Structure and function of the skin Flashcards
What are the 3 main layers of the skin?
Epidermis
Dermis
Sub-cutis
What is the epidermis?
This is the outer layer of the skin and is made up of stratified squamous epithelium
What cells make up the epidermis?
Keratinocytes (95%)
Melanocytes
Langerhan’s cells
Merkel cells
What are the 4 main layers of the epidermis?
Keratin layer - Stratum corneum
Granular layer - Stratum granulosum
Prickle cell layer - Stratum spinosum
Basal layer - Stratum basale
What layer of the epidermis is found only in the palms and soles?
Clear layer - Stratum lucidum
Describe the structure of the keratin layer?
The keratin layer is made up of corneocytes, which are overlapping, non-nucleated cell remnants, which form the insoluble cornified envelope
This layer contains 80% keratin and filaggrin
Lamellar granules release lipids, forming a tight waterproof barrier
What is the function of the stratum lucidum in the hands and soles?
To reduce friction between the keratin and granular layer
Describe the structure of the granular layer of the epidermis
It is made up of 2-3 layers of flatter cells, containing large keratohyalin granules, which store structural filaggrin and involucrin proteins
These cells also contain Odland (Lamellar) bodies, which are secretory organelles that contribute to the skin barrier
They have a high lipid contain with a lost cell nucleus
They are the origin of the cornified envelope
Describe the structure of the prickle cell layer of the epidermis
It is made up of large, polyhedral cells and contains many desmosomes
Intermediate filaments connect to the desmosomes
Describe the structure of the basal layer of the epidermis
It is comprised of small cuboidal cells with lots of intermediate keratin filaments
It is usually 1 cell thick
It is highly metabolically active
Describe the structure of the basement membrane
The epidermal basement membrane is made up of laminin and collagen IV
What are some factors that regulate epidermal turnover?
Growth factors
Cell death
Hormones
What are melanocytes?
Melanocytes are pigment producing dendritic cells
Where are melanocytes found?
In the basal layer and above
How do melanocytes cause skin pigmentation?
Melanocytes contain organelles known as melanosomes, which convert tyrosine to melanin pigment
Melanin absorbs light and acts as a neutral density filter
Full melanosomes, called melanin granules, are transferred to adjacent keratinocytes via dendrites to form a protective cap over the nucleus in basal cells
What are the 2 main types of melanin pigment?
- Eumelanin - Brown or black
- Phaeomelanin - Red or yellow
What is a tumour of melanocytes called?
Melanoma
What determines skin colour?
Amount of melanin (Size of melanosomes) Number of keratin layers
Eumelanin and phaeomelanin levels
NOTE: The number of melanocytes stays the same!!
What causes darker coloured skin?
More cells in the keratin layer
Larger melanosomes in the keratin layer
More eumelanin
Movement of melanosomes from the basal layer
What causes lighter skin?
Fewer cells in the keratin layer
Smaller melanosomes in the keratin layer
More phaeomelanin
Melanosomes restricted to basal layer
What is vitiligo?
An autoimmune condition in which there is a loss of melanocytes, leading to skin depigmentation
Where do Langerhan’s cells form?
They are mesenchymal in origin, forming in the bone marrow
Where are Langerhan’s cells found?
In the prickle cell level of the epidermis, and also in the dermis and lymph nodes
What is the function of Langerhan’s cells?
They are involved in the skin immune system, functioning as APCs, picking up an antigen in the skin and circulating to the lymph nodes via the lymphatic system
What is an example of a condition affecting the Langerhan’s cells of the skin?
Langerhan’s cell histiocytosis
What is the unique organelle found in Langerhan’s cells?
Racquet organelles (Birbeck granules)
What is the function of racquet organelles in the Langerhan’s cells?
They are thought to be involved in the trafficking of antigens
Name the organelle
Racquet organelle (Birbeck granules)
What are Merkel cells?
These are mechanoreceptor cells found in the basal layer, between the keratinocytes and nerve fibres
What are some examples of mucosal membranes?
- Eyes (Ocular mucosa)
- Mouth (Oral mucosa)
- Nose
- Genito-urinary
- GI tract
What are some of the regions of the oral mucosa?
Masticatory mucosa
Lining mucosa
Tongue papillae
How is the masticatory mucosa adapted for function?
It is keratinised to deal with friction and pressure of mastication
What are some structures present in the ocular mucosa?
Lacrimal glands
Eye lashes
Sebaceous glands
What is the name of the junction between the epidermis and dermis?
Dermo-epidermal junction
What are some of the functions of the dermo-epidermal junction?
- Support, anchorage, adhesion, growth and differentiation of basal cells
- Semi-permeable membrane acting as barrier and filter
What is the name of the junctions between the cells of the epidermis and the extracellular matrix, holding the epidermis to the dermis?
Hemidesmosomes
What is the name of the junctions connecting the epidermal cells together at the dermo-epidermal junction?
Desmosomes
What are some conditions that affect the demo-epidermal junction?
- Bullous pemphigoid
- Epidermolysis bullosa (Genetic defect affecting molecules involved in skin adhesion)
What are some structures that make up the dermis? (9)
- Ground substance
- Cells (fibroblasts,macrophages, mast cells, lymphocytes, Langerhans cells)
- Collagen (Type I and III)
- Elastin
- Blood vessels
- Lymphatics
- Nerves
- Muscles
- Ground substances (Hyaluronic acid + Chondroitin suphate)
What is the function of fibroblasts in the dermis?
Fibroblasts maintain ground substance by producing collagen
What are the 2 main regions of the dermis?
- Papillary dermis - Thin, just beneath depidermis
- Reticular dermis - Thick, bundles of type 1 collagen, containing the appendages
How does the dermis of the skin differ in different ethnic backgrounds?
Asian, Hispanic/ Latino and African/ American skin types have a thicker dermis (proportional to degree of pigmentation) than caucasian skin types and smaller collagen fibres
African/ American skin has larger and more numerous fibroblasts
What structure allows the bypassing of superficial capillaries in cold weather?
AV shunts (Anastomoses)
How are lymphatic vessels arranged in the skin?
Lymphatic vessels in the skin are arranged into sub-epidermal meshed networks
Smaller non-contractile vessels feed into larger contractile lymphatic trunks, allowing for the continual drainage of plasma proteins, extravasated cells and excess interstitial fluid
What are the functions of the lymphatic vessels in the skin?
These lymphatic vessels have important immune functions, such as allowing the passage of Langerhans cells to lymph nodes and the channelling of micro-organisms / toxins towards the lymph nodes
What is a condition affecting the lymphatic vessels of the skin?
Lymphoedema
What types of nerves are present in the skin?
Somatic sensory nerves
Motor nerves
Autonomic nerves
What structures provide signals to somatic sensory nerves in the skin?
Free nerve endings
Special receptors
What are some examples of special receptors found in the skin?
Pacinian corpuscles (Pressure sensors)
Meissner’s corpuscles (Vibration sensors)
What structures in the skin are supplied by the autonomic nervous system?
Blood vessels
Nerves
Glands
What are the 3 main types of glands in the skin?
- Sebaceous glands
- Apocrine glands
- Eccrine glands
What type of gland is shown?
Sebaceous gland
Where are sebaceous glands most concentrated in the body?
Face
Chest
Back
What is the function of sebaceous glands?
They perform holocrine secretion of sebum into the opening of pilary canal (Hair canal)
What makes up sebum?
- Squalene
- Wax esters
- TG
- FFA
What stimulates sebaceous gland activity?
Hormones (Androgens), they are quiescent pre-puberty
What is the function of sebum?
Sebum helps to control moisture loss and protect against fungal infection
What is a common pathology of the sebaceous glands?
Acne vulgaris
What type of gland is shown?
Apocrine sweat gland
Where are apocrine sweat glands most commonly found?
Axillae, groin, eyelids, ears, mammary and perineal regions
Where in the skin do apocrine glands travel?
From the dermis of the skin into the pilosebaceous unit (Hair follicle)
What stimulates apocrine sweat glands?
Androgens
What is the function of apocrine sweat glands?
They secrete an oily fluid, the function of which is unknown
What causes body odour?
Bacterial decomposition of the oily fluid produced by apocrine sweat glands
What is a condition affecting the apocrine sweat glands?
Hidradenitis suppuritiva - Apocrine inflammation
What gland is shown?
Eccrine sweat glands
What simulates eccrine sweat glands?
Sympathetic, cholinergic nerve supply
What are some stimuli to eccrine gland action?
Mental stimulation
Thermal stimulation
Gustatory stimulation
Where are eccrine glands found?
They cover the whole skin surface
What is the function of eccrine sweat glands?
Eccrine sweat glands play a role in ultrafiltration of the body’s fluids and contribute around the same amount as 1 kidney
They function in cooling by evaporation and moistening of palms and soles to aid grip
What is a condition affecting the eccrine sweat glands?
Miliaria (Sweat rash) - Heat induced blockage of the eccrine glands
What are the components of the pilosebaceous unit?
Hair follicle
Hair shaft
Arrector pili muscle
Sebaceous glands
What are the 3 main segments of the hair follicle?
Infundibulum
Isthmus
Hair bulb
What causes hair pigmentation?
Melanocytes producing melanin pigment: More melanin means darker hair
Where does hair pigmentation by melanocytes occur?
Above the dermal papilla
Describe the features of hairs in those of Asian descent
Straight, round, large cross-sectional area, fastest growth rate; strong and durable
Describe the features of hair in those of African descent
Lowest hair density, curly/ spiral (ellipsoidal cross section), slowest growth rate; most fragile
Describe the features of hair in those of Caucasian descent
Straight to curly; round/ oval cross section; highest hair density, most moisture content; comparable strength with Asian hair
What are the 3 main types of hair?
Lanugo hair
Vellus
Terminal
What is lanugo hair?
Fine long hairs covering the foetus (Shed about 1 month before birth)
What is vellus hair?
Fine, short hair covering much of the body surface (Replaces lanugo hairs before birth)
What is terminal hair?
Long, coarse hairs (E.g. scalp and pubic area), the growth of which is influenced by androgen levels
What are the 3 phases of hair growth?
Anagen - Growing
Catalan - Involuting
Telogen - Resting
How does hair stability change in pregnant women?
In pregnancy, increase in hormone levels causes increased hair stability, however, around 2 weeks post partum, hair stability and hair growth in anagen phase will plummet
What are the patterns of male pattern hair loss?
Typical
Vertex
Anterior
What is the name of the folds at the edges of the nails?
Lateral nail folds
What is the name of the white semicircle at the bottom of the nail?
Lunula
What is the name of the skin flap that lays over the nail plate at the bottom?
Cuticle
What is the name of the fold of skin at the bottom of the nail?
Posterior nail fold
What is the name of the structure beneath the the end of the nail?
Hyponychium
What are the 3 layers of the nail plate?
Dorsal - Top
Intermediate
Ventral - Bottom
Describe the structures of the nail and nail bed
What are the functions of the skin? (6)
- Barrier function
- Metabolism and detoxification
- Thermoregulation
- Immune defence
- Communication
- Sensory functions
What does the skin form a physical barrier against?
Friction
Mechanical trauma
UV radiation (Via melanin caps)
What does the skin provide a chemical barrier against?
Irritants
Allergens
Toxins
What are some conditions that affect the barrier function of the skin?
- Steroid-sulphatase deficiency X-linked ichthyosis
- Cumulative irritant hand dermatitis
- Ulcer
What are some of the metabolic functions of the skin?
- Vitamin D metabolism
- Thyroid hormone metabolism
- Defence against chemicals, drugs, pollutants and sunlight
Describe the skins function in vitamin D metabolism
Ultraviolet B radiation is responsible for conversion of 7-dehydrocholesterol into vitamin D3 (Cholecalciferol) in the skin
Vitamin D3 is stored as hydrocycholecalciferol in the liver and is converted to 1,25-dihydroxycholecalciferol in the kidneys
How does the skin function in thyroid hormone metabolism?
The skin plays an important role in the conversion of thyroxine (T4), to Triiodothyronine (T3), which is biologically active
How does the skin detect temperature changes?
Thermoreceptors (E.g. Krause end bulbs cold, Ruffini ends warm)
What are some ways in which skin can react to temperature changes?
Behaviour control
Sweating
Shivering
Av anastomoses in the skin changing blood flow
How does the skin provide immune defence?
Secretion of anti-microbial oils and the presence of Langerhan’s and T cells within the skin
What are some failures of skin immune defences?
- Crusted scabies
- Tuberculoid leprosy (Hansen’s disease)
- Granulomatous disease
- Eczema herpeticum (Disseminated HSV infection)
- Chronic discoid lupus erythematosus
How does the skin play a role in communication?
Through visual changes, odour and sociosexual behaviour
What is meant by stigma?
The situation of an individual who is disqualified from full social acceptance
What are some of the sensory functions of the skin?
- Touch
- Pressure
- Vibration
- Pain
- Itch
- Hot/Cold
What are some of the consequences of barrier function failure?
- Fluid loss → Dehydration
- Protein loss → Hypoalbuminaemia
- Infection
What is a consequence of thermoregulatory failure of the skin?
Heat loss -> Hypothermia
How can skin diversity present
- Differences in how the same disease presents in different people
- Different diseases more likely in certain peoples
- Variation between severity in diseases between people
- Differences in presentations caused by practises of specific cultures (e.g. cupping)
What are some skin differences that can affect how skin diseases affect someone?
- Skin colour
- Density of keratin layer
- Function of the dermo-epidermal junction
- Susceptibility to irritants
What are some of the immune structures found in the skin?
- Kertin layer (Stratum corneum)
- Stratification
- Immune cells (Langerhans, T-cells, dendritic cells)
- Keratinocytes
- Cytokines, chemokines, eicosanoids
- Antimicrobial peptides
How does the keratin layer form a barrier against pathogens?
It is a tough, lipid rich physical barrier against infection, formed by the terminal differentiation of keratinocytes to corneocytes
They produce antimicrobial peptides (AMPs) that can directly kill pathogens and cytokines and chemokines, which recruit and regulate cells of the adaptive and innate immune system
What is the function of Langerhan’s cells in the skin?
They act as sentinels in the epidermis and process lipid antigen and microbial fragments, before presenting them to effector T cells
From what lineage do Langerhan’s cell arise?
From the macrophage lineage
What lymphocytes are mostly present in the epidermis?
CD8+ T-cells
What lymphocytes are found in the dermis?
CD4+ and CD8+ T-cells
What skin condition is associated with Th1 cells?
Psoriasis
What skin condition is associated with Th2 cells?
Atopic dermatitis
What skin conditions are associated with Th17 cells?
Psoriasis and atopic dermatitis
What are the functions of Th1 cells?
Activates macrophages to destroy microorganisms, secrete IL2 and IFN-gamma
What are the functions of Th2 cells?
Help B cells to make antibodies, secrete IL4, IL5, IL13
What are the functions of the Th17?
Modulate skin and mucosal immunity, secretes IL17
What is the main type of dendritic cell found in the dermis?
Plasmacytoid dendritic cells
What is the function of plasmacytoid dendritic cells?
They are the main source of alpha-interferon and transmit information to T and B cells
They constantly rotate between the skin and the lymph nodes and will activate T and B cells once they present antigens
