintegrate, and the cells die. Further dehydration creates a tightly interlocked layer of cells that consists of keratin fibers surrounded by keratohyalin.

Also contain profilaggrin+keratin filaments+loricrin Filaggrin- interfilamentous glue which aggregates and aligns keratin filaments in

cornified layer Eventually degraded into urocanic acid and pyrollidine carboxylic acid(filter UVR)

Stratum lucidum:• Few cells thick, and seen in palms and soles

• Cells are refractile, eosinophilic or unstained.

• There is no nucleus and the cytoplasm is almost fully keratinized.

Stratum corneum:• Many layers of flattened cornified, or keratinized cells.

• All the cytoplasm is replaced by 8-10nm filaments of keratin embedded in an amorphous filaggrin matrix.

• Cell membrane is thickened by deposition of non-keratin material on inner aspect of membrane (cornified envelope).

• The intercellular space is filled by material from lamellar granules.

• Desmosomes are modified and do not hold cells together.

• Surface layers of dead cells desquamate and are lost.

Keratinization• Formation of keratin within the keratinocytes

• KERATINS: filamentous skeleton of all cells comprises of Actin, tubulin and intermediate filaments• Epithelial cells- Keratin is the intermediate filament

• keratin filaments extend from the nuclear membrane to desmosomal plaques at the cell membrane

• keratins enable keratinocytes to sustain mechanical and non-mechanical stress

• KERATIN – INSOLUABLE PROTEIN • BASIC ( 1-8 ) ACIDIC (9 -19 ) • To be functional: 1 acidic+ 1 basic• SIMPLE EPITHELIA – K8/ K18• STRATIFIED SQUAMOUS – K5/K14• In SKIN, SUPRABASAL KERATINS – K1/K10

Cornification

- process that begins in cells of the upper spinous layers

- the induction of proteins forming the cornified cell envelope and lipid barrier– dependent on calcium

• At the molecular level, the cornified envelope is formed by proteins that are highly crosslinked by transglutaminases, with specific lipids on the outside

Steps• Spinous layer- INITIATION• Envoplakin, periplakin and involucrin crosslinked by TG 1 &5 over the

desmosomes- scaffold for CE

• Granular layer-REINFORCEMENT• Loricrin and SPRs crosslinked by TG1 & TG3• Lipids from lamellar bodies are attached to above crosslinked

proteins- LIPID BILAYER

• Corneum • Crosslinking of fillagrin bound keratin filaments and other proteins

• Desquamation• Extrusion of ceramides, FFAs

Lipid BarrierFORMATION AND FUNCTION OF THE EPIDERMAL LIPIDS• The lipid composition and moisture content change with increasing

differentiation • Stored in the Odland bodies which are the precursors of the skin-

specific barrier lipids in the form of lamellar bilayer lipid membranes.

• Stability• Desquamation• Permeability barrier • Regulation of the water and fluid content - elasticity and firmness of

the horny layer depend on moisture content.• AMP activity• Toxin exclusions

1- Odland body with bilayer membrane2- Keratinocyte3- Exocytosis4- Epidermal lipids5- Horny cells

COMPOSITION OF THE EPIDERMAL LIPIDS• Ceramides (40%) - barrier-forming and moisture-binding functions • FFA (25%) • Cholesterol (25%) • Cholesteryl sulphate.

THE PERMEABILITY BARRIER • It prevents invasion by microorganisms and certain substances such

as chemicals and allergens.

• It minimizes transepidermal water loss (TEWL) and thus protects the body from dehydration.

Natural moisturizing factors (NMF)• The ability of the skin to store water depends in large part on

the make-up of the barrier lipids in the horny layer.

• Substances, which occur naturally in the body and retain water in the horny layer, are called natural moisturizing factors (NMF). They originate from the cornification process (differentiation) of keratinocytes and from sweat and sebum

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