Epithelia and the Integumentary System Flashcards
what are epithelial tissues?
- they are at the boundaries of the body
- they cover surfaces, line cavities and form glands
- they separate controlled internal environment from uncontrolled external environment
where does epithelia develop from?
3 germ layers:
- endoderm = GI lining
- mesoderm = lining of cardiovascular system
- ectoderm = epidermis/skin
what functions do epithelia perform?
- protection
- skin is first defence against microbes - diffusion
- alveoli in lungs - absorption
- small intestine - secretion
- glands
what are the 4 common properties of epithelia?
- epithelial cells have polarity
- they have a basement membrane
- they are involved in cell adhesion and communication
- they are involved in cell replacement
what is the polarity of epithelial cells?
- apical surface is to external environment (cilia, microvilli)
- basolateral surfaces have specialised function for absorption
- polarity determines specialisation
what is the basement membrane of epithelial cells?
- separates epithelia from underlying tissue
- allows access to nutrients, ions and proteins to regulate growth of cells
- restricts migration and development of cancer
what are the 2 layers of the basement membrane and what are their roles?
- Basal lamina (BL)
- provides mechanical support by tethering epithelial cells together to resist stretching and tearing - Reticular lamina (RL)
- reticular fibres anchor BL to underlying connective tissue via collagen and elastin
how are epithelial cells used in cell adhesion and communication?
- gap junctions allow passage of water, ions and small molecules: lateral communication
- modification of lateral membranes allow passage between cells
- cell matrix attachments bond epithelial tissues to connective tissue beneath basal surfaces
- adhesive junctions link cytoskeletons from cell to cell
- adhering junctions form bundles of active filaments around cells
- myosin filaments enable contractioon of cells
how are epithelia involved in cell replacement?
- the more hostile the external environment, the greater the cell death
- tissue homeostasis via cell replacement from stem cells
intestine epithelium renews within 5 days
interfollicular epidermis takes 4 weeks to renew
- lung epithelium takes 6 months to renew
what are gap junctions?
- transmembrane claudins near the apical surface
- control paracellular diffusion of materials between epithelia
- claudins vary from cldn-18 to cldn-3
what are hemidesmosomes?
- most prominent attachment between cell and cell-matrix
- anchoring junction
- anchors intermediate filaments in a cell to the ECM
what are integrins?
- proteins which anchor the cell the laminin
- attach to extracellular keratin
what are adherens junctions?
- connect actin filament bundle in one cell with the actin filament bundle of another cell
what are tight junctions?
- they cell the gap between epithelial cells
- prevent paracellular transport
what are desmosomes?
- connects intermediate filaments of one cell to those in the next cell
- found in tissues under high amounts of mechanical stress
how is epithelial cell fate regulated?
- switched expression of adhesion cadherins induces mesenchymal cells to form epithelium
- epithelium is in close contact with mesenchyme
- mesenchymal tissues control epithelial cell fate
- formation of epithelium is reversible
- epithelial-mesenchymal transitions are important in embryonic development and pathology e.g. cancer
what are the 4 major types of epithelium?
- simple: single layer of cell
- lungs - stratified: many layers
- skin - pseudostratified
- upper respiratory tract - transitional (urothelium)
what is simple squamous epithelia?
- appearance of thin scales: flattened nuclei and cells
- facilitates rapid passage of molecules e.g. alveoli, serosa in intestine
what is simple cuboidal epithelia?
- secretion and absorption of molecules requiring active transport
- kidney tubules, ducts of glands
- nuclei in centre of cell layer
what is simple columnar epithelia?
- cuboidal presentation - elongation
- with/without cilia/microvilli on apical surface
- absorb/secrete molecules using active transport
- majority of GI tract
- ciliated surfaces line fallopian tube to move egg, and in respiratory tract to remove particulates
what is pseudostratified columnar epithelia?
- single layer of cells with appearance of multiple layers due to nuclei at different levels and irregular nature
- all cells contact the basement membrane
- ciliated or unciliated
- ciliated cells can be interspersed with goblet cells
what is stratified squamous epithelia?
- most common type of stratified epithelia in the body
- apical cells appear squamous (thin and flattened)
- basal cells are cuboidal or columnar
- top layer can be covered with dead cells containing keratin
- areas of high abrasion
- skin upper layers are keratinised, oesophagus layers are non-keratinised
what is stratified cuboidal epithelium?
- less common
- found in ducts and glands
what is stratified columnar epithelium?
- rare
- found in conjunctiva, pharynx, male urethra and embryo
- allows tissue to stretch and contract
- contains goblet cells to secrete mucin for lubrication
what is transitional epithelia?
- cells have round shape when relaxed
- allows change in shape distension without damaging epithelial lining
what is glandular epithelia?
- they have complex ducts
- contain single cell types of a variety of cell types
- they secrete different substances:
mucus = mucus gland protein = serous gland
what are the 2 types of gland?
- exocrine: secretion via ducts
2. endocrine: secretion without ducts
what are unicellular goblet cells?
- mucin-secreting glands
- scattered within simple epithelia, such as cuboidal, columnar, and pseudostratified epithelia
what are multicellular acinar/acinus cells?
- any cluster of cells
- acinar cells are found in the pancreas and have exocrine function
what is the integumentary system?
- skin and its accessory organs: hair, nails and cutaneous glands (sweat, sebaceous, mammary)
what is skin?
- largest organ in the body - 12-15% of body weight
- barrier between internal and external environments
- give insight to internal health via issues inside the body being displayed on skin
- regulates loss of fluid, electrolytes and proteins
what are the 3 layers of the skin?
- epidermis (uppermost)
- dermis
- hypodermis: areolar and adipose tissues between skin and muscles
- not part of integumentary system
what is the dermis?
- connective tissue layer beneath epidermis
- thicker than epidermis (0.2mm eyelids, 2mm palms)
- contains collagen, elastic and reticular fibres
- contain fibroblasts to produce laminin and fibronectin of the ECM
- contains nerve endings, blood and lymphatic vessels
- can induce arteriovenous anastomoses (thermoregulation)
what are the 2 zones of the dermis?
- papillary layer
- thin region of loose connective tissue (areolar)
- allows mobility of leukocytes, mast cells and macrophages - reticular layer
- thick layer of dense, irregular tissue
- less cells
- contains adipocyte clusters
- weight gain causes striae (stretch in skin)
what is the dermal-epithelial boundary?
- wave boundary of finger-like projections
- dermal papillae: raised areas
- epidermal ridges: sunken areas
- less risk of sheering between layers
what is the role of dermal papillae?
- they facilitate nerve fibres reaching close to surface of epidermal layer in highly sensitive areas
what is the epidermis?
- keratinised stratified squamous epithelium
- lacks blood vessels: diffusion of nutrients occurs from underlying dermal layer and connective tissue
- self-generation throughout life every 2-4 weeks
what are the 2 types of skin?
- thick skin
- palms of hands and feet
- no hair
- contains 5 stratum layers - thin skin
- rest of body
- contains hair
- contains 4 stratum layers
what are the 5 stratified epidermal layers, from base to uppermost?
- stratum basale
- stratum spinosum
- stratum granulosum
- stratum lucidum
- stratum corneum
what are the cells of the epidermis?
- stem cells
- keratinocytes
- melanocytes
- langerhans cells
- merkel cells
what are stem cells?
- undifferentiated cells which can divide to form keratinocytes in deep layer of epidermis
- the only cells in contact with basement membrane, so maintain their ability to replicate
what are keratinocytes?
- make up 80-90% of epidermis
- synthesise keratin which forms majority of upper surface
what are melanocytes?
- synthesise melanin pigment and release it
- melanin is UV absorbant, antioxidant and protect skin from UV damage
- melanin provides pigmentation but also depends on carotene pigments and the oxygen content of blood (red pigment from haemoglobin)
how do melanocytes protect against UV radiation?
- melanosomes are phagocytosed by keratinocytes to surround the nucleus on the sun side
- melanocytes spread amongst keratinocytes and shed fragments containing melanin
- keratinocytes phagocytose the fragments, internalise them and accumulate melanin around their nucleus to protect from UV radiation
what are langerhans cells?
- dendritic cells of the skin
- macrophages which originated from bone marrow and migrated to epidermis
- density of 800 per mm2
- immunological barrier to defend against pathogens and toxins that may penetrate the skin
- maintain homeostasis and decide whether to launch an immune response
what is the action of langerhans cells?
- migrate to lymph nodes for antigen presentation
- present antigens to skin lymphocytes
- skin lymphocytes circulate to the secondary lymphoid organs
- this activates an immune response
what are merkel cells?
- touch receptors
- associate with dermal nerve fibres to form a Merkel disc
- low numbers
what is the stratum basale (germinativum)?
- single layers of low columnal and cuboidal cells
- consists of undifferentiated mitotically active keratinocytes
- melanocytes give skin colour
- merkel cells connected to nerves
- dermal papilla dispersed with epidermal ridges at junction of tissues
- hemidesmosomes anchor intermediate filaments in cells to cell-matrix (only found in this layer)
- desmosomes also do this in all layers
what happens to the undifferentiated keratinocytes in the stratum basale?
- keratinocytes are attached to basement membrane and generate cells for superficial layers
- the keratinocytes migrate up to replace layers of skin
what are the two main melanin pigments?
- pheomelanin = red soluble pigment
- eumelanin = brown insoluble pigment
what is the stratum spinosum?
- several layers of keratinocytes
- usually thickest layer
- deepest cells are mitotic and are pushed upwards
- as cells get towards the surface they cease to divide
- produce keratin filaments which cause cells to flatten and become squamous at surface
- keratinocytes linked by desmosomes
- tight junctions allow water retention in skin
- dendritic cells are present
what is the stratum granulosum?
- 3-5 layers of flat keratinocytes
- post-mitosis so cells no longer divide
- dark-staining keratohyalin granules bind to cytoskeleton and convert to keratin
- tight junctions prevent leakage of solutes to intracellular space
- cells undergo apoptosis
- produce glycolipid-filled vesicles which spread over cell surface to form waterproof barrier between spinosum and granulosum
what is the stratum lucidum?
- thin translucent zone
- only exist in thick skin: hands and feet
- densely packed keratinocytes which have no nuclei or organelles, hence translucent
- protects areas prone to damage from mechanical stresses
- continual layer - there are no distinct cell boundaries
- granules of elerdin with proteoplasm (produce of keratohyalin)
what is the stratum corneum (uppermost layer)
- 15-30 layers
- comprised of dead keratinocytes which flake off - stratum disjunctum
- resistant to abrasion, penetration and water loss
- cohesive
what is in the cornified envelope of the stratum corneum?
- keratins
- enclosed with insoluble amalgam of proteins
- cross-linked by transglutaminases
- surrounded by lipid envelope
- lamella bodies are secretory organelles packed with lipids to stop water loss
what is the stratum disjunctum?
part of the corneum which flakes off:
- layers of flattened corneocytes are cornified from expressing keratin that has been crosslinked by transglutaminase
- corneo-desmosomes organise corneocyte adhesion to regulate desquamation
- mitotic cells replace flaked-off cells
what are the 3 accessory structures of the integumentary system?
- nails
- hair (pilus)
- glands
what are nails?
- derivatives of stratum corneum as it invaginates onto one side of finger
- composed of dead cells densely packed with hard keratin fibres
- new cells added by mitosis in the nail matrix, causing nail to lengthen
how can appearance of nails show underlying health issues?
- iron deficiency: nail becomes flat or concave
- long term hypoxemia (heart defect): nail becomes clubbed
what is hair in the integumentary system?
- slender filament of keratinised dead cells from follicle
- found almost everywhere in different densities: 50/cm2 in trunk, 100000/cm2 on scalp
how does hair change during lifetime?
- lanugo: fine, downy, unpigmented
- foetus - vellus: fine, unpigmented, 2/3 women, 1/10 men
- terminal hair: longer, coarser, pigmented
- eyebrows, eyelashes, scalp post-puberty, pubic, male facial
what is the hair follicle?
- hair bulb grows around bud of vascular connective tissue: dermal papilla
- dermal papulla provides nutrients to cells of hair shaft
- mitotically active hair matrix to push shaft up
medulla = loosely arranged cells cortex = keratinised cuboidal cells cuticle = surface layer of scaly cells
what are 5 types of glands?
- merocrine sweat glands
- watery perspiration, associated with myoepithelial cuboidal cells - apocrine sweat glands
- ducts lead to follicles
- scent glands respond to stress and sexual stimulation - sebaceous glands
- oily sebum lubricating skin and hair
- secrete lipids as antibacterial shield
- holocrine: cells break down and are replaced by mitosis - ceruminous glands: external ear wax
- mammary glands
what is barrier function?
- prevents water loss, physical injury and infection by microbes
- cross-linked keratin layer upon a scaffold of keratinocytes
- slightly acidic (pH 4-6)
- bacteriocidal agents: saturated and unsaturated fatty acid inhibit growth of bacteria
- lysozyme cleaves cross-linkages in bacterial cell walls
- C6H inhibits induction of antibiotic resistance
- immunological barrier
how does the integumentary system regulate temperature?
- skin contains thermoreceptors in epidermis
- counter current heat exchange between arterial and venous blood flow in dermis
- arteriovenous anastomoses assists thermoregulation
- hypothalamus regulates temperature regulation
how does the hypothalamus detect changes in temperature?
- insulation by piloerection of hairs
- cooling by sweating
- vasodilation/vasoconstriction by anastomoses to control temperature
