Epidermal Cells of Skin

Dr Yugandar

• Epidermal cells:1.Melanocytes2.Keratinocytes ( 80 %

cells in epidermis )3.Langerhans cells4.Merkel cells

Keratinocytes• Passive BRICK in the defensive wall surrounding the body

• Ectoderm derived cells

• Contains number of structural proteins (filaggrin, keratin),

enzymes (proteases), lipids and antimicrobial peptides

(defensins)

• It contributes to maintain the important barrier function of

the skin

Keratinocytes• Keratinization is part of the physical barrier formation

(cornification),it produces more and more keratin

• Finally undergo programmed cell death

• The fully cornified keratinocytes are constantly shed off

and replaced by new cells

• Epidermal turn over time ranges from 52 to 75 days

• Appr. Transit time from basal layer to st. corneum 12 to

19 days & through st. corneum is 14 days

Keratinocytes

• 80 – 85 % epidermal cells

• Epidermal cells self renewal maitained via stem

cells in basal layer of interfollicular epithelium &

bulge region of hair follicles

• Keratinocytes produce 6 keratin filaments

Keratinocytes

• Keratinocytes proliferate within the basal cell layer

• As differentiation proceeds, it progress upwards

through the different epidermal layers (the spinous

layer, granular layer and cornified layer)

• It becomes anucleated and increasingly compacted in

size, before being eventually lost from the skin surface

by desquamation

• The smaller black dots in the cells of the granular layer

represent keratohyalin granules.

Keratinocyte Differentiation

• Keratinocytes Present in suprabasal layer lose

characteristics of underlying layer D/t Terminal

Differentiation ,they become flatten out assume shape

of corneocytes

• Four structures identified by Electron Microscopy

1.Keratohyalin granules 2.Cornified Envelopes

3.Lamellar bodies 4.Desmosome plaques

Characteristics of Self renewing Epidermis:

• Keratinocyte Stem Cells

• Keratinocyte Proliferation

• Keratinocyte Terminal Differentiation

KERATINOCYTE STEM CELLS

• Stem cells are slowly replicating cells

• It divides into another stem cell or a

cell committed to differentiation

• Protected in a specific tissue

niche,Protected by cell-ECM and

cell-cell interactions

• Located within the bulge region of

the hair follicle & at the base of rete

ridges of inter follicular epidermis

• Stem cells are located

within the bulge region of

the hair follicle and at the

base of rete ridges of

inter follicular epidermis

• Stem cells are multipotent, can

generate hair, sebaceous gland or

epidermal keratinocytes

• They allow for the maintenance of the

cell population in the epidermis with

little to no damage to the TISSUE

genome

• At times of tissue damage, it increase

their rate of proliferation to repopulate

the wounded area

Epidermopoiesis

• Epidermal thickness & number , size of epidermal

cells remain constant with rate of cell production

matching rate of cell loss

• Mitotic divisions occur in keratinocytes every 18 to

19 days , up to 5 to 6 times of mitotic divisions

Epidermopoiesis

Stimulatory• Human EGF• TGF alfa• KGF induced by Dermal

fibroblasts,PDGF,IL-1 beta,TNF

• IL-1 & IL- 6

Inhibitory• IFN alfa & gama

Low Calcium inhibits keratinocytes differentiation & stimulate Proliferation

KERATINOCYTE STEM CELLS

Control of stem cell function by Wnt family or Delta / Notch path ways

Loss of stem cells may lead to phenotype of aged epidermis :

• Flattening of the epidermal/dermal junction• Keratinocyte cell size becomes variable• Nuclear atypia• Loss of melanocytes• Loss of Langerhans cells• Delayed injury response• Delayed chemical clearance• Decreased immune response• Dcreased resistance to mechanical stress• Increased incidence of cancer

KERATINOCYTE PROLIFERATION

• Proliferation done by transient

amplifyingcells (TA cells)

• In normal epidermis, remain

attached to basement memb.

• Transition from stem cell to TA

cell is the first step in

keratinocyte differentiation

• TA cells migrate laterally along

the basement membrane

• TA cells only divide 2-3 times before they withdraw from the cell cycle

• TA cells carry-out cell proliferation for tissue without risk for long-term genetic damage to tissue

• Inc expression of c-myc associated with transition from stem cell to TA cell

• Retinoids, vitamin D, AP1 regulate proliferation and differentiation

KERATINOCYTE TERMINAL DIFFERENTIATION

• when a keratinocyte releases from the

basement membrane, it undergoes

changes in morphology & gene

expression

• Gradual change in cell strength &

water impermeability

• Terminally differentiated keratinocytes

synthesize a cornified cell envelope &

undergo programmed cell death

• Stem cells & their differentiated progeny are organized into

columns k/a epidermal proliferation units

• keratinocytes permanently withdraw from the cell

cycle,initiate expression of epidermal differentiation

markers & move suprabasally become corneocytes in the

stratum corneum

• Corneocytes are keratinocytes that have completed their

differentiation program & have lost their nucleus and

cytoplasmic organelles,will eventually be shed off through

desquamation as new one come in

KERATINOCYTE TERMINAL DIFFERENTIATION

• stratum basale – cuboidal

cells

• cells with in this layer

proliferate

• all cells attached to the

basement membrane

• one cell layer thick

• keratins K5 & K14 integrins,

p63

Stratum spinosum –

• cells increase in size

• increased cytoplasm:nucleus

ratio

• cell layer4-6 cells thick, no

further cell division

• Keratins K1 &

K10 ,involucrin,envoplakin,peripl

akin

Stratum granulosum

• cells become elongated

• usually 1-2 cell layers thick

• accumulate amorphous

keratohyaline granules

• Keratins K1 & K10 loricrin,

filaggrin,transglutaminase3

Stratum corneum-

• Keratinocytes contain

thickened cell envelopes

• contain no nucleus

• imbedded in lipid matrix

• no new proteins expressed

Factors promoting keratinocyte differentiation

• A calcium gradient,with the lowest concentration in the

st.basale & inc concentrations until the outer stratum

granulosum,where it reaches its maximum

• Calcium concentration in the stratum corneum is very low

b/c dry cells are not able to dissolve the ions

• Those elevations of extracellular Ca concentrations induces

an inc intracellular free Ca concentrations in keratinocyte.

• Part of that intracellular Ca inc comes from Ca released

from intracellular stores& from transmembrane Ca influx

• Vitamin D3 regulates keratinocyte proliferation &

differentiation by modulating Ca concentrations & regulating

the expression of genes involved in keratinocytes

differentiation

• Keratinocytes are the only cells in the body with the entire

vitamin D metabolic pathway from vitamin D production to

catabolism and Vitamin D receptor expression

Factors preventing epidermal stem cells to differentiate

into keratinocytes

• The transcription factor p63

• Vitamin A and its analogues

• Epidermal growth factor

• Tumor growth factor

• Cholera toxin

KERATINOCYTE INTRACELLULAR STRENGTH

• Keratins are members of the intermediate filament (IF)

gene family

• There are over 50 members of the IF gene family that are

expressed in a tissue & differentiation specific manner

• IF proteins have a conserved central rod domain of helical

coiled-coil segments

• the amino-and carboxy-terminal sequences of IF proteins

are variable

KERATINOCYTE INTRACELLULAR STRENGTH

Keratins heterodimerize with specific pairing partners:

• Type I family• Type II family heterodimers then

oligomerize into longer fibrils

KERATINOCYTE INTRACELLULAR STRENGTH

• Keratins heterodimers

oligomerize into longer

fibrils

• fibrils continue to

assemble until IF is 10-

12 nm in diameter

Three types of interactions

hold keratinocytes together

in epidermal sheets:

• Hemi desmosomes

• Desmosomes

• Adherens Junctions

HemidesmosomesAdhesion site that links the keratin

cytoskeletal components of a cell

to extracellular matrix (basement

membrane)

• Transmembrane components:

integrins α6β4,BPAG2

• Plaque components:BPAG1

(BP230),plectin

• Cytoskeletal component: keratin

DESMOSOMES

Adhesion site that links the keratin cyto skeletal components of two cells

• Transmembrane components: desmogleins,desmocollins• Plaque components: desmoplakins,plakoglobin,plakophilin,keratoclamin• Cytoskeletal component:keratin

Adherence JunctionsAdhesion site that links actin

cyto skeletal components of two cells

• Transmembrane components: E-cadherin• Plaque components: β-catenin,α-

catenin,vinculin,VASP,p120ctn

• Cyto skeletal component: actin

KERATINOCYTE CORNIFICATION

Process that begins in cells of the upper

spinous layers

• the induction of proteins that comprise the

cornified cell envelope (CCE) are

expressed as intracellular [Ca2+] rise in

differentiating keratinocytes

• chromosome 1q21 contains cluster of

genes called the Epidermal Differentiation

Complex

newly synthesized envoplakin, periplakin, and involucrin form heterotrimers that associate with the cell membrane-transglutaminase1 crosslinks these proteins via ε-(γ-lysine) isopeptide linkages-this scaffold forms along the entire inner surface of the cell membrane

• Lamellar bodies (containing free-fatty acids, cholesterols, and ceramides) bud off from the Golgi complex

• At the transition from the granular layer to the cornified layer, the lamellar bodies fuse with the cell membrane and extrude their contents into extracellular space

• Also at transition,filaggrin becomes post-translationally modified & directs the aggregation of keratin proteins

Keratohyalin granules

• Dense,Irregular amorphous deposits along keratin

filaments in St.Spinosum & St.Granulosum• Composed of Profilaggrin – Bundling of Keratin Intermediate filaments

to form MacrofibrilsFilaggrin – Water binding in SCLoricin & Cystatin A – Transglutaminase activity &

Components of Cornified EnvelopeKeratins – K1 & K10

• Mutations that occur post zygotically lead to Aplasia cutis

• X- linked ichthyosis – failure of cornified cells to slough

results in retention hyperkeratosis

• In PV – antibodies to Desmoglien 3 & 1

• In SSSS – antibodies to Dsmoglien 1

• In psoriasis – cell cycle reduced to 36 hrs

KERATINOCYTE CORNIFICATIONCONSEQUENCES OF FAILURE

Sunburn cells

• A sunburn cell is a keratinocyte with a pyknotic

nucleus and eosinophilic cytoplasm

• It appears after exposure to UVC or UVB radiation

or UVA in the presence of psoralens

• It shows premature and abnormal keratinization

• It has been described as an example of apoptosis

• Derived From?• Function?

• Synthesize & secrete?

• Neural Crest• Pigmentation/ protection• Melanin

Melanocytes

• They enter Epidermis B/W 7 to 8 weeks of Gestation

•Distributed in Basal layer,Extends towards dermis

below level of basal cells above lamina densa

-Curved arrow shows Langerhans cells

- straight arrow shows Melanocytes

Melanocytes

• High density on face & male genitalia, Lowest on

trunk

• Production of melanin,transfer to keratinocytes &

hair follicle shaft through Melanosomes

• One Melanocyte is in contact with 36 keratinocytes

these group of cells k/a Epidermal Melanin Unit

Melanocytes• Lightly pigmented skin are Melanosmes

small,mostly aggregated,Membrane bound clusters

• Dark skinned- Inc Melanization,Dec Melanosme

degradation,Larger melanosomes,singly

distributed,Individual organelles,contains larger &

more dendritic melanocytes

Melanocytes

• Hair melanocytes: 1 M = 5 K,Graying d/t Dec in no

of follicular melanocytes,1 M = 10 K in basal

• Chronic sun exposure - larger melanosomes

• No Racial defferences in no of Melanocytes

• Difference in size,distribution & no of melanosomes

• All races have same density of Melanocytes

Melanocytes

Melanin : • Synthasized in melanosomes•Stimulated by MSH

Pheomelanin

• Red – Yellow in colour•Synthasized in Pheomelanosomes•Sperical structure

Eumelanin

• Brown to Black in colour•Synthasized in Eu Melanosomes•Oval shaped

Stages of Melanosomes 4 stages

St. 1 – round,membrane bound vesicles,No Melanin,Enzyme

activity in Concentric lamellae

St. 2 – Melanin deposited in cross linked longitudinal filaments

St. 3- Melanin deposition inc by enzyme & Non enzymatic

polymerization

St. 4 – fully developed,no enzyme activity

From 1 to 4 Inc Acid phospatase,Dec Tyrosinase

Melanocytes• Melanocortin 1 receptor controls type of melanin

produced

• Loss of function MC1R results in Inc PM & Dec EM

• Fair skin more prone to UV radiation absorption,Inc risk

of melanoma

• C – Kit activation plays key role in migration &

development of Melanocytes & Melanoblasts

Melanocytes

• Reduced - Piebaldism- Waardenburg syndrome

• Defective Pigment - Albinism- Chediak Higashi

syndrome

• Freckles result from localized inc in production of pigment by normal number of melanocytes•Naevi – benign proliferations of Melanocytes

The melanocyte is the dendritic pigment-producing cell of the epidermis and found in the basal layer.

•Different races have same # of melanocytes• amount of melanin produced differs•Melanin accumulates in keratinocytes

Langerhans cells

• Dendritic cells

• Dopa negative & ATPase Positive

• Distributed Basal & Spinous layers

• 2 to 8 % total Epidermal Population

• LC have folded nucleus,Clear cytoplasm,Well

developed Endoplasmic reticulum,Golgi complex

Langerhans cells

• Intracytoplasmic organelles k/a Birbeck

Granules/Tennis racket shaped/Rod shaped with

straited appearance

• Contains Actin & Vimentin

• Connected to Keratinocytes = E – Caderin Recp

• Exposure to UV radiation causes depletion of LC &

Dec ability to present antigen

• LC have become prospective vehicle for tumor

theraphy & tumor vaccines

• In Psoriasis,Contact Dermatitis, Sarcoidosis their

number is reduced

Langerhans cells

• LC histiocytosis seen in

Lettere siwe disease

Eosiniphilic granuloma ( skull bones)

Hand schuller christian disease : DI,Bone

lesions,Exophthalmos

Hashimoto Pritzker

• Derived From?• Function?• Contain?

• Neural Crest• Sensory• Dense-core graunles

Merkel Cells

More no of Merkel cells seen in high tactile

sensitivity areas such as high density of hair

follicles,digits,lips around oral cavity

Merkel Cells

• Oval in shape,15 micron,oriented parallel to BM

• Pale cytoplasm,lobulated nucleus with Cytoplasmic

spines towards Keratinocytes

• Merkel – Neurite complex along with sensory nerve

endings,serve as mechanoreceptors for touch

sensation

• Slow Adapting , type 1 Mechanoreceptors

Merkel Cells

• EM shows microvilli at cell surface with dense core

granules with intermediate filaments & lobulated

nucleus – Dot Like pattern

• Markers – Cytokeratin 20 ( CK 20)

• Contains battery of neuropeptides –

NSE,VIP,Chromogranin A,Calcitonin gene related

peptide

• Cells embedded in Basal layer & form desmosomal

connections with Basal Keratinocytes

• Merkel Hyperplasia - adnexal tumours Naevus

sebaceus,Trichoblastomas,Trichoepitheliomas &

nodular hidradenomas

Mast cell

• Most numerous in Subpapillary dermis,around blood

vessels,nerve & appendages

• Ovoid ,Mono or Binuclear,numerous round cytoplasmic

granules

• Play role in Innate & Adaptive immunity

• Mast cell reside in papillary dermis & undergo

migration through basal lamina in to epidermis

Mast cell

• Play role - Skin pathology in

AD,CD,Scleroderma,Immuno bullous disorders

Mast cell

Thank You