The document details the role and characteristics of dendritic cells, specifically their functions in the innate immune system, including antigen presentation, T cell activation, and immune memory maintenance. It discusses the different types of dendritic cells, their embryological development, histological identification, and their implications in various immune processes and diseases. Additionally, it covers melanocytes and Merkel cells, detailing their functions and significance within the skin's immune response and sensory mechanisms.

1. JAI KUMAR PILLAI
KIMS, BANGALORE
DENDRITIC CELLS
1

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

3. 3

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

6. TYPES OF DENDRITIC CELLS
 Langerhans cells
 Dermal dendritic cells
 Melanocytes
 Merkel cells
6

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

14. 14

15. Histology
 Light microscopy : pale staining & convoluted nuclei
15

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

17. 17

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

23. 23

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

26. 26

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

28. MITF affects melanoblast differentiation by
inducing tyrosinase, TRP1& dopachrome
tautomerase(TRP2)
28

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

37. 37

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

40. 40

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

42. 42

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

45. 45

46. 46

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

48. FACTORS INFLUENCING MELANIN
SYNTHESIS
 UV LIGHT : Increases melanocytes
 HORMONES:MSH,ACTH, oestrogen,
progestrone,Androgens,lipotropins,thyroxine
all have melanocyte stimulating property
 OTHERS:- pgd2,pge2,arachidonic acid,oleic acid
 AGEING : Causes decrease in follicular
melanocytes
48

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

51. 51

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

56. 56

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

58. 58

59. IMMUNOHISTOCHEMICAL
TECHNIQUE
ANTIGEN ANTIBODY POSITIVE
CELLS
ANTIGEN
LOCATION
S100 PROTEIN Anti-S100
PROTEIN
Melanocytes,lan
gerhans
cells,schwann
cells,adipocytes,
myoepithelial
cells of sweat
glands
Nuclear and
cytoplasmic
gp100 HMB-45 Fetal
melanocytes,acti
vate adult
melanocytes
Cytoplasmic
Melan-A/MART-1 A103/M2-7C10 Melanocyte Cytolplasmic
Tyrosinase T311 Melanocytes Cytoplasmic
PNL2 ANTIGEN PNL2
ANTIBODY
Melanocytes Cytoplasmis
59

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

65. 65

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

67. 67

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

69. 69

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

74. THANK YOU
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