KERATINISATION & CORNIFICATION

Keratinization

keratinization is a genetically programmed, complex series of morphologic changes and metabolic events whose endpoint is a terminally differentiated, dead keratinocyte that contains keratin filaments, matrix protein, and a protein-reinforced plasma membrane with surface-associated lipids.

EpidermisEpidermis is mainly composed of

keratinocytesThickness of epidermis- 0.05-0.1

mm formed by division of cells in

basal layer which undergoes terminal differentiation to give rise to stratum spinosum , granular layer and stratum corneum

Life cycle of epidermal cellsThree distinct phases:1. Mitosis2. Differentiation3. Exfoliation1.MITOSIS New epidermal cells are continuously

formed by stratum basale to compensate for those that exfoliate at the surface.Some basal cells may remain quiescent which permits them to reenter the cell cycle and continue proliferation when required.

Turnover time-is the amount of time for whole cell population to replace itself.This depends on the time taken for individual cells to divide and the proportion of basal cells dividing .

The mean turnover time of epidermis is estimated to be 39 days divided as follows:

-13 days for proliferative compartment(proliferation of cells in basal layer)

-12 days for differentiated compartment (transit time for a keratinocyte, from the time it detaches from the basal layer to the time it enters the stratum corneum)

-14 days for cornified layers(transit time within the stratum corneum and subsequent desquamation)

Psoriasis is a consequence of an increase in the proliferating cell compartment in the basal and suprabasal levels of epidermis and not due to shortened cell cycle. The number of cycling cells is increased approximately sevenfold.

Characteristics of epidermisTissue renewal- continuous self

renewal of keratinocytesStrength- intracellular and

intercellularCornification- formation of

corneocyte and cornified cell envelope in stratum corneum

Regulation of proliferation and differentiation of keratinocyte

Proliferation

Stimulatory β1 integrin TGF-α, c-Myc and p63 Inhibitory TGF-β

Differentiation

Notch signalling pathway and

The transcription

factors PPARα, AP2α/γ, and C/EBPα/β

Tissue Renewal of epidermis

Stratum basale 1. stem cells 2. transit amplifying cells (TA

cells) 3. postmitotic cells

Stem cellsLocated in the

bulge area of hair follicles, basal interfollicular epidermis and the base of sebaceous glands

Proliferationdivision of stem cells →

daughter cells, the transit amplifying cells

After undergoing several cell divisions(5-6), transit amplifying cells give rise to the postmitotic cells that undergo terminal differentiation which detach from the basal lamina to migrate to stratum spinosum

Recent evidence has suggested that asymmetric division of basal cells can directly give rise to a suprabasal differentiating daughter ce1l.

Epidermal Differentiation Epidermal cells undergo characteristic changes during

its ascent from undifferentiated basal cells to fully differentiated cornified cells

When keratinocyte is released from stratum basale:1. Cells lose their mitotic potential2. Synthesis of keratohyaline granules, odland bodies3. changes in the expression of intracellular lipids,

membrane glycoprotein, growth factor receptors and adhesion proteins

4. gradual change in cell strength and water impermeability

5. Loss of nuclei and cell organelles 6. synthesis of a cornified envelope in the stratum

corneum and programmed cell death

Keratinocyte morphology Stratum basale-

contains cuboidal cells, proliferating cells

Stratum spinosum- 8-10 layers thick, no further cell division, cells are polyhedral in shape with a rounded nucleus, in upper spinous layer, cells have odland bodies/lamellar granules

Stratum granulosum- 2-5 cells thick,keratinocytes become elongated, accumulate amorphous keratohyaline granules

Stratum corneum- 15-40 cells thick, contains thickened envelope, loss of nucleus and organelles, imbedded in lipid matrix

Differentiation-specific proteins expressedStatum basale-Keratins 5 and

14, p63Stratum spinosum-Retain k5/14

keratins, new synthesis of k1/10 keratins (keratinisation specific keratins), involucrin, periplakin, envoplakin.

Stratum granulosum- K1 and k10, loricrin, profilaggrin, transglutaminases

The sequences of differentiation will be dealt under

1. Fibrillar protein2. Keratohyaline granules3. Lamellar/ odland bodies4. Intercellular junction formation5. Cell membrane modification6. Resorption of nuclei and

organelles

FIBRILLAR PROTEINKeratinKeratins are Intermediate filamentsextend fron nuclear membrane to

desmosomal plaques of cell membraneForms the cytoskeletal component enable keratinocyte to sustain

mechanical and non-mechanical stressKeratins of increasing molecular weight

are synthesized as cells migrate from the basal layer to cornified layer

54 functional keratin genes(34 epithelial keratin and 17 hair keratin genes)

KERATINSKeratins are rod-

like proteins of two varieties, type I and type II and are paired to form dimers and are held together by hydrophobic interactions

Keratohyaline granulesSeen in keratinocytes of stratum

granulosumComposed of profilaggrin, keratin

filament and loricrinHistidine is required for the synthesis

of profilaggrinApplied aspect Icthyosis vulgaris -affected skin is deficient in histidine -profilaggrin is not synthesised

Function of profilaggrinkeratohyaline granules release

profilaggrin ↓ cleavage of profilaggrin ↓ Filaggrin→bind with keratin ↓ causes keratin aggregation (keratin

macrofibres) ↓ flattening of corneocyte

Odland bodies / lamellar bodies

present in the cytoplasm of cells of upper spinous layer and granular layer.

Lamellated membrane bound organellescontain ceramides, cholesterol, free fatty acids

and acid hydrolasesFound adjacent to the cell membrane with

alternating thick and dense lines seperated by lighter lamellae

In order to form odland bodies, three precursor lipids; namely, phospholipids along with cholesterol and glucosylceramides, are synthesized within the stratum spinosum and stratum granulosum cells

Intercellular junctionLink adjacent keratinocytes and responsible

for mechanical, biochemical and signalling interactions between cells

Components: 1. desmosomes 2. adheren junctions 3. gap junctions 4. tight junctions

Changes in the plasma membrane Cornified cell envelopechemically resistant, highly

insoluble proteinaceous structure in the inner surface of plasma membrane

Chromosome 1q21 contains cluster of genes known as epidermal differentiation complex

Epidermal differentiation complexchromosome 1q21 contains a number

of genes responsible for epidermal differentiation.This chromosomal region is named the Epidermal differentiation complex (EDC).

45 genes have been identified within the EDC which code for proteins involved in the formation of the cornified envelope, including loricrin, involucrin and SPRs .

Formation of cornified envelope: Initiation

Rise in intacellular calcium levels

↓ Expression of plakins

(envoplakin , periplakin) which form heterotetramers and associate with keratins and desmosomes

↓ As intracellular calcium

levels continue to rise,activation of enzyme tranglutaminase 1

↓ Cross-linking of the plakins

and involucrin

Reinforcement Loricrin: a cysteine-rich

protein that forms the major protein component of the cornified envelope(70%)

Loricrin and SPRs(small proline rich proteins) are cross linked by transglutaminase 3

which are again crosslinked to the existing scaffold with the help of transglutaminase 1

form a scaffold along the entire inner surface of plasma membrane

reinforced by the addition of other precursor protein- repetin, trichohyalin, cystostatin α, elafin and LEP/XP-5 (skin-specific protein)

Filaggrin bound keratin consisting almost entirely of keratin 1, keratin 2e and keratin 10, become crosslinked to the cornified cell envelope .

Formation of lipid layer

Lipid is synthesized and packaged in odland bodies in upper spinous layer and stratum granulosum

↓ odlland bodies cluster towards

plasma membrane ↓

fuse with plasma membrane ↓ disperse lipids into intercellular

space ↓ lipids are remodelled into neutral

lipids by hydrolytic enzymes, lipases and glycosidases

begin to organize into intercellular lamellae

lipid envelope-helps in barrier function and intercellular cohesion

Resorption of nuclei and cell organellesAfter the differentiation,

epidermal cells become transformed into horny cells. During this process, the nucleus and cell organelles disintegrate and are eliminated from the cells by lysosomal enzymatic lysis

ExfoliationThe horny cells at the surface of

stratum corneum are continuoucly shed .As the cells reach the surface, the cementing substance become less effective and desquamation ensures.

Cleavage between desmosomes in cornified layer results in shedding of corneocytes

Cholesterol sulphate is an intercellular cement substance

Cholesterol sulphate is hydrolysed to cholesterol by enzyme-cholesterol sulphatase which leads to desquamation of corneocytes

Applied aspect: X-linked recessive icthyosis:

cholesterol sulphatase deficiency→accumulation of cholesterol sulphate →corneocyte retention

Genetic defects in keratinization disorders

Stratum corneum proteins•Ichthyosis vulgaris FLG ( Filaggrin)

•Loricrin keratoderma LOR (Loricrin)

Gap junction proteins•Clouston’s syndrome (hidrotic ectodermal dysplasia 2)

•Erythrokeratoderma variabilis (EKV))

•EKV with erythema gyratum repens

•Keratitis, ichthyosis and deafness

•Palmoplantar keratoderma and deafness

Calcium pump defects•Darier’s disease ATP2A2

•Hailey–Hailey disease ATP2C1

Keratin disorders

•Bullous congenital ichthyosiform erythroderma (BCIE) KRT1, KRT10

•Diffuse non-epidermolytic palmoplantar keratoderma KRT1

•Epidermolytic palmoplantar keratoderma KRT9

•Focal non-epidermolytic palmoplantar keratoderma KRT16

• Ichthyosis bullosa of Siemens (IBS) KRT2E

•Ichthyosis hystrix (Curth–Macklin) KRT1

•Pachyonychia congenita type 1 KRT6A, KRT16

•Pachyonychia congenita type 2 KRT6B, KRT17A

Desmosome proteins

•Dilated cardiomyopathy with woolly hair andKeratoderma DSP Desmoplakin 1

•Ectodermal dysplasia with skin fragility PKP1

Metabolic and enzyme defects

•Autosomal recessive ichthyosiform erythroderma TGM1 (Transglutaminase)

•X-linked ichthyosis steroid sulphatase deficiency

•CHILD syndrome NSDHL, EBP 3

•Harlequin ichthyosis ABCA12

•Netherton’s syndrome SPINK5

•Refsum’s disease PAHX • Sjogren-larsson syndrome FALDH(fatty aldehydehye dehy)

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