Structure (& Function) of the Skin Flashcards
The integumentary system
The barrier between the internal organs and the outside environment (irritants, pollutants and pathogens)
Not just the skin but also hair, nails, glands and sensory receptors (the accessory structures of skin)
Primary organ: skin (>2 m2 of the body surface area, >4.5 mm thick and 4.5-5 kg
Function: protection, regulation and sensation
Skin: largest and fastest growing organ in body Cutaneous glands: sebaceous (sebum) and sweat glands Fingernails: densely packed cells covered in keratin Hair: pigmented filament (mostly keratinised cells)
Skin types
Two types of skin on the body: hairy and glabrous (non-hairy)
Hairy skin: covering more than 90% of the body. Hair contribute to tactile sensations, forms part of our social interaction and can detect foreign objects Glabrous skin: mainly on palms and soles of feet. Specialised nerves to detect tactile details. Thicker than hairy skin.
Skin function
Protection (against external environment including invasion from bacteria)
Thermoregulation
Humidity control
Produces vitamin D
Viruses, bacteria and fungi live on your skin (ecosystem)! Demodex
Epidermis
Stratified epithelium (usually thinner than dermis)
Outer layer of skin with no blood vessel supply (squamous epithelium receives blood by diffusion)
Layers (strata) of keratinocytes. Continuous cycle of cell division and death
Also contains melanocytes, Merkel cells and Langerhans cells
30-45 days for basal cells to mature and migrate to top of epidermis
Production of keratin
Basement membrane separates the epidermis and dermis
Epidermis- layers
Stratum corneum: flat dead keratinocytes and cross-linked keratin fibres (squamous cells) protect body from water loss. Often described as ‘bricks’ (corneocytes) and ‘mortar’ (intercellular lipids). Loss of cells from this layer (desquamation) causes barrier to become weak (entry point for irritants)
Stratum lucidum: rows of dead keratinocytes only in glabrous skin. Lipid-rich eleidin keeps out water
Stratum granulosum: mostly mature keratinocytes migrating from spinosum, keratohyalin granules and lamellar bodies (also to waterproof skin)
Stratum spinosum: mature keratinocytes linked by desmosomes producing keratin. Also contains Langerhans cells (protection form microbes)
Stratum basale: deepest layer. Basal cells push older cells
up. New keratinocytes made.
Also contains melanocytes (skin pigment)
Dermis
Two parts: papillary (20% of dermal thickness) and reticular (80% of dermal thickness)
Papillary: mostly loose connective tissue some elastic fibres, capillary loops and nerves Reticular: dense irregular connective tissue containing collagen fibres, elastin, proteoglycans and glycosaminoglycans Basement membrane separates the epidermis and dermis: connection between the two allows cells and bioactive molecules to move between the two regions
Cells of the dermis
Fibroblasts and keratinocytes: principal cell of dermis. Production of collagen, elastic and reticular fibres and extracellular matrix
Macrophages: assist the immune system Adipocytes: fat cells (insulation, energy storage and wound healing) Mast cells: inflammatory cells that help with collagen remodelling and wound healing Myofibroblasts: rich in smooth muscle actin and myosin (contractile cells) Myoepithelial cells: cells in sweat glands
Keratinocytes
Make up more than 90% of the cells in the epidermis
Essential for stratification and forming protective barrier
Changes in structure and function as you move towards outer layer
Keratins: 80% of internal cell structure of differentiated keratinocytes Desmosomes and hemi-desmosomes: anchors (cells to each other and to basement membrane Lamellar granules: help form lipid protective barrier
Desmosomes and hemi-desmosomes
Multiprotein complexes important in adhesion and signalling
Desmosomes: connect cells to each other (junctions) via cadherin proteins. Also provide strength as they link intracellularly (through the filaments in the cytoskeleton) Loss of connection can cause significant issues e.g., skin blistering (pemphigus- an autoimmune disease) Hemi-desmosomes: connect keratinocytes to the basement membrane via integrin receptors and also contribute to strength Loss can again lead to skin blistering (tissue separation)
Skin toughness
Why does our skin not constantly rupture from the mechanical forces of the outside world?
Keratinocytes >70 times stiffer than other cell types (even after they die!) Due to their cytoskeleton and keratin filaments
Keratin
Keratin bundles are major component of the cytoskeleton of the keratinocytes
Network of fibre bundles extending from periphery to nucleus Bundles composed of keratin subunits wrapped around each other (coil). Think of rope! Different types of keratin expressed Contribute to structural integrity of epidermis (dissipate force) Few drugs can disrupt keratin intermediate filaments (though mutations in the keratin genes can lead to weak filaments)
Lamellar granules
Also known as: lamellar bodies, membrane-coating granules, keratinosomes, Odland bodies
Produced by keratinocytes (also found in lungs)
Round/ oblong in shape (though some say they’re branched) approx. 300-400nm width and 100-150 nm length
Secretory organelles: contain glucosylceramides, lipids, enzymes and other proteins. These are required for desquamation
(skin shedding)
Skin function: help form the impermeable
lipid-containing barrier
Abnormalities can cause eczema
and other skin conditions
Melanocytes- Other cells of the epidermis
Produce melanin (skin colour) and protects against UV radiation while supporting the immune system.
Found in bottom layer of epidermis.
Melanosomes formed in melanocytes and contain melanin
Melanocytes transfer melanosomes to keratinocytes (through dendrites)
Melanosomes form melanin caps reducing UV damage
Epidermal melanin unit: anatomical relationship between melanocytes and keratinocytes (each melanocyte is in contact with approx. 40 keratinocytes)
Other cells of the epidermis- Langerhans cells
also known as dendritic
cells and contain Birbeck granules
(rod shaped with function not clear).
Found throughout epidermis and
support immune system.
Dendritic cells: ‘sense’ by extending dendrites through intercellular tight junctions towards stratum corneum
Able to interpret the external environmental threats (inflammation or tolerance)
Non-dangerous: coordinate immune tolerance (no immune activation)
Dangerous: work with keratinocytes to instruct T lymphocytes to mount a response
As they interact with keratinocytes to ‘activate’, any damage to keratinocytes could affect immune response
histiocytosis
Buildup of excess immature Langerhans cells form granulomas (tumors) in skin and other organs. Stimulus for excess proliferation unknown
Cancer? Labelling controversial
Rare (1 in 5 million)
Can severely affect the immune system (can involve multiple organs and can be fatal)
Treatments: depends on severity from topical to chemotherapy and surgery
Other cells of the epidermis- merkel cells
oval shaped and responsible
for light touch and sensation through their
connection to nerve fibres
Merkel cells: allodynia
Hypersensitivity to touch which can manifest as pain
A type of neuropathic pain
Everyday tasks become difficult, e.g., combing hair, putting on a t-shirt, even temperature changes
Causes: many e.g., diabetes, fibromyalgia, migraines
Hyperpigmentation
Discoloured or darkened skin
Causes: increased melanin production or increased number of melanocytes (less common as melanocyte number is approx. similar in all individuals) Epidermal: too much melanin or too many melanosomes transferred Dermal: melanin crosses damaged basement membrane. Deeper and harder to treat Treatment: varied from photoprotection to prevent new spots to inhibitors of melanin production
Hair
Part of the integumentary system
Hair grows in the dermis from the hair follicle (from downward extension of epidermis)
Growth of hair is like skin: keratinocytes get pushed up, become flattened, dead, keratinised cells
Shaft: Hair exposed on skin surface
Hair root: Rest of the follicle
Hair bulb: Bulge at the base of the hair root
(contains a layer of basal cells:
‘hair matrix’ which form the hair)
Function: protect from sun, prevent heat loss, trap particulates (hair in nose and ears), prevent sweat from running into eyes (eyebrows)
Much more sensitive than skin to changes in environment (see arrector pili muscle)
Hair grows at approx. 0.3 mm a day
Approx. 50 hairs lost per day
Like skin melanin gives it colour (melanin production decreases with age: grey hair)
Alopecia and baldness: genetic sensitivity to androgen hormone dihydrotestosterone (DHT): less blood flow at hair follicle Male pattern baldness: mutation on X chromosome
Nails
Part of the integumentary system
Nail body formed on the nail bed (rich in blood vessels making the nail appear pink)
Lunula: Crescent shape at the base of the nail
Nail folds anchor the nail to the body
Keratin-rich and densely packed dead keratinocytes
Function: protective as they are the furthest extremities that experience the maximum mechanical stress
Infections and trauma to nail easily seen Changes in shape could be a symptom of something else (e.g., iron deficiency)
Glands
Exocrine glands: excrete substances via a duct
Glands of the integumentary system: sudoriferous glands, sebaceous glands, ceruminous glands, mammary glands (all exocrine)
Subcategories of exocrine glands; Structure: Simple- a single duct Compound- multiple, branched ducts Secretion method: Merocrine- most common. Excretion via exocytosis Apocrine- cell membrane buds off into the duct Holocrine- cell membrane ruptures to release the product
Sudoriferous glands
Sweat glands: eccrine (open directly onto surface of skin) and apocrine (opens into hair follicles)
Eccrine glands cover most of the body. Opens to epidermal surface Starts at birth Merocrine excretion Thermoregulatory function and ion excretion 500-750mL water lost per day Apocrine glands mostly where hair follicles are abundant (scalp, beard, armpits, groin) Active from puberty (linked to emotions: pheromone excretion) Viscous and protein rich
Sebaceous glands
Oil-producing glands: release sebum into follicular duct
Sebum coats, moisturises and protects skin Several types of lipids. 57.5% triglycerides and fatty acids 26% wax esters 12% squalene Holocrine gland: contains parts of ruptured cell Present at birth but highly active at puberty (excess sebum can cause acne) Not present on palms and soles of the feet
Ceruminous and mammary glands
Ceruminous glands;
Modified apocrine glands: found in the external auditory canal
Together with sebaceous glands, produce cerumen (ear wax). Important protective
role but also trap and take particulates away for tympanic membrane (eardrum)
1000-2000 ceruminous glands in a normal ear
Mammary glands; Type of apocrine gland Production of milk after birth 2 (in both men and women) on either side of the front of the chest wall Semicircular when young but begin to lose shape Each weighs 500-1000g
Sensory receptors in the skin
Important to note we are focusing only on skin sensory receptors (other do exist in other parts of the body)
Sensory receptors are in all layers of skin Mechanoreceptors: physical change Thermoreceptors: temperature change Nociceptors: pain Mechanoreceptors Respond to physical changes Hair: light touch Meissner’s corpuscle: indentation and slipping Pacinian corpuscle: vibrations Merkel’s disk: texture Ruffini corpuscle: stretch
Thermoreceptors
Body has both cold receptors (sensing 25-30°C) and warm receptors (sensing 30-46°C)
When changes in temperature occur, the opposite receptor stops discharging
Nociceptors
Nociceptors
Signal pain related to pressure, temperature and chemicals
The receptors only signal when tissue damage occurs- inflammatory markers caused by tissue damage binding to the receptors are the trigger TRP (transient receptor potential) ion channels: one of the ion channel families present on nociceptor neurons A-delta fibres: relay pain information related to painful temperatures, uncomfortable pressures, and chemicals C fibres: relay pain information related to thermal and mechanosensitive pain
