Epithelia Flashcards
WHAT ARE EPITHELIA? • EPITHELIAL CELL TYPES • EPITHELIAL JUNCTIONAL COMPLEXES • EPITHELIAL STRUCTURAL FEATURES • POLARISED EPITHELIUM • LEARNING OUTCOMES
what are epithelia
Epithelial cells form tissue – an epithelium • One of four tissue types ( nervous, connective, epithelial, msucle ) • Many variations of epithelial tissue • Structure (cell shape) • Layers • The structure of an epithelium relate to and enables its functionEpithelia form a continuous sheets of cells. They are heterogenous cell types specialized for distinct functions . Airway lined with cilliated stratified pseudocolumnar epithelium , kidneys lined wih columnar too . Epithelia is all over the body , they act as barrirs between the external and body and between fluid compartments. They act to transport molecules via absorptive processes and secretory processes
Epithelial cell types
columnar, cuboidal, squamous
Every cell comes into contact with basement membrane seperating it from the underlying tissue
Simple cuboidal
Description: • Single layer of cube-like cells • Same cell height/width • Central nuclei Function: • Secretion and absorption Localisation: • Kidney tubules • Ducts • Secretory portions of small glands • Ovary surface
Simple columnar epithelium
Description: • Single layer of cube-like cells • Same cell height/width • Central nuclei Function: • Secretion and absorption Localisation: • Kidney tubules • Ducts • Secretory portions of small glands • Ovary surface
Simple squamous
Description: • Flat cells with disc-shaped nuclei Function: • Passage of materials by diffusion and filtration • Secretion of lubricating substances Localisation: • Kidney glomeruli, alveoli of lungs, blood vessels, lymphatic vessels • Often given another name as considered “specialised” e.g. endothelium, mesothelium, endocardiu
SImple pseudostratified
Description: • Look like there are multiple layers • But all cells are in contact with the extracellular matrix • Several “layers” of nuclei • Ciliated / non-ciliated Function: • Secretion of mucus Localisation: • Non-ciliated: vas deferens, large glands • Ciliated: trachea, upper respiratory tract
stratified cuboidal
Description: • Look like there are multiple layers • But all cells are in contact with the extracellular matrix • Several “layers” of nuclei • Ciliated / non-ciliated Function: • Secretion of mucus Localisation: • Non-ciliated: vas deferens, large glands • Ciliated: trachea, upper respiratory tract
Stratified columnar epithelium
Description: • Look like there are multiple layers • But all cells are in contact with the extracellular matrix • Several “layers” of nuclei • Ciliated / non-ciliated Function: • Secretion of mucus Localisation: • Non-ciliated: vas deferens, large glands • Ciliated: trachea, upper respiratory tract
stratified squamous epithelium
Description: • Thick membrane composed of multiple cell layers • Superficial portion of cells are squamous “flattened” • Basal cells are cuboidal/columnar and metabolically active • Keratinised and non-keratinised Function: • Protects underlying tissue Localisation: • Non-keratinized: oesophagus, mouth, vagina, urethra, anus • Keratinized: epidermis of skin
Transitional epithelium
Description: • Several cell layers • Resembles stratified squamous and stratified cuboidal epithelium • Basal cells are cuboidal/columnar • Superficial cells are dome-shaped or squamous-like depending on stretch Function: • Stretches readily permitting distension of urinary organ Localisation: • Ureter, bladder, parts of urethra
Junctional proteins in epithelia
Epithelial cells have unique structural features: • Junctional proteins • Connect cells to each other • Connect cells to basement membrane • Some allow INTERcellular communication e.g. transport of ions • Enable POLARISATION
tight junctions
Also known as “Zonula Occludens” • Most apical junction • branching network of strands of proteins • “zipper” together neighbouring epithelial cells • Defines apical/basolateral boundary • Prevent passage of molecules and ions • Permeability barrier – “gatekeeper” of the paracellular pathway
adhering junctions
Also known as “Zonula Adherens” • More basal than tight junctions • A “belt” that encircles epithelial cell directly beneath the tight junction • Allow cell-cell adhesion via interaction between extracellular domains • Connect intracellularly to the actin cytoskeleton
desmosomes
Also known as “macula adherens” • Spot like adhesions arranged on the lateral surface of cells • Allow cell-cell adhesion via interaction between extracellular domains • Intracellular cytoplasmic tail associates with adaptor and signalling proteins
gap junctions
Lateral edges of epithelial cells, allow intercellular communication • Permeable to large molecules ≤1 kDa • Ions, metabolites, signalling molecules • Permeability of GC varies between cells and can change in response to stimuli • Not specific to epithelial cells, nearly all cells have gap junctions • Consists of 2 connexons, one from each cell, each formed of 6 connexin subunits
hemidesosomes
Localised to the basal surface • Attach epithelial cells to the basement membrane (rather than to an adjacent cell) • Stud-like structure • Membrane-spanning proteins • Integrins
microvilli
Apical surface area can be increased by presence of a “brush border membrane” • Microvilli • Proximal tubule of kidney, small intestine • Enables absorption • Up to 20x greater surface area
basolateral surface
Basolateral surface area can also be increased by: • Lateral interdigitations • Basal infoldings • Important for transport across the epithelial layer • Particularly seen in basolateral surface of renal epithelium
cillia
Cilia also present on apical surface of some epithelia • Longer/larger than microvilli • Cilia move back and forth to move particles • Ciliated cuboidal/columnar: • Oviduct (though non-ciliated secretory epithelium here also) • Pseudostratified columnar: • Respiratory tract (lower larynx, trachea, bronchi) • Vas deferens, epididymis
keratin
Stratified squamous epithelium can contain keratin, but defined by how much • Lots – “Keratinized” • Very little – “Non-keratinized” • Keratinized stratified squamous epithelium • A layer of keratin at the surface in the dead squamous cells, appear anuclear • “waterproof” • Examples include the skin (epidermis)
The tight junction complex distinguishes an APICAL and BASOLATERAL membrane of the cell • Effectively a “top” and a “bottom” • Each domain expresses distinct lipid and protein components • This demarcation of the cell membrane is POLARISATION
Example: epithelium lining renal tubules Tight junctions form complexes between cell Gives rise to an apical surfaces And a basolateral surface
epithelial polarisation
So what? • Enables transport of ions, solutes and water • Transepithelial transport • Often AGAINST their ionic (electrical) and concentration (chemical) gradient
transepithelial transport is either
Paracellular transport occurs between the cells via tight junctions • Epithelia are considered “leaky” or “tight” depending on tight junctions • Established electrochemical gradients “drag” molecules
Transcellular transport • Through cell • Crosses both apical and basolateral membranes • Requires transporters/channels • Usually energised by a pump e.g. Na +/K + ATPase • Establishes an electrochemical gradient favourable for movement of ion/solute against its electrochemical gradient
Paracellular • Transcellular • (Or both)
Transepithelial transport examples - intestine - Cl- secretion into lumen, airway Cl - secretion into lumen , kidney sidum reabsorption into circulation
