Skin Flashcards
Main Functions of skin
Protection, sensory, thermoregulatory, metabolic and absorptive
Protective function explained
provides a physical barrier against pathogens and friction. Pathogens that do get in alert antigen presenting cells in the skin which initiate an immune response. Melanin protects nuclei from UV radiation. Prevents excessive water loss.
Sensory function explained
contains many sensory receptors and mechanoreceptors
thermoregulatory function explained
Fatty layer- helps maintain heat, hair follicles, sweat production
metabolic function explained
cells of skin synthesise vitamin D needed in calcium metabolism and bone formation
Absorptive function
skin patches can be placed, containing nicotine, hormone replacements, steroids etc.
3 main layers of skin
epidermis, dermis and hypodermis
What type of epithelium is the epidermis?
Stratified squamous
4 main epidermal layers
Basal layer (stratum basale), spinous layer (stratum spinosum), granular layer (stratum granulosum) and stratum corneum
Stratum Basale location
between the dermis and stratum spinosum. Bound to the basal lamina via hemidesmosomes
Stratum Basale cell type
simple cuboidal or columnar
How is the stratum basale characterised?
Intense mitotic activity. Contains progenitor cells for the epidermal layers and basal stem cells for the keratinocytes.
What sorts of keratin is expressed?
cytoskeletal keratins are 10 nm in diameter.
How often are the stratum basale epithelial cells renewed?
25-30 days
What is normally the thickest layer in the epidermis?
Stratum spinosum
What are the prickles on the stratum spinosum cells?
Tonofibrils assembled from keratin fibre bundles that terminate at numerous desmosomes, holding the cell layers together.
Where will epidermis have a larger stratum spinosum?
Skin where there is a lot of friction, such as the foot sole
What type of cells are in the basale granulosum?
flattened
What process are they undergoing?
Terminal differentiation process of keratinisation
What is the cytoplasm of the basale granulosum cells filled with?
basophilic keratohyaline granules
What are keratohyaline granules?
dense, non- membranous bound masses of filaggrub and other proteins associated with keratin
Keratohyaline granule functions
cross link proteins together in a big mass, forming a cornfield layer
What else occurs in the basle granulosum?
small keratinosomes are released, filled with lipids producing water proofing
What cells are present in the stratum corneum?
15-20 layers of squamous cells, that have been squashed and migrated from the stratum basale’s cuboidal cells
What is the stratum corneum?
The most superficial layer of the epidermis, containing remnants of dead keratinocytes, including desmosomes, monofilaments and cornified cell envelope.
What is the cornified cell envelope?
A structure formed beneath the plasma membrane in terminally differentiating stratified squamous epithelia that provides a physical barrier and consists of a thick layer of highly cross linked insoluble proteins.
What are the fully keratinised cells called?
squames
What is the process of cornification?
Metabolism ceases. Cells completely filled with keratin that forms the cytoskeleton by binding to desmosomes, hemidesmosomes and cell junction plaques. Loses nuclei.
What is keratin + structure?
Fibrous, insoluble scleroprotein. Two strands of alpha keratin parallel to each other wrap around one another fro form a supercoil.
Cells present in epidermis
keratinocytes, melanocytes, Langerhan’s, Merkel Cells
Function of keratinocytes
Constitute 90% of the cells of the epidermis. Form a barrier against environmental damage such as heat, UV light, water loss and pathogens.
Keratinocyte maturation stages
- Keratinocytes in the stratum basale begin to differentiate and migrate superficially
- Keratinocytes then permanently withdraw from the cell cycle
- Begin expressing different keratin molecules, such as 1, 5, 10 and 14.
- eventually become corneocytes, when the keratinocytes lose their nucleus and organelles, once differentiation has completed, forming the stratum corneum.
Melanocytes function
Melanocytes produce melanin in melanosomes. It’s different forms are responsible for different colours of hair and skin.
Two types of melanin + colour
Eumelanins (black/brown pigments) produced by eumelanosomes and phenomelanins (red pigments) produced by phenomelanosomes
Melanocytes origin and location
Neural-crest cell derivatives that migrate into embryonic epidermis’ stratum basale. One melanocyte accumulates for every 5/6 keratinocytes
melanocyte structure
Rounded cell bodies attached to the basal layer by hemidesomosomes, but lack attachments to keratinocytes. Contains several long cytoplasmic extensions that penetrate the epidermis and run parallel to the basal and spinous layers of cells
Melanin synthesis stages
- Tyrosinase enzymes, transmembrane enzymes in vesicles, convert tyrosine to 3,4-dihydroxyphenylalanine which is then further transformed into different melanin molecules.
- Melanin pigment, linked to the matrix and structural proteins, accumulates in the vesicles until they form mature elliptical granules called melanosomes.
- Melanosomes are then transported via kinesin to the tips of the cytoplasmic extensions
- Neighbouring keratinocytes phagocytose the dendrite tips, take in the melanosomes and then transport them by dynein towards their nuclei.
- Form a supranucledar cap surrounding the nucleus, protecting DNA from UV radiation
Langerhan’s cell function
Bind, process and present antigens to T lymphocytes, becoming antigen presenting cells that initiate an immune response.
Langerhan’s cell location and structure
Most clearly seen in the stratum spinosum layer. Cytoplasmic processes extend from the dendritic cells between keratinocytes.
Merkel cells function
Low threshold mechanoreceptors, essential for sensing gentle touch
Merkel cell structure
joined by desmosomes to keratinocytes of the basal epidermal layer. Same structure as keratinocytes however have no melanosomes. Contain small, Golgi derived, dense core granules in areas near the basolateral surface where the cells have synaptic contact with nerves.
Merkel cell location
Abundant in highly sensitive skin, such as the fingertips
What is the dermis?
Layer of connective tissue that supports the epidermis and binds it to the subcutaneous hypodermic layer
Two dermis sublayers
Upper papillary layer and lower reticular layer
Papillary layer components
Loose connective tissue, collagen fibres, fibroblasts, mast cells, dendritic cells and leukocytes
Structure of papillary
Contains dermal papillae which are projections that interdigitate with invagination epidermal ridges. Anchoring fibrils of Type VII collagen insert into the basal lamina, helping bind the dermis to the epidermis
What is the function of the epidermal ridges and dermal papillae?
Increase the strength of adhesion between the epidermis and dermis
Structure and components of reticular layer
Thicker layer, consists of dense irregular connective tissue with more fibres and less cells than the papillary layer. Network of elastic fibres also present, provide elasticity to the skin.
Between collagen fibres and elastic fibres there are a large number of proteoglycans rich in dermatan sulphate.
Which layer of the skin contains a rich network of blood and lymphatic vessels?
Dermis
Two Major plexuses
Subpapillary plexus and deep plexus
Location of subpapillary plexus
Between the papillary and reticular Demis layers. Capillaries branch off and extend into the dermal papillae
What sort of blood vessels are present in the subpapillary plexus?
A nutritive capillary network
Deep plexus location
Near the interface of the dermis and the hypodermis
What sort of vessels in the deep plexus?
Larger blood vessels and lymphatic vessels
Anteriovenous anastomoses function
The shunts decrease blood flow to the papillary layer to minimise heat loss and increase flow to maximise heat loss.
Location of the arteriovenous anastosomes
Between the the deep and subpapillary plexuses
Where are nerve fibres found?
Sensory afferent nerve fibres form a network in the papillary dermis and around hair follicles ending at dermal and epithelial receptors.
Autonomic effector nerves to dermal sweat glands. Smooth muscle fibres in the skin of some areas are postganglionic fibres of sympathetic ganglia.
What type of innervation is not present?
Parasympathetic
Hypodermis structure
loose connective tissue and adipocytes. Extensive vascular supply.
Hypodermis function
Bind loosely to the skin of subjacent organs making it possible for the skin to slide over them.
Two types of skin glands
Sebaceous and sweat glands
Sebaceous glands location
Embedded in the dermis all over the body apart from the soles and palms
Structure of sebaceous glands
Branched acinar glands with several acini converging at a short duct that usually empties into the upper portion of a hair follicle. In hairless regions, the sebaceous ducts open directly onto the epidermal surface
Pilosebaceous unit definition
a hair follicle and its associated sebaceous glands
What form of secretion occurs at acini?
Holocrine, where cells undergo terminal differentiation and form lipid producing sebocytes filled with small fat droplets. Eventually cells undergo autophagy and release the lipids called sebum.
What does sebum cover?
surface of epidermis and hair shafts
Continents of sebum
mixture of lipids that includes wax esters, squalene, cholesterol and hydrolysed triglycerides.
Sweat gland structures
Long epidermal invaginations embedded in the dermis
Two types of sweat gland
eccrine and apocrine
Eccrine sweat gland location
Mainly sole of foot
Three cell types in secretory part of gland and function
Clear cells, located on basal lamina and produce sweat.
Dark cells, filled with strongly eosinophilic granules line the lumen and do not contact basal lamina. Granules undergo merocrine secretion of glycoproteins with bactericidal action
Myoepithelial cells, on basal lamina contract to move watery secretion into duct
How is sweat produced?
Interstitial fluid from capillary rich dermis around the gland is transported through the clear cells into the gland’s lumen
Adaptations to the duct to minimise sodium ion loss
Ducts of eccrine sweat glands have two layers of acidophillic cells linked with mitochondria and cell membranes rich in the sodium potassium ATPase. Duct cells thus absorb sodium ions from the secreted water.
How is sweat lost from the gland?
Duct merges with stratum basale and sweat floes through a spiralling channel to an excretory sweat pore on the skin’s surface where it then quickly evaporates upon release.
Under what control is release from eccrine glands?
cholinergic sympathetic control
Apocrine sweat gland location
skin on the axillary and perineal regions
Under what control is release from apocrine glands?
adrenergic sympathetic control
What type of secretion occurs at apocrine glands?
merocrine secretion from secretory granules that undergo exocytosis into the lumen
What is secreted?
Viscous milky secretion which may acquire a distinctive door as a result of bacterial activity
What is hair?
elongated keratinised structures that form within epidermal invaginations, hair follicles. Keratin is harder and more compact than that of the stratum corneum
What is the hair shaft surrounded by?
A hair follicle, an epidermal downgrowth
Hair location
Bulb in dermis, however hair shaft in epidermis, so is an epidermal structure
What inserts into the base of the hair bulb + function?
Dermal papillae, containing a capillary network. Also covered in a layer of keratinocytes which form the matrix of the elongating hair root.
Cells in the hair root matrix + functions
keratinocytes - divide rapidly around dermal papillae, undergo keratinisation, melanin accumulation and terminal differentiation.
melanocytes- transfer melanosomes to epithelial cells which will later differentiate into hair cells
What are arrector pili muscles?
small bundles of smooth muscle cells that extend from midpoint of fibrous sheath to the dermal papillary layer.
Arrector pili muscle function
Contract to pull the hair shaft into a more effect position
What are nails?
Hard plates of keratin on the dorsal surface of each distal phalanx
Structure of nail
Nail root- at bottom of the nail covered by a fold of skin called the eponychium (proximal nail fold).
Cuticle- extension of epidermal stratum corneum
Nail plate bound to a nail bed, which contains only the basal and spinous epidermal layers
Brief nail growth
Nail root continuously forms from nail matrix containing keratinising cells that mature and harden. Continuous growth of the matrix pushes the nail plate forward over the nail bed. Epidermal fold at the tip of the finger is called the hyponychium (lateral nail fold)
