This document summarizes the histology of skin. It describes the layers of the epidermis and dermis, the cells found in skin including keratinocytes, melanocytes and Langerhans cells. It also discusses skin structures like hair follicles, sweat and sebaceous glands. Functions of skin like protection, sensation and thermoregulation are covered. Cancers of the skin and wound healing are summarized.

1. HISTOLOGY OF SKIN
Dr Evans Masitara PRAS 1
Dr Patrick Gicheru Ortho 1
Instructor : Professor OGENG’O

2. Objectives
Layers of the skin
Function of skin
Layers of Epidermis
Layers of Dermis
Cells of the Skin
Structures of the Skin
Clinical Relevance

3. Overview
• Largest Organ ,20% mass,2M2,
• Epidermis(Ectoderm),keratinized stratified squamous
• Dermis(Mesoderm),Dense CT,mechanical support,thickness
• Hypodermis, Adipose, Subcut Fascia
• Epidermal Derivatives – Hair follicles
- Sweat Glands
- Sebaceous Glands
- Nails
- Mammary Glands

4. Function
• Barrier and protection
• Immunologic
• Homeostasis
• Sensory
• Endocrine
• Excretion
• Absorption

5. Classification
• 1mm to 5 mm
• Thick – Hairless, palms and soles
• Thin - Hair follicles
• Haired vs Glabrous
• Glabrous – Eyelids ,lips,nipple,areola,palms,soles
• Anatomically Thickest skin found on back.Thick dermis

6. Layers of Skin
• Epidermis
Stratum Basale /Germinativum – Stem cells
Stratum Spinosum /Prickle cell - LM appearance –short processes
Stratum Granulosum – Keratohylin granules
Stratum Lucidum – Thick skin, Transluscent,subdivision S Corneum
Stratum Corneum – Thickest,Keratinized cells

7. Epidermis and Dermis

10. Stratum basale
• Single layer ,rests on Basement membrane
• Stem cells give rise to keratinocytes,mitosis
• Small cuboidal to low columnar
• Closely spaced nuclei,basophilic cytoplasm
• Melanocytes scattered
• Attached to each other and keratinocytes – Desmosomes
• Attached to BM- Hemidesmosome

11. Stratum Spinosum aka Squamous Layer
• Several Cells Thick
• Larger than S basale
• Numerous cytoplasmic processes(spines)
• Attached by desmosomes (NODE OF BIZZOREZO)
• Contain bundles of Keratin in cytoplasm
• Cells referred to as PRICKLE cells

13. Stratum Granuosum
• Most superficial layer of non keratinized epidermis
• One to three cells thick
• Keratohylin granules – Cysteine and histidine rich
- Aggregates Keratin filaments in S Corneum
- Intense basophilic stain
• Some have lost nuclei ,clear outline

14. Stratum Corneum
• Thickest layer, Varies most in thickness
• Horny cells
• No nucleus and organelles
• Keratin Filled
• Outer membrane coated with lipids – Water Barrier

15. Stratum Lucidum
• Considered subdivision of S Corneum
• Thick skin
• Stains poorly, LM difficult
• Advanced keratinization in eosinophilic layer
• Disrupted nucleus and cytoplasm

16. Dermis
• Composed of connective tissue
• Suppports epidermis
• Dermal papillae and Epidermal ridges
• Thickness varies according to region
• Collagen(Tough) Elastin( Elastic)
• True Dermal Ridges (Thick Skin) – DERMATOGLYPHICS
• 2 LAYERS

17. Dermal Epidermal Junction
• Epidermis supported by BM
• BM – Plasma membrane basal cells
- Lamina Lucida
- Lamina Densa
- Sublamina Densa Zone
- Dermal Papilla Interdigidate with epidermal ridges
CLINICAL
Pemhigoid and bulous pemphigus are conditions affectin derma –
epidermal junction – Detachement of epidermis from BM

18. Papillary Dermis
• More Superficial
• Loose CT , below epidermis
• Thinner collagen 1 and 3
• Elastic fibres
• Blood vessels – Terminate but serve epidermis
• Nerve processes enter Basal lamina
• Meisner corpuscules – Light touch

19. Reticular Layer
• Deep to papillary layer
• Thicker and less cellular
• Irregular dense CT ,little ground substance
• Collagen and elastic fibres form Langers lines
CLINICAL
Skin incisions made parallel to Langers lines heal with least scarring

20. Panniculus Adiposus
• Deep to reticular layer
• Storage site/insulation
• Thick in obese and cold climate
• Errector pilli muscles- Goose bumps
• Panniculus carnosus – Platysma and some muscles of facial
expression
• Panniculus adiposus + CT = Hypodermis/Subcut layer

21. Dermal Hypodermal junction
• Contains large vessels which supply dermis
• Paccinian Corpuscles –Deep pressure receptors ,mechanical/vibration

22. Cells of the skin
• Keratinoctes
• Langerhans’ cells
• Melanocytes
• Merkels cells

23. Keratinocytes
• Predominant cell type of epidermis( Ectodermal)
• Origin- Basal epidermal layer
• Keratin – Major structural protein in epidermis 85%
- Participate in water barrier
• Contain numerous ribosomes(Basophilic),RER,Golgi
• Ribosomes – Keratin filaments(Intermediate) Tonofilaments

24. Keratinocytes CNT
• Keratin filament synthesis continues in S Spinosum
• Secretes Keratohyalin granules in S Spinosum
• K Granules contain Filaggrin and Trichohyalin
• Produce Tonofibrils, Thicker eosinophilic
• Soft keratin formed in S Corneum
• Synthesize lamellar bodies – Intercellular epidermal water barrier
(Cell n lipid envelope)
• Desquamate in S Corneum and shed off

26. CLINICAL
• Keratinization 2 to 6 Hours
• Time it takes cells – S granulosum – S corneum
• Clinical
Psoriasis – Turnover of keratinocytes greatly reduced

27. Melanocytes
• Neural cell crest derived, dendritic cells
• Pigment producing cells
• TYROSINE – DOPA – MELANINE (TYROSINASE)
• Maintain ability to replicate
• Epidermal- Melanin Unit
• Transfer melanin to keratinocytes via pigment donation
• UV irradiation protection

28. SKIN COLOR
• Number of melanocytes same in all races
• Age,gender,race,hormornes,diet,genetics,climate,toxins
• Melanin degraded more rapidly in light skinners by lysosomes
• Eumelnin (Brown) Pheomelanin(Reddish Yellow)
• INCREASE – Sun exposure
• - Hormonal imbalance (Adisons)
• Number decreases with age
• Albinism – Deficiency of Tyrosinase
• Vitiigo – Absence of melanocytes
• Others – Oxyhemoglobin, Carotenes,Bilirubin,hemosiderin

29. Langerhans cells
• Mesordemal origin
• Dendritic APCs
• CD34 stem cell Bone marrow
• Mononuclear phargocytic system
• S Spinosum predominant
• Uneven nuclei , BIRBECK GRANULES
• Antigen presenting cells, Delayed type sensitivity rxn,contact dermatis
• Express MHC 1,2 ,CMPLMT C3b,CD1a
• HIV Resevoir

31. Merkel cells
• Epidermal cells that function in cutaneous sensation
• Fingertips, palms,soles,genital mucosa
• Modified epidermal cells in basale
• Neural crest
• Bound to keratinocytes with desmosomes
• 80nm dense cored neurosecretory granules- Adrenal medulla,carotid
body
• Neuron + Epidermal cell = Merkels corpuscle(Merchanoreceptor)

33. Merkel cell carcinoma
• Uncontrolled proliferation in sun exposed areas
• Early metastasis via lymph

34. Aging of skin
• Epidermal and dermal atrophy
• Reduced number of melanocytes, Langerhan cells and hair
• Dermis – Flacidity,reduced elasticity, collagen synthesis, fibroblasts
Decreased vascularization and innervation
UV – Stimulatory to melanocytes,
inhibitory to Langerhans –Photo aging

35. Cancers of Epidermal origin
• Basal cell carcinoma – Resemble cells from stratum basale
Follicular bulge of hair follicle sheath
No metastasis
Surgical excision
Squamous cell carcinoma – Atypical cells pan epidermis(cis)
Disruption of Basement memb (Mets)
Treatment stage n tumor dependent
Mohr's excision

36. Cancers of Epidermal Origin Cont
• Malignant Melanoma
• From melanocytes
• May be only in epidermis (melanoma in situ)
• Radial growth
• Irregular pigmented multicolor lymph nodes
• Metastasize via lymph
• A B C D RULE

37. Wound healing
• Primary Union – Surgical, Clean, Approximated edges
• Secondary union – Separated edges, loss of cells,trauma
- Granulation
• Healing requires epidermal and dermal repair

38. Dermal repair
• blood clot formation
• removal of damaged collagen fibers, through the effort of
macrophage activity that is associated with inflammation,
• formation of granulation tissue
• re epithelialization of the exposed surface,
• proliferation and migration of fibroblasts and differentiation of
myofibroblasts involved in

39. EPIDERMAL REPAIR
• Involves proliferation of
• basal keratinocytes in the stratum basale in the undamaged
• site surrounding the wound .
• Mitotic activity is markedly increased within the first 24 hours
• wound site is covered by a scab that represents dehydrated blood clot
• The migration rate may be as much as 0.5 mm/day, starting within 8
to 18 hours
• proliferation and differentiation occur behind the migration front,
leading to restoration of the multilayered epidermis.

41. SCAB

42. Structures of the skin
Nerve endings
skin appendages
Dr. P. Gicheru

43. Nerve supply
• Skin is endowed with various sensory and motor nerve endings
• Sensory nerves terminate in receptors
• Free nerve endings
• Encapsulated nerve endings
• Pacinian corpuscle
• Meissner’s corpuscle
• Ruffini’s corpuscle
• Krause end-bulb
• Motor nerve termination – vessels, erector pili muscles, sweat glands

44. Sensory Nerve supply

45. Free nerve endings
• Endings lack capsulation
• Terminate in stratum granulosum
• Sensitive for multiple sensory modalities
• Including fine touch, heat, cold
• Mechanoreceptors for hair follicles – attach to outer root sheath of
follicles

46. Pacinian/lamellar corpuscles
• Locate in the dermis and hypodermis
• Numerous in finger tips
• Sensitive to vibration and pressure
• Resemble cut onion, 1mm
• Lymph like fluid between lamellae
• Rapidly adapting fibres
• Other sites: joint, periosteum,
internal organs such as pancreas

47. Pacinian/lamellar corpuscles

48. Meissner’s corpuscle
• Located in the dermal papillae
• Appear as tapering cylinders
• Touch receptors
• Particularly sensitive to low frequency
stimuli in hairless skin – lips, palms
• Capsule made by flattened Schwann cell
with irregular laminae

49. Meissner’s corpuscle

50. Ruffini’s corpuscles
• Simples encapsulated mechanoreceptors
• Sensitive to stretch and torque
• Rapidly adapting
• Location : dermis/hypodermis
• Fusiform shape with fluid filled cavity
• Axons arborized within the capsule

51. Epidermal appendages
• From down-growth of epithelium during
development
• Include;
• Hair follicle and hair
• Sabaceous glands
• Eccrine sweat glands
• Apocrine sweat glands
• nails

52. Hair follicle and hair
• Absent only on palms, soles, lips, around
urogenital orifices
• Distribution affected by sex hormones
• Hair color determined by type of melanin
contained
• 3 Segments of hair follicle;
• Infundibulum
• Ithsmus
• Inferior segment – supra bulbar & bulb

54. hair
• Elongate filamentous structure
• Growth phases – anagen, catagen, telogen
• Composed of keratinized cells from hair follicle
• Emerges as hard keratin
• Arrector pili muscle - Smooth muscle
• Hair has 3 layers
• Medulla
• Cortex
• Cuticle of hair shaft - keratinized
• Also has melanin pigment

55. Clinical aspects
• EPITHELIAL follicular BULGE – has epidermal stem cells
• Migration and re-epithelialization in full thickness epidermal
injury/removal
• Grafting in deeper wounds
• Alopecia – areata, universalis, tractional

56. Sebaceous glands
• Dev’s as outgrowth of external root sheath
• Holocrine secretion – sebum
• Basal cells continuously divide
• Mitosis secretion = 8days
• Abundant SER – lipid synthesis
• Role – bactericidal, emollient, barrier,
pheromone
• Role in Acne

57. Sweat glands
• Classified according to structure
and nature of secretion
• Eccrine
• apocrine

58. Eccrine sweat glands
• Blind ending coils tubular structures
• Not ass. With hair follicles
• Secretory segment – deep dermis/hypodermis
• Duct segment – epidermis
• Role – thermoregulation, excretion
• Secretion – merocrine
• initially isotonic then hypotonic - Na, Cl, Ur, uric acid, ammonia

59. Eccrine sweat glands cont.
• Cell types in secretory segment
• Clear cell
• Dark cells
• Myoepithelial cells
• Duct segment cells
• Stratified cuboidal epithelium

60. Apocrine sweat glands
• Develop from same epidermis that give rise to hair
• Connected to hair follicle
• Coiled tubular
• Secretions - initially thought to be apocrine, however confirmed to be
merocrine
• Secretion –pheromones, Protein, CHO, lipids, ammonia, usually oduorless.
• Oduor due to bacterial action
• Become functional at puberty with hair growth

61. Apocrine sweat glands cont.
• Secretory portion
• One cell type – simple epithelia
• protein synthesis features
• Myoepithelia cells
• Duct cells – stratified columnar
• Urea frost

63. Nails
• Plates of keratinized cells with hard keratin
• nail bed – continuous with st. spinosum and basale
• Lunula – white cresent shaped
• Eponychium/cuticle – has hard keratin; doesn’t desquamate
• Hyponychium – thickened epidermal layer – secures free edge at the
finger tip

64. Nail cont.
• root covers matrix
• Matrix has stem cells, melanocytes,
merkels and Langerhans cells
• Nail has corneocytes; lack organelles
• Ingrown toe nails – improper nail plate-
groove fit.
• onychomycosis

65. references
• Ross textbook and atlas of histology 6th edition
• Kimani’s histology