Skin Structure And Function Flashcards
What are the 3 main layers of the skin
Epidermis
Dermis and
Subcutis/hypodermis
Briefly explain the epidermis ?
Self regenerating
Stratified squamous epithelium
Produces non living surface layer of the protein keratin
Direct contact with external environment
Is the epidermis water resistant ?
Yes
The main cells involved in the epidermis are called what ?
Keratinocytes
What are the different layers of the epidermis starting from deepest
Stratum basale -> stratum Spinosum -> stratum granulosum -> stratum Corneum
Explain thee basement membrane ?
Separates the epidermis from the dermis
Stratum basale attached to it by hemidesmosomes
What does the stratum basale layer look like
Cuboidal/low columnar cells
Attached to the basement membrane by hemidesmosomes
Basal cells have highly indented+ folded basal cell membrane with numerous hemidesmosomes
What is the function of the stratum basale ?
Contain stem cells
Mitotically active/ proliferates continuously
Allows maturation to renew/regenerate other layers
Basal cells -> mature into stratum spinosum
Explain the stratum spinosum
Prickle cell layer
Multilayered
Contains polyhedral shaped keratinocytes - these synthesise cytokeratins
What are cytokeratins ?
Cytoplasmic intermediate filaments which accumulate to form tonofibrils
What are tonofibrils ?
Bundles of tonofilaments
Tonofibrils bind to desmosomes to form strong contact between adjacent keratinocytes
Why is the stratum spinosum called the prickle cell layer
Due to the desmosome junctions which are seen as prickles or spines
What do the keratinocytes mature into from the stratum spinosum ?
The stratum granulosom - granular layer
What occurs to the keratinocytes in the granular layer ?
Cells become flattened
Contain basophillic keratohyalin granules
What is the function of keratohyalin granules ?
Contain amino acids and protein that interact with tonofibrils which produces keratin -> keratinisation
What happens with cells in the stratum granulosum
Cells start to lose their nucleus and cytoplasm and become the stratum corneum
Explain the stratum corneum ?
Keratin squames -> large flat plate like envelopes filled with cross linked keratin
Why is the stratum corneum water resistant
Due to lipids
Lipid containing anti wetting agents synthesised
Where is stratum lucidum found ?
Only in thick skin
What can the epidermis do ?
Self renew
Where does skin regeneration start ?
Stem cells in the basal layer continuously divide and differentiate migrating upwards
Explain the maturation of keratinocytes ?
1.Cells start process of differentiation in basal layer
2. Differentiating cells pushed upwards into stratum spinosum (produce prickle cells)
3.cell shape changes as this occurs and desmosomes become more pronounced
4.prickle cells continue to be pushed upwards and reach granular layer
5.cells degraded - loses nucleas and cytoplasm
6. Highly keratinised flattened cell
7.corneocytes - eventually shed off and die
What is the thickness of skin related to ?
The dermis and subcutis
What is the dermis composed of ?
It is connective tissue
Composed of collagen I
and elastin fibres and ground substance
What are the main cells found in the dermis ?
Fibroblasts
What are the main functions of the dermis ?
Immune role
Provides a blood supply
Tensile strength -> collagen
Allows stretch -> elastin
What are the to layers of the dermis ?
Superficial papillary dermis
Deep reticular dermis
Explain the structure of papillary dermis ?
Contain fine, interlacing collagen and elastic fibres
Also contain blood vessels and nerve endings
Explain the structure of the reticular dermis ?
Thicker layer -> collagen binds much more strongly
Elastin fibres much thicker too ad longer
What is the dermis epidermal junction ?
Attachment of epidermis to dermis through hemidesmosomes
It aligns cells of the epidermis
What is function of the dermo-epidermal junction
Serves as a base for repithelialisation in wound healing
Barrier function -> from and into epidermis
Explain the subcutis
Mostly composed of adipose tissue
Compartmentalised by vertical fibrous septa
What is the use of subcutis
Fatty layer of the skin
Acts as energy source
Shock absorption and insulation
How does the skin act as a sensory organ
It has free nerve endings in the skin
What is the function of the free nerve endings ?
Occupy the papillary dermis and extend into epidermis where they associate with Merle cells and act as mechanoreceptors
What are meisnners corpuscles
Rapidly acting mechnoreceptors which are responsible for touch
Prominent in papillary dermis of hands and feet
What are pacinian corpuscles ?
Detect deep pressure and vibration
Where are pacinian corpuscles found ?
Deep into subcutis
What are examples of adnexal structures ?
Hair and nails and glands
What is hair
Long thin cylinderal shafts composed of keratin
What is the function of hair and fur?
Providing colour and shape but particularly thermoregulation -> heat conservation
What is the hair follicle ?
Tubular structure of specialised connective tissue and epithelium
How is a hair follicle reduced ?
During active hair growth the epithelial cells around the hair papilla proliferate to form layers of the follicle
What gives hair their colour ?
Melanocytes
As melanin is incorporated in cells that form hair shaft
What causes hair to stand up ?
Contraction of the arrest or pili smooth muscle -> thermoregulated response
Explain the structure of the nail briefly
Dense keratinised plate which rests on stratified squamous epithelium - ‘the nail bed’
How do nails grow ?
Proliferation and differentiation of the epithelium underlying the nail root
What is the role of the eccrine glands ?
Temperature control glands -> excrete sweat and therefore lower body temperature
Where are eccrine glands found ?
Everywhere on human skin part from nail beds, lips, external auditory canal and some parts of the genitalia
Most abundant -> palms, soles and axillae
What are the role of the apocrine glands ?
Scent glands - role unclear in humans
Where can the apocrine glands be found ?
Axillae and genitals
What are sebaceous glands and their function
Formed from hair follicle
Produce sebum- lipid which can lubricator the skin
Greatly enlarge at puberty in response to androgens
Where can sebaceous glands be found ?
Everywhere except palms and soles
Label the skin
What are the cells in the dermis ?
Fibroblasts
Lymphocytes
Dermal dendritic cells
Mast cells
What are common cells in the epidermis ?
Keratinocytes
Stem cell
Merkel cell
Langerhans cell
Melanocytes
Where can keratinocytes be found ?
Keratinised stratified squamous epithelium
Where can keratinocyte be found ?
Protection and barrier
Involved with melanocytes in vitamin d function
Where can stem cells be found ?
Stratum basale
What is the function of stem cells ?
Self renewal and repopulation of epidermal layers
Allow skin regeneration and cell turnover
Where can merkel cells be found ?
Epidermis
What is the function of touch cells ?
Touch cells
Relay touch related information
Where can Langerhan cells be found ?
Epidermis
Stratum spinosum and upper dermal layer
What is the function of langerhans cell
Antigen presenting cells
Allow phagocytosis
Where can melanocytes be found ?
Basal layer
What is the function of melanocytes
Protection from radiation
For example ultraviolet light
Provide colour to skin and hair
How do melanocytes protect from uv radiation ?
Melanin produced from tyrosine within melanosomes -> transferred to keratinocytes
Form cap over nucleas protecting the dna
What is uv radiation implicated in ?
Development of melanoma and non melanoma skin cancers
What can chronic uv exposure in humans lead to ?
Loss of skin
Abnormal pigmentation
Haemorrhage of blood vessels
Wrinkles + premature ageing
What is vitamin d useful for ?
Vitamin d synthesis
How is vitamin d produced
255-330nm UVB converts 7-dehydrocholesterol in the plasma membrane of keratinocytes to pre vitamin D3
How do melanocytes and keratinocytes work together to protect cells from UV damage
- Melanin in synthesised by melanocytes within organelles called melanosomes
- Melanosomes transferred into neighbouring keratinocytes By melanocytes cytoplasmic processes
- Melanosomes with keratinocyte -> a cap is formed over nucleus which forms protective layer over the dna
Where are keratins found
All epidermal layers
What is the function of keratins ?
Major structural protein
Intermediate filaments
What molecules are found in keratohyalin molecules ?
Profilaggrin
Involucrin
Loricrin
What is the function of profilaggrin
Converted to fillaggrin -> aggregates keratin filaments into tight bundles
What is the function of involucrin?
Formation of cell envelope around cells in the stratum corneum
Where is the function of loricrin
Cross links to involucrin
What molecules form the lamellar granules
Polysaccharides
Glycoproteins
Lipids
What is the function of the polysaccharides+ glycoproteins and lipids in the lamellar granules
Form the cement that holds together the stratum corneum cells -> impermeable intracellular lipid layer
What is the function of fibroblast
Synthesis of collagen,elastin + other extra cellular matrix compounds -> involved in sending and receiving signals from other molecules
What is the function of lymphocytes
Immunosurveillance -> detecting foreign antigens
What is the function of dermal dendritic cells
Phagocytosis
Antigen presentation
What is the function of mast cells
Produce inflammatory mediators such as histamine or heparin
For eosinophils and neutrophils
Important in wound healing - immediately released after injury
What is collagen type 1
Major structural fibrous protein - provide tensile strength
What is the function of elastin
Confer elasticity and recoil -> allow stretch and recoil in response to movement
What do proteoglycans and glycosaminoglycans provide
Hydration
What are the key molecules in the dermis
Collagen type I - stronger than type III
Elastin
Proteoglycans
Glycosaminoglycans
What is a wound
The breakdown in the protective function of the skin
A wound causes the loss of what thing ?
The loss of continuity of epithelium
What is the depth of injury related to ?
The ability to heal
Describe what would mean by erosion wound
Only epidermis lost - healing faster
What would mean by ulceration ?
Wound deep into epidermis
What is partial thickness wound
Epidermis and varying parts of dermis are damaged
In a partial thickness wound what acts as a reservoir of epithelial cells
Adnexal structures because they allow repopulation of the wound
What is a full thickness wound ?
Epidermis
All of dermis
Deeper structures affected
Wound edge cells only
What are the three phases of wound healing
Inflammatory
Proliferation
Remodelling
Explain the inflammation phase ?
1.Platelets initiate haemostasis/blood clot and healing cascade
2. Attracts other cells to. The wound to fight infection and transition from inflammation to repair 3. Neutrophils and macrophages phagocytose dead tissue and bacteria/pathogens
What are the characteristic of inflammation ?
Red/swelling
Explain the proliferative phase
1.Reepithelialisation
2. Formation of granulation tissue mainly type III collagen
3. Neovascularisation occurs : proliferation and migration of endothelial cells occur
Explain what reepithelisation is
1-2 days of wounding
Epithelial cells loosen cell to cell adhesions and migrate to the wound site, cover the granulation tissue and then meet in the middle
2. Once wound is covered in keratinocytes - keratinocytes start to proliferate
Explain simply what reepithelisaton is
Cells dividing to replace damaged or loss of cells
What are the two methods of keratinocyte migration
Leapfrog method
Train method
How is collagen type III formed when granulation tissue is formed
Underneath cells fibroblasts are recruited that replace collagen
What is the remodelling phase
When granulation tissue becomes mature tissue
How does remodelling phase occur
Collagen is organised into thick bundles
Cross linked -> forms mature scar
Switch in type of collagen
What is the switch in collagen in te remodelling phase
Type III to type I
Compare type I collagen to type III collagen
Type I collagen is much more organised and stronger
Explain the strength of the skin
Final -> only 70-80% of preinjured skin
And after 1 year
Strength is 5% at 1 week 20% at 3 weeks
How do cells in wound healing know what to do ?
They respond to signalling molecules
What is the function of epidermal growth factor
Allows reepithelialisation -> proliferation and maturation of keratinocytes
What is the function of platelet derived growth factor
Matrix formation -> activates increased numbers and activity of fibroblasts
Also important in remodelling due its role in production of proteases
What is the function of vascular endothelial growth factor
Angiogenesis which is endothelial cell proliferation and migration
Which 3 signalling molecules are involved in inflammation
Interleukin -1 (II-1)
Interleukin-6 (II-6)
Tumour necrosis factor
What do epidermal growth factors do in terms of keratinocytes
Signal to the keratinocytes to divide and migrate to close over the wound
What are local factors that’s affect wound healing
Infection
Foreign body
Oxygenation - cells highly active need o2 for respiration
Vascular supply
What are systemic factors that’s defect wound healing
Age
Diseases
Alcohol and smoking
Immunocomprimised conditions
Obesity - affects vascular flow
Medications
