Download Functions of the skin 1 Physical barrier against friction and shearing

DERMATOLOGY
Dr.Pawana Kayastha
FUNCTIONAL ANATOMY,&
PHYSIOLOGY OF SKIN
INTEGUMENT :
skin(epidermis,dermis) and associated appendages
(sweat glands,sebaceous glands,hairs,nails).
 Largest organ in the body.abt. 16% of total body weight.
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EPIDERMIS :
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Outermost layer of the integument
Stratfied squamous epithelial layer of ectodermal origin
Devoid of vessels
Pic: Epidermal Layers
Pic: Skin, layers and its appendages
FUNCTIONS OF THE SKIN
Function
Structure/cell involved
Protection against:
Chemicals, particles,
dessication
Stratum corneum
Ultraviolet radiation
Melanin produced by melanocytes and transferred to
keratinocytes
Antigens, haptens
Langerhans cells, lymphocytes, mononuclear
phagocytes, mast cells
Microbes
Stratum corneum, Langerhans cells, mononuclear
phagocytes, mast cells
Preservation of a balanced internal environment
Prevents loss of water,
electrolytes and
macromolecules
Stratum corneum
Shock absorber
Strong, yet elastic and
compliant covering
Dermis and subcutaneous fat
Sensation
Specialist nerve endings mediating pain leading to
withdrawal, and itch leading to scratch and hence
removal of a parasite
Vitamin D synthesis
Keratinocytes
Temperature regulation
Eccrine sweat glands and blood vessels
Protection, and fine
manipulation of small
objects
Nails
Hormonal
Testosterone synthesis from
inactive precursors
andtestosterone conversion
to other androgenic steroids
Hair folliclesSebaceous glands
Pheromonal (of unknown
importance in humans)
Apocrine sweat glands
Psychosocial, grooming
and sexual behaviour
Hair, nails, appearance and tactile quality of skin
APPROACH TO THE PATIENT
HISTORY
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time course of rash
distribution of lesions
symptoms (e.g. itch or pain)
family history (especially of atopy and psoriasis)
drug/allergy history
past medical history
provocating factors (e.g. sunlight or diet)
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previous skin treatments.
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EXAMINATION
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looking at and feeling a rash
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assessment of nails, hair, and mucosal surfaces, even
if these are recorded as unaffected.
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The following terms are used to describe distribution:
flexural, extensor, acral (hands and feet), symmetrical,
localized, widespread, facial, unilateral, linear,
centripetal (trunk more than limbs), annular and
reticulate (lacy network or mesh like).
TERMS USED TO DESCRIBE SKIN LESIONS
Term
Definition
PRIMARY LESIONS
Macule
A small flat area of altered colour, e.g. freckle
Papule
A discrete lesion, usually raised above the surface of the skin and therefore visible.
There may be a change in colour. Some, particularly if they arise from the subcutis, are
felt rather than seen. Larger papules are referred to as nodules. Although there is no
agreed definition a nodule is usually bigger than 1 cm, e.g. a melanocytic naevus or a
nodular melanoma
Plaque
A raised area of skin with a flat top, typically, several cm or more across. Scale is usually
present, e.g. psoriasis
Vesicle
and bulla
A small (∼ several mm) and a larger blister (∼ several cm) respectively. Blisters are
collections of fluid; if a small needle is inserted fluid drains out. They form either
within the epidermis or just beneath it, e.g. a thermal burn or pemphigoid
Pustule
A focal visible accumulation of pus in the skin. Usually yellow or green, e.g. acne
Abscess
A localised collection of pus in a cavity, more than 1 cm in diameter
Weal
An evanescent discrete dermal collection of fluid. The fluid is diffuse, unlike in a
blister. Weals are usually white due to masking of the local blood supply by fluid, e.g. a
nettle sting
Papillom
A projecting nipple-like mass, e.g. skin tag
Petechiae, Petechiae are pinhead-sized macules of extravascular blood in the dermis. They are flat.
purpura
Larger ones,
and
ecchymosi referred to as purpura, may be palpable. If bleeding involves deeper structures then it is
s
an ecchymosis ('bruise')
Burrow
A linear or curvilinear papule, caused by a burrowing scabies mite
Comedon
e
A plug of keratin and sebum wedged in a dilated pilosebaceous orifice
Telangiect The visible dilatation of small cutaneous blood vessels
asia
SECONDARY LESIONS (which evolve from primary lesions)
Scale
Crust
A flake arising from the stratum corneum, e.g. psoriasis Exudate of blood or serous
fluid, e.g. eczema or tissue fluid
Ulcer
An area of skin from which the whole of the epidermis and at least the upper part of
the dermis has been lost
Erosion
An area of skin denuded by complete or partial loss of the epidermis
Fissure
A slit-shaped deep ulcer, e.g. irritant dermatitis of the hands
Sinus
A cavity or channel that permits the escape of pus or fluid
Scar
The result of healing, in which normal structures are permanently replaced by fibrous
tissue, e.g. post-biopsy scar
Atrophy
Loss of substance due to diminution of the epidermis, dermis or subcutaneous fat, e.g.
atrophy due to excess topical corticosteroids
Stria
A streak-like, linear, atrophic, pink, purple or white lesion due to changes in the
connective tissue, e.g. Cushing's syndrome or pregnancy-induced
INVESTIGATIONS
Test
Use
Clinical example
Skin swabs
Bacterial culture
Impetigo
Blister fluid
Electron microscopy and viral
culture
Herpes simplex
Skin scrapes
Fungal culture
Tinea pedis
Microscopy
Scabies
Nail sampling
Fungal culture
Onychomycosis
Wood's light
Fungal fluorescence
Scalp ringworm
Erythrasma
Blood tests
Serology
Streptococcal cellulitis
Autoantibodies
Discoid lupus
erythematosus
HLA typing
Dermatitis herpetiformis
DNA analysis
Epidermolysis bullosa
Histology
General diagnosis
Immunohistochemistry
Cutaneous lymphoma
Immunofluorescence
Immunobullous disease
Culture
Mycobacteria/fungi
Skin biopsy
Patch tests
Allergic contact eczema
Hand eczema
Urine
Dipstick (glucose)
Diabetes
mellitus
Cytology (red cells)
Vasculitis
Assessment of pigmented
lesions
Malignancy
Dermatoscopy (direct microscopy of
skin)
DIASCOPY
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A glass slide is pressed firmly on the skin lesion. If a red lesion
blanches, it implies that the red colour is secondary to blood
within the vessels. By contrast, blood outside the vessels, such
as that from a bruise or from vasculitis, will not blanch.
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Success in blanching is a more useful physical sign than failure
to blanch.
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Granulomatous lesions a glass slide reveals an appearance
commonly referred to as 'apple jelly nodule'.
EPILUMINESCENCE MICROSCOPY
(DERMATOSCOPY, DERMOSCOPY)
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This refers to surface microscopy using an illuminated lens
with oil immersion directly on to the skin's surface. The
presence of oil reduces specular reflection and reduces 'errors'
due to the different refractive indexes of the various
superficial layers of skin.
WOOD'S LIGHT
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This involves irradiation with a UV light source that causes
normal skin, particularly dermis, to fluoresce (in the visible
light range).
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The basis for this is that in the ultraviolet A wavebands used
by Wood's light, pigmentation has a greater degree of
absorption than at longer wavebands, resulting in a greater
degree of difference in fluorescence between pigmented and
depigmented skin.
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Wood's light also enhances the examination of cutaneous
pigmentary abnormalities such as in patients with vitiligo,
where areas of subtle depigmentation are more easily seen.
MYCOLOGY SAMPLES
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Cutaneous scale, nail clippings and plucked hairs can be
examined by light microscopy when mounted in 20%
potassium hydroxide.
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The keratin is dissolved, allowing fungal hyphae to be
identified.
SWABS
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Bacterial swabs Bacterial swabs taken in an appropriate culture
medium are sometimes useful.
PRICK TESTS
 Prick tests are a way of detecting cutaneous type I
(immediate) hypersensitivity to various antigens such as
pollen, house dust mite or dander. The skin is pricked
with a dilution of the appropriate antigen solution.
After 10 minutes a positive response is indicated by a
weal and a flare. The weal is due to a local increase in
capillary permeability and the flare a result of activation
of the axon reflex.
 . In individuals with a clear history of particular type I
hypersensitivity a systemic reaction may follow a prick
test and resuscitation facilities should be available. As
an alternative, specific IgE levels to antigens can be
measured in serum by a specific radioallergosorbent test
(RAST).
PATCH TESTS
 Patch tests detect type IV (delayed or cell-mediated)
hypersensitivity. It is common practice for a 'battery' of
around 20 common antigens, including common
sensitisers such as nickel, rubber and fragrance mix, to
be applied to the skin of the back under aluminium
discs for 48 hours. The sites are then examined for a
positive reaction 24 hours later and possibly again a
further 24 hours later. An eczematous reaction, in the
absence of an irritant reaction, suggests a type IV
hypersensitivity to that particular allergen.
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A negative patch test does not exclude a pathogenic
role for a particular antigen nor does the presence of a
particular response to an antigen mean that this antigen
is causing the clinical disease.
HISTOLOGY
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Skin biopsies for routine histological examination are
usually fixed in 10% formalin and stained with
haematoxylin and eosin. Immunocytochemistry may
also be performed on formalin-fixed sections but may
require frozen sections . Immunocytochemistry is
particularly useful for tumour diagnosis.
IMMUNOFLUORESCENCE
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A portion of the skin biopsy can be frozen in liquid
nitrogen for direct immunofluorescence (IF). This
involves visualising antigens that are present in skin by
identifying them with fluorescein-labelled antibodies.
Similarly, indirect immunofluorescence can identify
circulating antibodies in the serum by an additional step
of adding the serum to a section of normal skin or
other substrate. Immunofluorescence plays a major role
in the diagnosis of the autoimmune bullous disorders.
ELECTRON MICROSCOPY
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This investigation has played an important role in the
diagnosis of some of the rare blistering disorders such as
epidermolysis bullosa, although the availability of a range of
antibodies to basement membrane zone antigens has in part
replaced it.
PHOTOTESTING
 Phototesting involves exposing skin (often on the back) to a
graded series of doses of ultraviolet radiation (UVR) of known
wavelength, either on one occasion or repeatedly.
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In many photodermatoses erythema will occur at a
lower dose of UVR than occurs in the normal
population (e.g. drug-induced photosensitivity), or the
time course of erythema may be prolonged (as in
xeroderma pigmentosum). Alternatively, UVR will
provoke lesions with the morphology of the underlying
photodermatosis, such as may occur in lupus
erythematosus or solar urticaria. Diagnostic
phototesting is an essential component of the
investigation of patients with presumed photosensitive
drug reactions and idiopathic photodermatoses such as
solar urticaria.