Structure, function and disorders of the skin Flashcards
What are the main macroscopic variations in human skin?
Colour - ethnicity, site, UV
Hair- site, sex, age, ethnicity
Laxity/wrinkling - site, age/UV
What are some example skin conditions that have a psychosocial impact?
Vitiligo - autoimmune depigmentation
= if it affects visible areas in dark-skinned races its much more noticeable than in whites
Alopecia areata - autoimmune hair loss= effects scalp, especially in women
What are UV induced abnormalities?
Sunburn causes aging of the skin in white people but doesn’t occur in black people
- skin cancer: basal cell carcinoma, malignant melanoma
How does UV light cause skin ageing?
UV induces damage to dermal collagen and elastin causing “photo ageing”
What is the epidermis of the skin made up of ?
stratified squamous keratinised epithelium, mainly made up of keratinocytes and their products
What are the layers of the epidermis?
Stratum corneum (horny layer) Stratum lucideum (only in hard skin) Stratum granulosum (granular layer) Stratum spinousum (prickle cell layer) Stratum basale (basal layer)
Which layer of the skin does keratinocyte mitosis occur in mainly?
basal layer
What happens to the daughter cells produced in the basal layer?
Move upwards to form the prickle cell layer, where terminal differentiation begins and keratinocytes lose ability to divide
What do keratinocytes produce?
keratins (heterodimeric fibrous proteins) which contribute to the strength of the epidermis
Keratins = main constituent of hair and nails
Why is the stratum spinsosum called the “prickle cell layer”?
prickle cells joined by prickle like desmosomes (intercellular junctions)
What happens to keratinocytes in the stratum granulosum?
abrupt changes
- lose their plasma membrane
- begin differentiating into corneocytes= main cells of stratum corner
What is present in the granular layer?
Keratohyalin granules = aggregations of
- keratins
- fibrous proteins (filaggrin, involucrin)
- enzymes (which degrade plasma membrane; cross link proteins = fillaggrin and involucrin)
What are the roles of filaggrin and involucrin?
Filaggrin: aggregates keratins
Involucrin: forms a major part of corneocyte envelope
What is the stratum corneum made up of?
layers of flattened corneocytes - skin barrier function
What is the transit time of a keratinocytes from basal layer to horny layer?
30-40 days
What are melanocytes and where are they found?
Dendritic cells of neural crest origin
Occur at intervals along basal layer
Produce melanin
In black or tanned skin, they produce more melanin but there are not increased numbers of them
What are langerhan cells and where are they found?
Dendritic cells of bone marrow origin
Scattered throughout prickle cell layer
Specialised at presenting antigens to T lymphocytes
Mediate immune reactions - e.g. allergic contact dermatitis
What is psoriasis?
abnormal epidermal growth and differentiation
How common is psoriasis and what causes it?
2% of the population
cause is unknown but runs in families and thus is influenced by genetic factors
What happens in psoriasis ?
Extreme proliferation of the epidermal basal layer, causing gross thickening of the prickle cell layer and production of excessive stratum corner cells - clinically manifesting as excessive scaling
What is allergic contact dermatitis?
mediated by langerhan cells - these cells are responsible for presentation of antigens to T lymphocytes and for cutaneous delayed type hypersensitivity reactions
What can cause allergic contact dermatitis?
Nickel (10% of women) - earrings
hair spray
What is malignant melanoma?
malignant growth of melanocytes
- most common primary site is in the skin
- retention of tumour cells above the epidermal basement membrane is associated with a good prognosis
- more penetrating nodular melanomas may have a more poor prognosis
What is vitiligo?
autoimmune destruction of melanocytes - immune system attacks melanocytes, usually in symmetrical localised areas of skin, causing well-demarcated pigmentation
- much more visible in dark skin
- no clear explanation for the curious symmetrical nature of vitiligo
- it can sometimes affect the whole body
What happens to melanin?
it is produced in the melanocytes and fed out to the surrounding keratinocytes through dendrites
What happens to melanocytes in hair follicles as we age?
they stop functioning and this causes greying (but not in the epidermis)
Where is the epidermal basement membrane ?
at the derma-epidermal junction below the basal layer = best seen with PAS stain
What are the characteristics of the dermis?
tough, fibrous and vascular layer - main component of leather
What makes up the main part of the dermis and what is it made up of?
extracellular matrix - includes collagens (esp. type 1 collagen), elastin and other ECM components
What synthesises the ECM?
Fibroblasts - spindle shaped cells scattered throughout the dermis
Other than the ECM, what other dermal components are there?
Blood vessels
Lymphatic vessels
Mast cells
Nerves
What is “solar elastosis”?
damage to collagen and elastin due to excessive UV exposure
What is another name for stretch marks?
striae
What are keloids?
Traditional skin markings
- Scars following wounding and comprise mainly of collagen synthesised by fibroblasts
Where are the blood vessels located in the skin?
Smaller blood vessels are found in the superficial dermis (mainly capillaries, small venues and arterioles)
Interconnecting vessels
Larger blood vessels in the deeper dermis
What is a port wine stain?
it is a birthmark = congenital malformation of dermal blood vessels
Where are mast cells located ?
they are distributed around dermal blood vessels and contain histamine
What happens when mast cells are activated?
During a type 1 immediate hypersensitivity reaction, histamine is released causing increased vascular permeability and leakage of plasma into extravascular sites = local oedema
- in the skin = urticaria and angio-oedema
What are the skin appendages?
hair follicles Sebaceous glands - hair follicles + sebaceous glands = pilosebaceous unit sweat glands = eccrine and apocrine nails
What happens if the sebaceous duct can no longer communicate with the hair follicle?
if it can’t communicate any longer because it becomes obstructed acne can occur - this causes holocrine secretion
What causes acne?
Skin disease affecting the sebaceous glands
- abnormal differentiation of sebaceous gland ducts which become obstructured
- infection with normally harmless skin bacteria
- changes in these glands occur at puberty
What is hyperhidrosis?
increased sweating cause is usually unknown - may affect only palms and soles
- embarrassing wet hands, - wet, malodorous feet
What are apocrine sweat glands?
large sweat glands most abundant in axillae, genital and submammary areas
- no function of value
- produce odourless, protein rich, apocrine secretion
- digestion of this by cutaneous microbes produces body odour
What are the main functions of the skin?
- barrier function
- sensation
- thermoregulation
- psychosexual communication
How does the skin act as a barrier?
Outer epidermis forms a major barrier preventing percutaneous absorption of exogenous substances
- studied a lot as it has to be overcome during percutaneous drug delivery
- can be seriously disrupted in many diseases = e.g. psoriasis
What does poor barrier function lead to?
- loss of fluid
- loss of protein
- loss of other nutrients
- loss of heat
Could lead to excessive absorption of potentially harmful exogenous agents
What are some disorders of skins sensory function?
- leprosy
- diabetic sensory neuropathy
How is the skin involved in thermoregulation?
vascular thermoregulation and thermoregulatory eccrine sweating are both critically important in the maintenance of body temp
- failure of either may have serious consequences
What is involved in vascular thermoregulation?
- dilation of skin blood vessels = heat loss
- constriction of skin blood vessels = pallor and heat conservation
What happens if vascular thermoregulation fails?
widespread vasodilation or erythrodermic psoriasis and inability to vasoconstrictor in a cold environment = heat loss - patients are often shivery and may become hypothermic
What is thermoregulatory eccrine sweating?
evaporation of eccrine sweat causes cooling - critically important in the maintenance of body temp
What is psychosexual communication?
skin itself and its appendages are manipulated in many ways as a means of communication and expression
