Structure and function of the skin Flashcards
How does the skin provide a protective barrier?
By the use of hair, stratum corneum, sebum and sweat
Protection against desiccation via the stratum granulosum
How does the skin form a chemical barrier?
Inorganic acids, proteins and fatty acids in the skin protect the animal
How does the normal skin flora inhibit the infectious agents?
Coagulase-negative staphylococci, micrococcus spp, Propionibacterium acnes and actinobacter spp. Break down lipids to produce antibacterial free fatty acids.
How does the skin help regulate temperature?
Control of blood flow close to the surface and sweat production (esp. in horses)
What cells in the skin detect sensory stimuli?
Sensory nerve endings in the dermis and lower epidermis detect environmental temperature, vibrations touch and pain
Merkel cells detect pressure
Meissner’s corpuscles detect gentle pressure
Pacinian corpuscles detect deep pressure
How does the skin function as an excetetory organ?
Skin excretes small amounts of urea, uric acid, ammonia and lactic acid
How does the skin provide photoprotection?
Urocanic acid and melanin absorb UVA and UVB but UVB is still needed in small amounts for vitamin D production
Melanin caps the DNA to protect it (“nuclear capping”)
What immune cells live in the skin?
Keratinocytes are immunologically active producing interleukin-1 and thymic stromal lymphopoetin and also antimicrobials such as beta-defensin and cathelicidins
Langerhans cells provide antigen presenting and direct inflammation
Dermal dendritic cells, T-cells and mast cells are all present in the dermis
How does the skin help regulate blood pressure?
The body stores Na+ in the skin which is regulated by immune system cells and lymph capillaries
How does the skin produce vitamin D?
Previtamin D3 from 7-dehydrocholesterol (7-DCH) is isomerised to vitamin D3 in the basal and suprabasal layers of the skin which is then transported to the liver for further metabolism
Where are nutrients stored in the skin?
Mainly in the subcutis (aka hypodermis) particularly fat water and vitamins
The skin provides an absorbtive surface for what type of drugs?
Lipid soluble
In plated animals eg. armadillio what is the armour formed of?
Dermal bone covered in relatively small overlapping epidermal scales called scutes (composed of bone covered in horn)
What is the follicular ostium?
The hole where the hair comes out, it is fully lined with epithelium
Where does the sebaceous gland lie?
Halfway down the hair shaft
The subcutis is also known as the ……………
Hypodermis
What are papillae apiposae?
Waves of subcutis that project into the dermis following the hairs and serve to protect these hair follicles, sweat glands ad vessels
What does the subcutis comprise of and what is it made up of?
The subcutis is made up predominantly of lipocytes and they serve as an energy reserve, providing thermoregulation protective padding and insulation.
Why are infections in the subcutis hard to clear?
There is limited blood supply to this area so infections are hard to clear
Where does the connective tissue of the dermis arise from?
Connective tissue of mesenchymal origin and provides structural strength
What does the connective tissue of the dermis consist of?
It consist of insoluble fibres (collagen and elastin) to give tensile strength which are surrounded by soluble polymers (proteoglycans and hyaluronan) to resist and absorb compressive forces
What does the thickness of the dermis comprise of and what two components can it be divided into?
The thickness of the dermis depends on the thickness of the skin and the dermis can be divided further into deep (solid collagen, thicker strands) and superficial (wispier collagen).
What structures does the dermis contain?
The dermis contains the epidermal appendages, arrector pill muscles, blood vessels, lymph vessels and nerves. NB no blood supply to the epidermis
What stratum layer can be absent in haired skin?
S. Granulosum
Where is the water barrier
top of the stratum spinosum
How do free nerve endings lie in the skin?
Free nerve endings enter the epidermis, interdigitating between the keratinocytes ending at the level of the water barrier (top of the stratum spinosum)
What are the rapid adapting mechanoreceptors and how are they laid out in the skin?
The rapid adapting mechanoreceptors are laid out in more of a mosaic pattern and include Meissener’s and Pacinian corpuscles and tylotrich hair
What are the slow adapting mechanoreceptors and how are they laid out in the skin?
Slow adapting mechanoreceptors also follow this pattern and include Merkel cells, Ruffini corpuscles, sinus hairs and tylotrich pad
What motor nerves lie in the skin? What is their function?
Adrenergic and cholinergic fibres will control things like heat loss (by controlling atrichial & epitrichial sweat glands) and blood vessels
What is the deep dermal vascular plexus and where does it supply?
Large vessels that sit at the junction between the dermis and the subcutis, they supply blood to the lower hair follicle and epitrichial sweat glands therefore are responsible for the growth of new hairs
What is the mid- dermal plexus and where does it supply?
Smaller vessels that sit at the level of the sebaceous glands and supply the arrector pili muscles, mid hair follicle and sebaceous glands, this is the primary blood supply to hairs in the telogen phase.
What is the superficial dermal vascular plexus and where does it supply?
This sits just below the epidermis and is made up of many capillary loops that supply the epidermis and upper hair follicle. These capillaries can be bypassed for thermoregulation.
What is the function of sebaceous glands?
Their main function is to grease the hair, so it can emerge intact and keep growing smoothly
What do sebaceous glands secrete and how?
Sebaceous glands produce a holocrine secretion that is predominantly lipids/triglycerides (e.g. Linoleic acid) but also contains transferrin, IgA and IgG for immune function.
The sebum is excreted by a squamous duct into the hair follicle
What is an apocrine secretion?
The apical portions of the cell are pinched off and lost during the secretory process
What is a holocrine secretion?
Involves death of the cell, the secretory cell is released as it breaks apart and the contents of the cell become the secretory product.
What is a merocrine secretion?
Merocrine- Formation of membrane bound secretory vesicles internal to the cell.
Name some specialised sebaceous glands
Meibomian gland (eyelid)
Circumanal (hepatoid) gland
Preputial glands in horses
Infraorbital, inguinal and interdigital glands (in close association with sweat glands) in sheep
Name the two types of sweat gland
Epitrichial and Atrichial
Describe Epitrichial sweat glands
What type of secretion are they?
Associated with a hair follicle
Not innervated or under neural control
Made up of a single layer of flattened cuboidal cells surrounded by myoepithelial cells
Coiled/tubular or saccular structures distributed throughout haired skin
Located below the sebaceous gland and excrete sweat into the pillary canal just above the sebaceous gland opening, thought to be apocrine secretion.
Have pheromonal and antimicrobial properties
Describe atrichial sweat glands
What type of secretion are they?
Not associated with a hair follicle
Also made up of a single layer of flattened cuboidal cells surrounded by myoepithelial
Small, tightly coiled glands
Restricted to not haired areas e.g. footpads
Thought to be merocrine secretion
How thick is dog/cat skin?
0.5-7mm
where are epidermal rete ridges seen?
Epidermal rete ridges are seen in pigs and horses at the mane and tail base.
What are keratinocyes?
Large cells of epithelial origin with complex internal cytoskeleton (mainly keratins and actin)
The cytoskeleton of keratinocytes is attatched to what?
Tight junctions (cell to cell)
Desmosomes (cell to cell)
Hemidesmosmes (cell to dermal proteins)
How do the keratins differ in keratinocyes?
The keratins differ in different layers: K5 and K14 in basal layers, K10 in upper layers. This differentiation is under the influence of calcium
What do keratinocytes produce?
The keratinocytes produce extracellular lipids (ceramides, cholesterols and fatty acids) along with sebaceous glands
Name some specialised atrichial glands
Mammary glands
Interdigital glands of small ruminants
External ear canal: Cerumen is a mixture of sebaceous and ceruminous glands
Nasolacrimal glands : Aqueous component of the tear films
Apocrine glands of the anal sac
What is the stratum basale?
A Single layer of cuboidal cells, about 1/3 will be proliferating and the rest are just ‘padding’
What influences stratum basale division?
Growth factors and hormones e.g. epidermal growth factor (GF), insulin-like GF, keratinocyte GF, cortisol, T4
Inflammatory mediators eg interleukin-1, IL-6 and transforming growth factor alpha
Drugs and vitamins eg vitamin D and retinoid
How are the stratum basale cells attatched to the dermis and themselves?
They are attached to the dermis by Hemidesmosmes and then to each other by desmosomes
The Hemidesmosmes provide a strong link between the keratin intermediate filaments and the interstitial collagens in the dermis. The number and complexity of these molecules allows a number of congenital and auto immune diseases.
The desmosomes provide a strong link between the KC’s linking keratin intermediate filaments and providing intracytoplasmic signalling. They are very complex links made up of cadherin’s (calcium dependant adhesion proteins) such as desmogleins and desmocolins and therefore subject to autoimmune disease.
What do the stratum spinosum cells look like? What do they produce?
Around 1-20 cells thick, the cells are polyhedral with prominent desmosomes
The upper layer produces involucrin which forms part of the cornified layer
What do the stratum granulosum cells look like? What do they produce?
These cells are much less metabolically active and their nucleus is shrinking
They are variably present in haired skin and appear as flattened cells with small nuclei, intracellular keratohyaline granules which contain profilaggrin (biological glue) and loricrin
Various lipids and enzymes are secreted extracellularly to create a watertight seal
Where can you find the stratum lucidum? What does it look like and what does it produce?
Not present in haired skin but often present in hairless (glabrous) thickened skin for example the nasal planum
Quite rare to see as it contains so much fat that during staining is cannot be seen
Slightly flattened cells with shrunken nuclei, intracellular keratohyaline (filagrin-keratin aggregates) and increases intracellular lipids, ceramides, cholesterols and fatty acids.
What does the stratum corneum consist of?
Composed of enucleated flattened cells (aka Squamates) of variable thickness
In the stratum corneum what has the plasma membrane been replaced with?
The plasma membrane is replaced by a cornified envelope comprising of intracellular proteins (involucrin, loricrin, periplakin) and are extensively linked with the aid of fillagrin and keratin
Hydrophilic bonding of the lipids forms organised mortar between squames (lipids from lamella bodies)
Where are melanocytes derived from and what is their function?
Derived from the neural crest they sit in amongst a group of keratinocytes which they serve and produce the pigment for, they also have an immune function however this is poorly defined
What are the two types of melanin?
Eumelanin black = brown
Phaeomelanin = reddish brown
Where does the pigment produced in melanocytes come from?
Melanocytes produce pigment in melanosome organelles which are membrane bound structures involved in the transfer of pigment to keratinocytes.
How many KC’s does each melanocyte serve?
10-20
What else will influence skin pigment?
Carotenoids – yellow pigments from plants (SQ fat and stratum corneum)
Oxyhaemoglobin
Deoxyhaemoglobin
Melanin – melanosomes deposit M onto lamellar structural proteins under the influence of tyrosinase (copper dependant)
The absence of pigments will cause….
The absence of these pigments cause leukoderma and leukotrichia
What causes hairs to have pigment?
Hair pigment is also caused my melanocytes associated with the hair matrix
What can change skin pigmentation?
Skin pigmentation is influenced by genetics, UV light and hormones such as melanocyte stimulating hormone (MSH)
Melanin functions to screen solar radiation and acts as a free radical scavenger
What is the function of langerhans cells in the skin?
Act as professional antigen presenting cells
How do langerhans cells appear on H&E?
The appear as clear cells on H&E or as distinctive tennis racket shaped or rod shaped cytoplasmic organelle or electron microscope
Once activated where do the langerhans cells of the skin migrate to?
Migrate to the dermis and regional lymph nodes on activation causing T-lymphocyte activation
What reduces the effect of langerhans cells in the skin?
Their effect is markedly reduced when exposed to UV (e.g. on the white legs of horses) and this is the cause of increased melanoma/carcinoma in pale skin
A mutation of transglutaminase will cause….
Causes severe hyperkeratosis and thin cornified envelopes
