9.1 - Introduction to Dermatology Part 1 Flashcards
What are the main juxtaposing embryological layers of the skin?
- epidermis - originates from ectoderm
- dermis - originates from mesoderm that comes into contact with inner surface of epidermis
- mesoderm essential for inducing differentiation of epidermal structures e.g. hair follicle
Describe the stages of development of epidermis and dermis
- week 5 - epidermis forms as single basal layer of cuboidal cells
- secondary layer of squamous, non-keratinising cuboidal cells (periderm) develops - cells with projecting globules covered with small protrusions (microvilli)
- week 11 - basal layer of cuboidal cells (stratum germinativum aka stratum basale) proliferates to form multilayered intermediate zone
- weeks 9-13 - development of hair follicles in stratum germinativum and appearance of lanugo hair
- weeks 12-14 - epidermal ridges protrude as troughs into developing dermis beneath (rete ridge)
- neurovascular supply develops into dermal papillae
- week 20 - further proliferation and differentiation into four more superficial strata
What does periderm generate?
- white, waxy protective substance - vernix caseosa
- protects foetus from amniotic fluid and also from bacteria after birth
What are the names of the four strata differentiated from multilayered intermediate zone in week 20?
From deep to superficial:
- stratum spinosum (spinous)
- stratum granulosum (granular)
- stratum lucidum (clear - found on palms of hands and soles of feet only)
- stratum corneum (horned)
What is a basic overview of the structure of the skin?
- epidermis
- basement membrane (dermal-epidermal junction)
- dermis
- subcutaneous fat
What is the structure of the epidermis?
- composed of keratinocytes (most abundant cell)
- cells in basal layer are small and cuboidal and undergo progressive differentiation and flattening:
- stratum spinosum
- stratum granulosum (granules of keratohyalin)
- stratum lucidum (only palms and soles, no nuclei or organelles)
- stratum corneum (no nuclei or organelles)
- cellular progression from basal layer to surface in 30 days - accelerated in skin diseases e.g. psoriasis
What is the structure of keratinocytes?
- filamentous cytoskeleton comprising:
- actin-containing microfilaments (7nm)
- tubulin-containing microtubules (20-25nm)
- intermediate filaments (keratins) (7-10nm)
What are the roles of keratins?
- structural properties
- cell signalling
- stress response
- apoptosis
- wound healing
What are desmosomes? (junctions that connect keratinocytes)
- major adhesion complex in epidermis
- anchor keratin intermediate filaments to cell membrane and bridge adjacent keratinocytes
- allow cells to withstand trauma
- contain many protein components
What are gap junctions? (junctions that connect keratinocytes)
- clusters of intercellular channels (connexons)
- directly form connections between cytoplasm of adjacent keratinocytes
- essential for cell synchronisation, cell differentiation, cell growth and metabolic coordination
What are adherens junctions? (junctions that connect keratinocytes)
- transmembrane structures
- engage with the actin cytoskeleton
What are tight junctions? (junctions that connect keratinocytes)
- role in barrier integrity and cell polarity
What are melanocytes? (other cells in epidermis)
- dendritic
- distribute melanin pigment (in melanosomes) to keratinocytes
- number of melanocytes equal among skin types (just amount of melanin made varies)
What are Langerhans cells? (other cells in epidermis)
- dendritic
- antigen-presenting cells
What are Merkel cells? (other cells in epidermis)
- mechanosensory receptors
- mast cells are also present in the epidermis
What is the structure and function of the basement membrane/dermal-epidermal junction?
- made up of proteins and glycoproteins like collagens (IV, VII), laminin, integrins
- roles include:
- cell adhesion
- gate-keeping functions - cell migration, diffusion of bioactive molecules
- rete ridges - strength, niche for epidermal stem cells
- roles in development, morphogenesis, wound healing and skin remodelling
What are the two layers of the dermis?
- papillary dermis - superficial, loose connective tissue, vascular
- reticular dermis - deep, dense connective tissue, forms bulk of dermis
What substances make up the dermis?
- proteins - 80-85% collagen (mainly types I and III), 2-4% elastic fibres (fibrillin, elastin)
- glycoproteins - fibronectin, fibulin, integrins - facilitate cell adhesion and motility
- ground substance - between dermal collagen and elastic tissue - glycosaminoglycans and proteoglycans - maintain hydration of dermis due to high water binding capacity of hyaluronic acid
What are the cells in the dermis?
- fibroblasts - primary cells within dermis
- histiocytes
- mast cells
- neutrophils
- lymphocytes
- dermal dendritic cells
What is the blood supply to the skin like?
- deep and superficial vascular plexus
- does not cross into epidermis
- pilosebaceous unit
- arrector pili muscle / arterioles / shunts
What is innervation to the skin like?
- sensory - free nerve endings, hair follicles and expanded tips
- one million afferent nerve fibres that form branching network often accompanying blood vessels, to form a mesh of interlacing nerves in superficial dermis
- distribution varies by body site e.g. face, extremities and genitalia are more richly innervated than rest of skin
What is innervation to sweat glands like?
- autonomic
- cholinergic innervation to eccrine sweat glands (salty water non-smelling sweat)
- adrenergic innervation to eccrine and apocrine (body odour sweat in armpits and genitals) sweat glands
What two types of afferent nerve are there?
- free-ended that end in non-encapsulated receptors in the epidermis e.g. Merkel cell
- corpuscular that end in encapsulated receptors in the dermis e.g. Pacinian, Meissner’s
What are Meissner’s corpuscles (aka tactile corpuscles)?
- encapsulated, unmyelinated mechanoreceptors in dermal papilla
- lamellated capsule
- superficial dermis
- most concentrated in thick hairless skin (finger pads and lips)
- detect light touch and slow vibration - low frequency stimulation
What are Ruffini corpuscles (aka bulbous corpuscles)?
- slow acting mechanoreceptors deeper in dermis
- spindle-shaped, encapsulated
- highest density around fingernails
- sensitive to skin stretch
- monitor slippage of objects
What are Pacinian corpuscles (aka lamellar corpuscles)?
- encapsulated, ovoid
- rapidly adapting (phasic) mechanoreceptor
- detects deep pressure and vibration (deep touch)
- also detects surface texture - vibrational role
- dermal papillae of hands and feet
What are Merkel cells?
- non-encapsulated mechanoreceptors
- detect light/sustained touch and pressure
- oval-shaped
- modified epidermal cells
- found in stratum basale
- most populous in fingertips, also found in palms, soles, oral and genital mucosa
Summary of skin innervation
- light touch - Meissner, Merkel, Free
- touch and pressure - Merkel, Ruffini, Pacinian, Free
- vibration - Meissner, Pacinian
- temperature - thermoreceptor
- pain - nociceptor (free nerve endings)
What is the skin microbiome?
- microbiota - bacteria, fungi and viruses
- microbiome - genome of microbiota
- predominantly actinobacteria, firmicutes, bacteroidetes and proteobacteria
What are the functions of the skin?
- immunological barrier
- physical barrier
- thermoregulation
- sensation
- metabolism
- aesthetic appearance
How does skin act as an immunological barrier?
- Langerhans cells
- tissue-resident T cells, macrophages and dendritic cells carry out immune surveillance in dermis to give rapid, effective immunological backup if epidermis is breached
- keratinocytes make their own endogenous antibiotics (defensins and cathelicidins) - part of innate immune defence against bacteria, viruses and fungi
What do Langerhans cells do?
- dendritic cell / macrophage family, both innate and adaptive immunity
- sentinel cells in epidermis
- initiate immune response against microbial threats
- also contribute to immune tolerance
- form dense network with which potential invaders must interact
- specialised at ‘sensing’ environment
- extend dendritic processes through intercellular tight junctions to sample outermost layers of skin (stratum corneum)
- interpret microenvironmental context = determine appropriate quality of immune response
- absence of danger - promote expansion and activation of skin-resident regulatory cells (Tregs)
- when toll-like receptors sense danger via PAMPs –> rapid initiation of innate antimicrobial responses
- induction of adaptive response - power and specificity of T cell/B cell and antibodies
How does skin act as a physical barrier?
- physical barrier against external environment
- cornified cell envelope and stratum corneum restrict water and protein loss from skin
- subcutaneous fat - important role in cushioning trauma
- UV barrier
- melanin in basal keratinocytes - protection against UV-induced DNA damage
How is skin involved in thermoregulation?
- vasodilation or vasoconstriction in deep/superficial vascular plexuses regulate heat loss
- eccrine sweat glands give an evaporative cooling effect
- role in fluid balance
How is skin involved in metabolism?
- vitamin D synthesis - conversion of 7-dehydrocholesterol into cholecalciferol
- subcutaneous fat has calorie reserves - 80% of total body fat in non-obese people is subcutaneous
- hormone (leptin) release - acts on hypothalamus - regulates hunger and energy metabolism
How is skin involved in aesthetic appearance?
- psychosexual function
- increased risk of suicide with people with skin condition