Structure and Function of the Skin Flashcards
what are the lines of normal skin development called
Blaschko’s lines
epidermis layer
- what does it arise from
stratified squamous that arises from dividing basal keratinocytes
varies at different body sites
rete ridges
downward projections of the epidermis into the dermis
name the layers of the epidermis
keratin
granular
prickle
basal
keratin layer of epidermis
- also called stratum corneum
- Composed of layers of flattened, scale-like cells. Thousands of these dead cells shed from the skin surface every day and are replaced by new ones from deeper layers.
- Corneocytes – terminally differentiated keratinocyte, are toughened and provide protection to the skin. Compose most of the stratum corneum
- contains filaggrin, involucrin and keratin (80% keratin and filaggrin)
where is teh keratin layer of epidermis thicker
areas of increased mechanical pressure eg sole of foot
what does the keratin layer of epidermis provide
a tight waterproof barrier
granular layer of epidermis
- Consists of 3/4 flattened layers of cells
- Keratohyalin granules that are the precursor to keratin, producing the keratin layer above.
- Odland bodies
prickle layer of epidermis
contains polyhedral cells with lots of desmosomes
basal layer of epidermis
- single layer of cells in conact with dermis
- cells in the epidermis proliferate from the basal layer - most metabolically active during foetal development
- contains melanocytes, merkel cells, keratinocytes and langerhans cells
keratinocytes
- can secrete a variety of cytokines in response to tissue injury or certain skin disease. play a role in immune function (innate and adaptive), cutaneous inflammation and tissue repair
- Produce keratin, which toughens and waterproofs the skin
what can keratinocytes be activated by
UV or sensitizers (eg allergic contact dermatitis)
which cell carry out vitamin D metabolism
kertatinocytes
what does HPV infection of keratinocytes cause
warts
melanocytes
dendritic cells formed in the basal layer and secrete the pigment melanin
where do melanocytes migrate from
neural crest to the epidermis in the first 3 months of foetal development
found above basal layer
what is the function of melanocytes
provide DNA protection from UV radiation
- convert tyrosine to melanin which absorbs light
- full melanosomes transferred to adjacent keratinocyte via dendrites and form protective cap over nucleus.
what are racial differences in skin pigmentation due to
- Found in equal numbers in black and white skin, but vary in melanin production
vitilgo
autoimmune disease with loss of melanocytes
nelson’s syndrome
excess of melanin stimulating hormone
merkel cells
found in the basal layer of the epidermis between keratinocytes and nerve fibres
act as mechanoreceptors - numerous on finger tips and oral cavity
basement membrane zone
- A structure consisting of collagen, hemidesmosomal proteins, integrins and laminin
- collectively, they hold the skin together, keeping the epidermis attached to the dermis
dermis
contains blood and lymphatic vessels, nerves, muscles, appendages and a variety of immune cells (mast cells and lymphocytes etc)
also contains, type I and III collagen, elastic fibres, ground substaces, fibroblasts
what cells are involved in collagen synthesis
fibroblasts
langerhans cells
- what are they characterised by
characterised by the Birbeck granule
why does skin sag as one gets older
collagen fibres are lost - they provided structure and tensile strength
dermographism
exaggerated wealing tendency when the skin is stroked
what are the 2 layers of the dermis
papillary - thin and lies just below epidermis, intertwines with rete ridges
reticular - thicker bundles of type I collagen, contains appendage structures
pacinian corpuscles
somatic sensory nerves that sense pressure
Meissners
somatic sensory nerves that sense vibration
what is a pilosebaceous unit
The structure consisting of hair, hair follicle, arrector pili muscles and sebaceous gland is an epidermal invagination known as a pilosebaceous unit.
what causes the pigmentation of hair follicles
melanocytes above the dermal papilla
what are the 3 types of hair
terminal (eg scalp, beard, pubic)
vellus (face of women, prepubertal children)
lanugo (anorexia nervosa and newborns)
what are the 3 stages of hair growth
anagen = growing
catagen = involuting
telogen= resting
what is the telogen phase of hair growth like in humans
asynchronous
sebaceous skin gland
responsible for secreting sebum or grease onto the skin surface (via the hair follicle) and are found in high numbers on scalp and face
hormone sensitive and inactive until puberty
control moisture loss and prevent from fungal infection (maintain skin barrier)
apocrine glands
scent glands found in the axilla and perineum
are androgen dependent - dont function until puberty
eccrine glands
sweat glands found on the whole skin surface, except mucosal surfaces
they are most common on the face, palms and soles of feet
A sebaceous
B apocrine
C eccrine
D = eccrine sweat gland, E = sebaceous gland, F = arrector pilli muscle, G = apocrine sweat gland
subcutaneous layer of the skin
consists predominantly of adipose tisuse as well as blood vessels and nerves
provides insulation and acts as a fat store
name some functions of the skin
metabolism and detoxification
thermoregulation
immune defence
communication
sensory
describe the skin’s role in vitamin D metabolism
UV exposure allows the conversion of 7 dehydro-cholesterol in the skin to vitamin D3 (cholecalciferol)
arthus reaction
localised type III hypersensitivity
skin testing in type III leads to arthus reaction - localised vasculitis as a result of immune complex deposition
compare the growth rate of nails to toe nails
nails - 0.1mm day
toe nails are slower
are nails or toe nails more commonly affected by disease
toes
describe nail anatomy
what are the physiological factors of skin that allow it to act as a barrier
low pH (5.5)
low oxygen tension
define skin failure
Skin failure is defined as loss of normal temperature control with inability to maintain the core body temperature, and failure to prevent percutaneous loss of fluid, electrolytes and protein, with resulting imbalance, and failure of the mechanical barrier to prevent penetration of foreign materials.
innate and adaptive immune system in skin
- The innate immune system of the skin comprises neutrophils and macrophages as well as keratinocyte-producing antimicrobial peptides. Expression of these peptides is both constitutive and induced by skin inflammation and they are active against infection and play a role in wound healing
- Adaptive immune response is slower in onset, results in immunological memory and is mediated by T and B lymphocytes
role of keratinocytes in immune protection
- secrete a variety of cytokines in response to tissue injury or certain skin disease
- produce AMPs which kill pathogens directly
role of Langerhans cells in immune protection
- Langerhans cells are a type of dendritic cells (antigen presenting) interspersed with keratinocytes in the basal layer of the epidermis.
- migrate throughtout the epidermis and dermis and travel via lympahtics to present antigens in lymph nodes - activate T cell response
where are Langerhans cells formed
bone marrow
which T cells are found where in the skin
- T cells (epidermis/dermis), mainly CD8+ T cells found in the epidermis and CD4+ and CD8+ found in the dermis.
T cell action
- CD4+ T helper cells
- TH1 activate macrophages to destroy micro-organisms. Their effector cytokines are IL-2 and IFNg
- TH2 activate B cells to produce plasma cells which produce antibodies. Their effector cytokines are IL-4, -5 and -6
- CD8+ T cells
- Can kill infected cells directly via apoptosis
where are T cells produced and where do they mature
produced in bone marrow and mature in thymus
which T cells are associated with psoriasis and atopic eczema
- CD4+ TH cells associated with inflammation include TH1 (psoriasis), TH2 (atopic dermatitis) & TH17 (psoriasis and atopic dermatitis).
what immune cells are found in the dermis
macrophages, neutrophils, mast cells, dermal dendritic cells
plasmacytoid DC
- DC that are found in inflamed skin, secrete IFN alpha
- Those who suffer from psoriasis typically exhibit skin lesions where pDCs accumulate
which chromosome do MHC molecules occur on
6
MHC class II cells
- Found only on antigen presenting cells e.g. phagocytes, macrophages, dendritic cells
- An APC takes up an antigen, processes it and retains a fraction called an epitope on the APC surface.
- This coupled within an MHC class II molecule mediates antigen presentation.
- Present to helper T cells – CD4 receptors
immunology of psoriasis
- The KC under stress releases factors that stimulate the pDC to produce IFN alpha, and release IL-1 beta, IL-6 and TNF
- Chemical signals activate the DC, which migrate to skin draining lymph node to present to and activate T cells: TH1 and TH17
- T cells are attracted to the dermis by chemokines and secrete IL-17 and 22. This stimulates KC proliferation, AMP release and neutrophil-attracting chemokines
- Dermal fibroblasts become involved
immunology of atopic eczema
- Impairment of skin barrier is a key factor:
- Mutations in the filaggrin gene is associated with severe/early onset disease
- Decreased AMP, may account for susceptibility to infection
- It allows access/sensitisation to allergen and promotes colonisation by micro organisms
- Immunological cells are found in lesions, probably attracted by the stressed keratinocytes
