Structure (& Function) of the Skin Flashcards
Function
protection, regulation and sensation
The integumentary system
Skin, cutaneous glands, fingernail, hair
what is the 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)
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
Small muscles (arrector pili) contract when
you experience physical/ emotional changes to give you goosebumps
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
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-
connection between the two allows cells and bioactive molecules to move between the two regions
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)
corgis lick grandpas saggy bums - make up the epidermis
stratified corneum, Stratum lucidum, Stratum lucidum, Stratum granulosum, Stratum spinosum, Stratum basale
Dermis Two parts
papillary (20% of dermal thickness) and reticular (80% of dermal thickness)
Dermis- Papillary
mostly loose connective tissue some elastic fibres, capillary loops and nerves
dermis-Reticular
dense irregular connective tissue containing collagen fibres, elastin, proteoglycans and glycosaminoglycans
Fibroblasts and keratinocytes
principal cell of dermis. Production of collagen, elastic and reticular fibres and extracellular matrix
Cells of the dermis
Adipocytes
Cells of the dermis
fat cells (insulation, energy storage and wound healing)
Mast cells
Cells of the dermis
inflammatory cells that help with collagen remodelling and wound healing
Myofibroblasts
Cells of the dermis
rich in smooth muscle actin and myosin (contractile cells)
Myoepithelial cells
Cells of the dermis
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
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).
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)
Desmosomes
anchors (cells to each other and to basement membrane
Multiprotein complexes important in adhesion and signalling
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)
Lamellar granules-Also known as: lamellar bodies, membrane-coating granules, keratinosomes, Odland bodies
help form lipid protective barrier
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
Hemi-desmosomes
anchors (cells to each other and to basement membrane
Multiprotein complexes important in adhesion and signalling
connect keratinocytes to the basement membrane via integrin receptors and also contribute to strength
Loss can again lead to skin blistering (tissue separation)
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
Melanocytes
Produce melanin (skin colour) and protects against UV radiation while supporting the immune system.
Found in bottom layer of epidermis.
transfer melanosomes to keratinocytes (through dendrites)
Langerhans cells
also known as dendritic cells and contain Birbeck granules (rod shaped with function not clear).
Found throughout epidermis and support immune system.
Merkel cells
oval shaped and responsible
for light touch and sensation through their connection to nerve fibres
Melanosomes
formed in melanocytes and contain melanin
Epidermal melanin unit
anatomical relationship between melanocytes and keratinocytes (each melanocyte is in contact with approx. 40 keratinocytes)
Hyperpigmentation- causes
increased melanin production or increased number of melanocytes (less common as melanocyte number is approx. similar in all individuals)
Hyperpigmentation-epidermal
too much melanin or too many melanosomes transferred
Hyperpigmentation- dermal
melanin crosses damaged
basement membrane. Deeper and harder to treat
Hyperpigmentation-Treatment
varied from photoprotection to prevent new spots to inhibitors of melanin production
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
Langerhans cells and immunity
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
Langerhans cell 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
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 cell: ‘hair matrix’ which form the hair)
4 different hair growing stages
anagen: growing phase
catagen: transition phase
telegen: resting phase
exogen: shedding phase
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 (women can be carriers!)
Nails - anatomical
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
nail 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)
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
Subcategories of exocrine glands; Secretion method
Merocrine- most common. Excretion via exocytosis
Apocrine- cell membrane buds off into the duct
Holocrine- cell membrane ruptures to release the product
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
Oil-producing glands
release sebum into follicular duct
sebum
Sebum coats, moisturises and protects skin
Several types of lipids.
57.5% triglycerides and fatty acids
26% wax esters
12% squalene
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 birth2 (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
Mechanoreceptors; sensory receptors
Respond to physical changes
Hair: light touch
Meissner’s corpuscle:
indentation and slipping
Pacinian corpuscle:
vibrations
Merkel’s disk: texture
Ruffini corpuscle: stretch
Thermoreceptors : sensory receptors
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 Some proteins involved are known (TRPV1, TRPM3, ANO1 etc.) but largely mechanisms are unknown
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
