dendritic cells are part of innate immune system, antigen presenting cells in skin, activation of t cells and inducing and maintaining immune tolerance, 4 types- langerhans cells, dermal dendritic cells, merkel cells, melanocytes
2. Innate immunity-Immune mechanisms that are
used by the host to immediately defend itself
Comprises of bariers, complements, antimicrobial
peptides, cytokines, macrophages, DCs , NK cells,
PMNs
Dendritic cells (DCs) are component of innate
immune system. Their main function is to process
antigen material and present it on the surface to T-
cells, thus functioning as antigen-presenting cells.
2
4. Location
Dendritic cells -skin ( Langerhans cells,dermal
dendritic cells)
inner lining of the nose, lungs, stomach and
intestines.
They can also be found in an immature state in
the blood.
4
5. FUNCTIONS OF DENDRITIC
CELLS
Antigen presentation and activation of T cells.
Inducing and maintaining immune tolerance.
Maintain immune memory in tandem with B cells.
5
7. LANGERHANS CELLS
First described by Paul langerhans
in 1868
They are bone marrow derived , dendritic ,antigen
presenting cells situated in middle of epidermis
They are present in
epidermis,dermis,thymus,tonsils,lymph nodes,
epithelia of oral and genital mucous membranes.
7
8. Embryology
They appear by fourteenth week of fetal life
Derived from mesenchymal precursors in bone
marrow
• until the 12th wk, cd1a- & lack birbecks granules
not present
8
9. They are distributed in basal ,spinous and
granular cell layers,
They constitute 2% to 8% of total epidermal cell
population.
They vary in distribution according to their
anatomical sites, their number ranging between
460 and 1000/sqmm of epidermis.
9
10. Within the epidermis LCs are anchored to
surrounding keratinocytes by E-cadherin mediated
homotypic adhesions
Dermal CD14/Cd11c have the potential to
differentiate into LCs under TGF-B1
10
11. Plasmacytoid dendritic cells- In conditions SLE,
virus infectn, psoriasis, allergic CD. Non-indigenous
DCs precursors characterized by highly dev
ER(plasma cell like appearance) enters the skin
Produce natural IFNs in respone to TLR ligand
Hence named principal type IFN producing cells
11
12. In conditions like Atopic dermatitis, contact eczema
another non indigenous DC precursors originate
from myeloid precursors
Inflammatory dendritic epidermal cells
Characterized by expression of CD1a, CD1b, CD23
& CD36
Immune response triggered by these is in TH112
13. Danger signals
TLR ligands, chemical haptens, hypoxia
LCs enlarges,downregulation of Fc receptors & E-
cadherin, increased MHC 2
Migrate downward into dermis-enter afferent
lymphatics- reach T cell zones of lymph node
TNF-a & IL-1B stimulates this process
To penetrate BM, LCs attach to it via a6 containing
integrin receptors producing type 4
collagenase(MMP9)
Osteopontin
13
16. On EM: lobulated nucleus
• Absence of tonofilaments, desmosomes
• Flat plate like structure with hemispherical blebs at
one end (birbecks granule)
• C/S app: tennis racquet shape.
`
arise from infolding of plasma membrane function as
phagosomes that digest extracellular material
16
18. Identified by special techniques
like impregnation by gold
chloride ,staining by ATPase
and alpha-D-mannose.
Immunohistochemical Markers
ATPase +ve , DOPA negative
S100, CD1a +ve
HLA-DR,DP,DQ antigen +ve
Recently identified
Langerin(CD207)-single best
marker
18
19. Other importance
Membrane receptors for C3b & Fc portion of IgG
Recognition of antigens& cell mediated immunologic
reactions as allergic CD
Allograft rejection(1a antigens)
Immune surveillance against UV neoplasia(UV
decreases no. of langerhans cells)
19
20. FUNCTIONS
cutaneous immune mechanism,especially in
antigen presentation ,
stimulation of T cell response
phagocytosis
regulation of epidermal differentiation
20
21. APPLIED ASPECTS
Implicated in various immune processes
allergic contact dermatitis,
HIV,
allograft rejection,
immune tolerance and
surveillance against neoplasia.
Psoriasis, sarcoidosis, CD their no. in skin is reduced
Because of their antigen presenting ability they have
become prospective vehicle for tumor therapy and
tumor vaccines
21
22. Dermal Dendritic Cells
Bone marrow derived leukocytes having migratory and
AP property
Markers include CD1b, CD1c, subunitA of clotting factor
X111
Differentiated from macrophages
• expression of MHC 2
• CD205
• absence of phagolysosome
DDC are located in the vicinity of superficial plexus
DDC proliferate constitutively in human dermis
22
24. MELANOCYTES
Dendritic cells that synthesize and secrete
melanin contaning organelles called
melanosomes.
Appearance of melanocytes in epidermis takes
place in craniocaudal direction
in accordance with the development of neural
crest from which they are derived.
24
25. EMBRYOLOGY
Neural crest, they migrate to epidermis, various
mucous epithelia, hair follicle, dermis,
leptomeninges inner ear, uveal tissue
In retina originate from optic cup of primitive
forebrain
Primitive melanocytes in skin are first found
during eighth week of fetal life
At 5mon melanosomes begin to transfer pigment
to keratinocytes
25
27. `
They express integrin receptors and use them to
migrate to epidermis during development
Melanoblast migration and differentiation into
melanocytes
• Wnt , ET3, BMPs
• Steel factor, c-kit , HGF
27
29. BMPs- disulfide linked dimeric proteins
belongs to TGF-B family
Signaling suppresses neural crest differentiation
into melanoblast
Functions as Wnt antagonists
29
30. Endothelins bind & activate transmembrane G
protein linked receptors EdnrA , B
Ednr B receptors imp for melanoblast migration
and proliferation
Defect causes waardenburg synd &
Hirschsprung synd
30
31. SF-early melanoblast development requires
cytokine SF and its TK transmembrane receptor
c-kit
Mutn of c-kit/Sf causes Piebaldism
HGF is ligand for transmembrane TK receptor
met, also essential for melanoblast proliferation &
differentiation
31
32. Types
Dendritic :- these are capable of pigment transfer to
other cells
Non-dendritic :- they retain the melanosomes
synthesized
Eg: uvea, retina , leptomeninges
32
33. Site specific melanocytes
1]Cutaneous Melanocytes
• Largest no. of melanocytes
• Skin & hair follicle
• Part of “epidermal melanin
unit”
2] Melanocyte stem cells
• present in hair follicle bulge
• Express TRP2 & nestin
• Other transcription factr-
SOX10 & Pax5
33
34. 3]Ocular melanocyte
uveal tract; they do not transfer their
melanosomes
required for proper routing of I/L & C/L neural
fibres in optic chiasm
Imp: visual abn in pts with albinism
34
35. 4]Otic Melannocyte
• reside in cochlea & imp for hearing
• Maintainence of endolymph through regulation of
K transport
• Imp: Waardenburg synd
5]CEPHALIC Melanocyte
distributed through out the meninges & more
dense in leptomeninges above pons & medulla
oblongata.
35
36. Epidermal melanin unit
Assn between dendritic melanocytes and keratinocytes
within epidermis
One melanocyte is in contact with 36 basal and
suprabasal keratinocyte
Ratio of melanocyte to keratinocyte in basal layer of
epidermis varies frm 1:4 to 1:10
36
38. DISTRIBUTION
Melanocytes are more abundant in skin of face
and male genitalia upto 2900/mm2 than on trunk
upto 1250/mm2
As a rule greater amount of melanin is present in
basal keratinocytes than in melanocytes
basal cells at tip of rete ridges are more38
39. MELANIN SYNTHESIS AND
MELANOSOMES
Melanin is produced in melanosomes(pigment
granules or organelles)
A key protein involved in melanosome assembly is
NCKX5 encoded by SLC24A5.
The major hormone controlling melanin synthesis is
MSH from the pitutary gland.
39
41. Melanin Synthesis
Synthesis of melanin starts with the conversion
tyrosine to dihydroxyphenyalanine(dopa) by
tyrosinase, a copper containing, aerobic enzyme
Critical, rate limitting step {Raper-mason pathway}
Then dopa is oxidized to dopaquinone by tyrosinase
41
43. These reactions occurs in melanosomes
then passed on to surrounding keratinocytes
undergo series of developmental and biochemical
stages;
STAGES OF MELANIZATION
Stage1:melanosomes are spherical, membrane bound
vesicles 0.3um composed of longitudinally oriented
concentric lamellae.
No melanin.
43
44. Stage2: melanosomes are ellipsoidal 0.5um
Melanin deposited within cross-linked long filaments .
Stage 3: melanin deposition increases by enzymatic
and non-enzymatic polymerizatn
Stage 4: melanosomes are fully developed and electron
opaque because of dense deposits of melanin.
mainly by polymerization
44
47. Between these electron dense melanized cores and
their outer membranes mature melanosomes house
distinct vesicles 40nm in diameter called
vesiculoglobular bodies.
Inv in internal organization & melanization of eu- and
pheomelanosomes
During progression from stage 1 to stage 4 tyrosinase
decreases and acid phosphatase increases( helping in
degradation)
47
49. TYPES OF MELANIN
Eumelanin
• insoluble
• produced in
eumelanosomes
• Large,elliptical
• Highly structured
fibrillar glycoprotein
matrix
• black and brown
colour of skin and
hair
Pheomelanin
• Sulfur containing,
soluble
• Pheomelanosomes
• Smaller, spherical
• Disorganised and
loose glycoprotein
matrix
• lighter colour of hair
49
50. Melanosome transport
Once melanosomes are formed, melanized and reach
tips of dendrites,
they are transported from melanocytes to
keratinocytes by apocopation
Keratinocytes phagocytize the melanosome laden tips
of melanocytic dendrites
In the epidermis melanosomes become concentrated
in a umbrella like array above nuclei of keratinocytes
on the side towards skin surface i.e. on the sunny side
of nuclei
50
52. Following transfer to keratinocytes
fully melanized melanosomes are conveyed
upwards as basal keratinoctes mature
are eventually degraded by lysosomal enzymes
and
shed as cornified cells are desquamated.
52
53. COLOUR OF SKIN
Absolute number of melanocytes in human skin
is same for both sexes and all races.
Differences in color among the races result
from differences in the
1.number
2.size
3.degree of melanization
4.distribution
5.rate of degradation of melanosomes
within keratinocytes
53
54. NORMAL MICROANATOMY
On staining with
H&E they contain
round to oval dark
stained nuclei and
clear halo of
surrounding
cytoplasm
54
55. On electron microscopy,these cells characteristically
‘hang down’ from basal cell layer and are devoid of
tonofilaments or desmosomes.
abundant melanosomes in varying stages of
melanisation.
numerous mitochondria , well developed RER Golgi55
57. Melanin can be bleached by strong oxidizing
agent such as H2O2 or KMNO4
DOPA reaction- unfixed tissue of enzymatically
seperated epidermal sheets are incubated in
0.01% soln of 3-4 dihydroxyphenylalanine this
stains functional active melanocyte as dark brown
or black.
57
60. FUNCTIONS OF MELANIN
Absorption of UV (phototoxic and photosensitize)
Thermoregulation
Free radical quencher
Protection against lipid peroxidation
Photoprotection, photoageing
Protection from photocarcinogenesis,
Regulation of vit D synthesis.
Camouflage and sexual appeal.
60
61. APPLIED ASPECTS
In vitiligo melanocytes are destroyed.
In albinism melanocytes are normal in number but
unable to synthesize fully pigmented
melanosomes because of defective enzymatic
formation of melanin.
Freckles result from increase in production of
pigment by normal number of melanocytes.
Nevi are benign proliferations of melanocytes
Malignant counterpart - melanoma
61
62. MERKEL CELLS
In 1875,FRIEDRICH MERKEL identified at
base of rete ridges cells that were in
contact with nerve fibrils and named them
tastzellen or touch cells.
Slow adapting type1 mechanoreceptors in
sites of high tactile sensitivity
62
63. Embryology
Appear in fetal skin by sixteenth week of
gestation.
They originate in the epidermis itself, presumably
from germinative keratocytes
63
64. Found in basal layer of epidermis
In hairy skin & glabrous skin of digits, lips, regions
of oral cavity , outer root sheath of hair follicle
They are arranged in groups at tips of rete ridges.
64
66. STRUCTURE
Electron lucent cytoplasm rich in organelles
Lobulated nuclei
Margins of cells project cytoplasmic “spines” towards
keratinocytes
Poorly developed desmosomes,delicate cytoplasmic
microfilaments.
Characteristic spherical granules which are
membrane limited with a dense central core
66
68. Merkel cells are supplied by myelinated nerves
as they near epidermis lose their myelin sheaths and
continue as unmelinated axons
surrounded by cytoplasm and basement membranes
of schwann cells.
The nerve fibres terminate in flat,meniscus like
contacts that are studded along basal aspects of
merkel cells.
68
70. On silver impregnated sections, the meniscoid
nerve terminal that covers basal portion of each
merkel cell can be seen as merkel disk.
Immunohistochemical markers :-
K8, K18, K19 & K20
K20 is highly specific for merkel cells.
70
71. Merkel cells express neuroendocrine markers
such as chromogranin A and synaptophysin.
Neurosecretory substances in particular
neuropeptides that are stored in densecore
granules include VIP , CGRP, Serotonin,
substanceP.
71
72. FUNCTIONS
They are slowly adapting , low threshold type 1
mechanoreceptors.
They may enhance or induce the excitability of
sensory nerve endings via release of
neuropeptides.
72
73. APPLIED ASPECTS
Merkel cell hyperplasia with keratinocyte
hyperproliferation is seen in adnexal tumors such
as naevus sebaceus, tricoblastomas,
trichoepitheliomas, nodular hidradenomas.
Merkel cell hyperplasia with hyperplasia of nerve
endings neurofibromas
neurilemmomas,
nodular prurigo
neurodermatitis
73