Structure & Function of Skin Flashcards
skin layers
epidermis
dermis
epidermis
outer layer, stratified squamous epithelium (skin proper)
demis
beneath epidermis, connective tissue
which granular layer does epidermis come from & how?
ectoderm cells form single layer periderm, gradual increase in layers of cells & periderm cells are cast off
which granular layer does dermis come from?
formed from mesoderm below ectoderm
where do melanocytes come from and what is their function?
pigment producing cells from neural crest
if there is a defect in one stem cell, how will the lesion appear?
it will follow Blaschko’s lines & will be asymmetrical
what does the skin consist of?
epidermis, appendages, derma-epidermal junction, dermis & sub-cutis
which cells make up the epidermis?
mostly keratinocytes
melanocytes
Langerhans cells
Merkel cells
how many layers does the epidermis have?
4 defined layers: keratin layer granular layer prickle cell layer basal layer
how do the epidermal stem cells divide?
divide at bottom & migrate to the top
palms & soles skin
thick coat of keratin over normal layer
abdominal skin
very thin epidermis, glands & hair follicles more pronounced
what is epidermal turnover?
how fast keratinocytes travel upwards
how is epidermal controlled?
growth factors
cell death
hormones
what happens if control of epidermal turnover is lost?
skin cancer
psoriasis
how long does migration of keratinocytes usually take?
28 days from bottom to top
how long does migration of keratinocytes take in psoriasis?
45 days
describe the basal layer
usually one small cuboidal cell thick, lots of intermediate filaments (keratin), highly metabolically active
describe the prickle cell layer
larger polyhedral cells, lots of desmosomes which intermediate filaments connect to
how does intra-epidermal blistering occur?
a build up of hydrostatic pressure which busts apart the relatively delicate cells (desmosomes detach) within the epidermis, more water gets pumped in & a fragile roof is formed on top which will eventually pop with any change of pressure. e.g. baby nappy rash
describe the granular layer?
very thin layer of 2-3 flattened cells, large keratohyalin granules contain flaggrin & involcurin proteins, has a high lipid content & Odland bodies. no cell nuclei.
where do cells die?
the granular layer where they spit out material which will be used to seal off & prevent water loss
the main component of this material is flaggrin
what is missing from the skin in eczema?
flaggrin
describe the keratin layer?
final top layer. Has an insoluble cornified envelope. mostly keratin and some filaggrin. lamellar granules release lipids. creates a tight waterproof barrier.
what are corneocytes?
overlapping non-nucleated cell remnants
describe the oral mucosa
masticatory - keratinised to deal with friction and pressure
lining mucosa - non-keratinised
specialied mucosa (tongue papillae) - taste
describe the ocular mucosa
has lacrimal glands & sebaceous glands and eye lashes for specialist functions
white lesions in skin (nail beds & mucosal membranes) is what?
thickening of normally thin epidermis so that you can’t see blood vessels anymore
what’s the difference between skin & scar tissue?
scar tissue has no appendages (hair follicles/nails)
is the dermis or epidermis thicker?
dermis is thicker
what are the majority of epidermal cells?
keratinocytes
what are the other epidermal cells?
melanocytes
Langerhans cells
merkel cells
which epidermal layer are melanocytes found in?
basal & suprabasal
which epidermal layer are Langerhans cells found in?
suprabasal
which epidermal layers are Merkel cells found in?
basal
when do melanocytes migrate & where during foetal development?
in the first 3 months
from the epidermis to neural crest
what are melanocytes?
pigment producing dendritic cells
how do melanocytes produce pigment?
convert tyrosine to melanin pigment;
- eumelanin (brown or black)
- phaeomelanin (red or yellow)
what are melanin organelles/granules called?
melanosomes
Full melanocytes are transferred where and how from epidermal cells?
to adjacent keratinocytes via dendrites
what do melanocytes form a protective cap over?
the nucleus
what cell type are melanocytes?
neuronal as they are evolved from the neural crest.
similar properties to astrocytes
what is vitiligo?
an autoimmune disease with loss of melanocytes, can be seen in both black/white people but more obvious in black people.
what is albinism?
the partial loss of pigment production
what is Nelson’s syndrome?
a melanin stimulating hormone is produce in excess by the pituitary causing an overproduction of eumelanin.
where do Langerhans cells originate from?
mesenchymal origin - bone marrow
where are langerhans cells found?
strategically dispersed in prickle cell level in epidermis. Also found in dermis & lymph nodes.
what do Langerhans cells do?
involved in skin immune system;
- Ag presenting cells
- pick up Ag in skin & circulate to lymph nodes via lymphatics.
which granules are present in Langerhans cells & why?
birbeck granules but we don’t know why
what are Merkel cells for?
responsible for sense of touch - mechanoreceptors
where are Merkel cells found?
in the basal layer between keritanocytes & nerve fibres
where do Merkel cells synapse?
directly with free nerve endings
what part of the free nerve endings does diabetes affect?
diabetes affects the myelin sheath which causes reduced/no/funny feeling.
is Merkel cell carcinoma more common the melanomas?
no but is more deadly than melanoma.
what is a pilosebaceous unit?
a hair follicle;
- epidermal component + dermal papilla
- specialised keratins
- adjacent sebaceous gland
how is hair pigmented?
via melanocytes above the dermal papilla of a pilosebaceous unit which melanise the matrix of a hair
name some functions of a pilosebaceous unit
- sensory
- reduces need for melanocytes
- is the origin of the gland which supplies moisture to the skin surface
where are the stem cells of a pilosebaceous unit?
in the dermal root sheath
what are the phases of growth of hair?
anagen, catagen & telogen
what happens during anagen?
hair grows
over 1-8 years
what happens during catagen?
the hair involutes
what happens during telogen?
shedding phase
what can affect the speed of hair growth?
hormones but hair will usually go through phases like clockwork
how is a hair shed?
through dermal papilla dissociation which always takes about 4 weeks
what is virilisation?
excessive & rapid hair growth
e.g. due to excess androgen from a tumour
what is alopecia areata?
autoimmune hair loss
normal hair -> grey hair -> hair loss
what are nails?
specialised keratins
which part of a nail is a hair bulb similar to?
nail matrix/root
where is the nail matrix found?
just below the skin surface proximal to the cuticle
what can a crumbly nail be due to?
- fungus
- repeated small traumas
what is the cannula of a nail?
the thick layer which creates a white semi circle at the base of the nail
what is the derma-epidermal junction?
the interface between epidermis & dermis, complicated structure
what does the DEJ have a key role in?
epithelial-mesenchymal interactions:
- support, anchorage, adhesion, growth & differentiation of epidermal cells
- semi-permeable membrane acts as a filter & barrier
where does the epidermis get it’s blood supply?
the dermis
also brings nutrients & takes away waste
name the two layers of the DEJ
lamina densa & lamina lucida
where are hemidsemosomes found?
in basal cells of epidermis
what are hemidesmosomes?
filaments that attach to proteins in the lamina lucida
what happens in an inherited disease of the DEJ?
a mutation in one of the proteins in the DEJ causes skin fragility
what happens in an acquired disease of the DEJ?
auto-antibodies against proteins in the DEJ are formed
describe epidermolysis bullosa simplex
caused by inherited defect of DEJ
lots of blistering, very fragile skin, mum can’t hold it properly, tearing of skin, can become severe enough to cause webbing of toes & fingers
describe bullous pemphigoid
can come on at any point in life (acquired)
defect in skin tethering, not stuck together very well causing blistering & a severe itch
controlled by steroids
What are the components of the Dermis?
- cells
- fibres
- ground substance
blood vessels, lymphatics, nerves
which cells are found in the dermis?
mainly fibroblasts macrophages mast cells lymphocytes Langerhans cells
which fibres are found in the dermis?
collagen - more common
elastin - less common but very important
what is ground substance?
a jelly like mass of mucopolysaccharides & glycosaminoglycans
what do fibroblasts do?
make collagen;
pro-collagen is made first which is cleaved by enzymes to make collagen
what does collagen do?
becomes fixed in the dermal layer & gives strength to it
what does elastin do?
gives the skin elasticity
what do macrophages do?
they are immune scavengers & Ag presenters
what do Langerhans cells of?
Ag presentation
what are mast cells?
chemical messengers
what happens to skin in ageing?
- elastin fibres become less well formed
- collagen gets depleted
- fibroblasts & DEJ become much flatter
- there are less nutrients being delivered to the epidermis
what makes ageing worse?
UV light & smoking
how is blood transported to the skin?
arteriole-> precapillary sphincters -> arterial capillaries -> venous capillaries -> post capillary venules -> collecting venules
(more blood delivered than metabolic need)
why is the surface of the epidermis undulating?
for a greater surface area
what does a localised overgrowth of blood vessels cause?
port wine stain, “stork marks”
often on face
can be liked to epilepsy
pretty harmless
how is lymph drained from the skin?
through sub-epidermal meshed networks from smaller non-contractile vessels to larger contractile lymphatic trunks
what are the important immune functions of the lymphatics?
- immune surveillance by circulating lymphocytes and Langerhans cells
- channelling of micro-organisms / toxins
how do lymphatics run?
they mirror the blood supply
how is chronic lymphoedema caused & treated?
caused by a variety of things e.g. blockage/lack of lymph nodes or infection damaging lymph vessels
squeeze leg with bandages but skin is irreversibly damaged for life
which somatic sensory nerves are found in the skin?
- free nerve endings
- special receptors: pacinian (pressure) & meissners (vibration) copuscles
describe messiness’ corpuscles
sense light touch
found in fingertips & lips
what do free nerve endings detect?
pain
what do motor nerves do in the skin?
innervate sweat glands
describe pacinian corpuscles
onion shaped nerve ends that detect deep pressure
what is neurofibromatosis?
overgrowth of nerve ending which are abnormally thickened
benign growths unless affecting brain or spine
name the 3 types of skin glands
eccrine
sebaceous
apocrine
when are eccrine glands stimulated & where are they?
stimulated when the body is working hard
found over the whole skin surface, particularly on palms, soles & axillae
where are sebaceous glands usually found?
often found linked to hair follicles, widely distributed across body mostly on face & chest
are hormone sensitive
what does the over stimulation of a sebaceous gland cause?
acne
where are apocrine glands found & what do they do?
found in the perineum & axillae
secrete odours containing pheremones
what do sebaceous glands produce?
sebum: squalene, wax esters, triglycerides & free fatty acids
what are the functions of a sebaceous gland?
control moisture loss & protect against bacterial & fungal infection
what can hair follicles do if the skin is damaged?
they can repopulate the top part of the skin using the epithelium in the hair follicle
how do apocrine sweat glands develop?
as part of the pilosebaceous unit
how are apocrine glands stimulated?
by androgen
how are eccrine glands controlled?
by sympathetic cholinergic nerve supply in response to mental, thermal & gustatory stimulation
what functions do eccrine glands have?
can perform ultrafiltration
cooling by evaporation
moisten palms & soles to aid grip
where do eccrine glands secrete to?
directly onto skin surface
name some of the functions of the skin (at least 3)
- barrier function
- metabolism & detoxification
- thermoregulation
- immune defence
- social communication
- sensory functions
give an example of acute skin failure
toxic epidermal necrolysis
what can cause erythoderma?
dermatitis
psoriasis
drug reaction
T cell problmes
what is erythoderma?
inflammatory skin disease with erythema and scaling that affects nearly the entire cutaneous surface. causes a loss of water from the skin.
what does skin failure do in terms of the barrier function?
fluid loss -> dehydration
protein loss -> hypoalbuminaemia
infection
what does skin failure do in terms of thermoregulation?
heat loss -> hypothermia
what does skin failure do in terms of immune defence function?
spread of infection
what does skin failure do in terms of the metabolic function?
disordered thyroxine metabolism
what does skin failure do in terms of the communication function?
inability to display healthy skin -> stigma
what does skin failure do in terms of the sensation function?
pain sensation working -> pain
what is the skin a barrier to?
physical factors: friction, mechanical trauma UV radiation
chemical factors: irritants, allergens, toxins
pathogens: bacteria, viruses, fungi
name the 4 stratum layers of the skin, top to bottom
stratum corneum
stratum granulosum
stratym spinosum
stratum basale
how does our skin protect us from UV radiation?
melanin pigment formed in basal cells absorbs UV rays to protect DNA in the cell’s nuclei
how is the skin metabolically active?
Vit. D metabolism
Thyroid hormone metabolism
what is the detoxification function of the skin?
defence against chemicals, drugs, pollutants & sunlight
where is Vit. D3 stored?
as hydroxycholecalciferol in the liver
what is hydroxycholcalciferolconverted to?
1,25-dihydroxycholecaliferol in the kidney
how is Vit. D metabolised?
UV light stimulates the conversion of cholecalciferol to Vit. D3
how is thyroid hormone metabolised?
thyroxine to triiodothyronine
where does thyroid hormone metabolism occur?
mostly in the peripheral to the thyroid tissues including skin
some occurs in thyroid glands
how is temperature sensed?
through thermoreceptors
describe how the superficial & deep plexuses of blood vessels help with temperature control
depending on how much going through each decides how much heat you lose from the skin
e.g. central body temp too high, more blood flow through superficial plexus
what is eczema herpeticum?
disseminated herpes simplex virus infection
what are the sensory functions of the skin?
touch, pressure, vibration, pain, itch, heat, cold
