Epithelia

Question 1

what are epithelia

Answer 1

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

Question 2

Epithelial cell types

Answer 2

columnar, cuboidal, squamous

Question 3

Every cell comes into contact with basement membrane seperating it from the underlying tissue

Question 4

Simple cuboidal

Answer 4

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

Question 5

Simple columnar epithelium

Answer 5

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

Question 6

Simple squamous

Answer 6

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

Question 7

SImple pseudostratified

Answer 7

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

Question 8

stratified cuboidal

Answer 8

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

Question 9

Stratified columnar epithelium

Answer 9

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

Question 10

stratified squamous epithelium

Answer 10

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

Question 11

Transitional epithelium

Answer 11

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

Question 12

Junctional proteins in epithelia

Answer 12

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

Question 13

tight junctions

Answer 13

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

Question 14

adhering junctions

Answer 14

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

Question 15

desmosomes

Answer 15

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

Question 16

gap junctions

Answer 16

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

Question 17

hemidesosomes

Answer 17

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

Question 18

microvilli

Answer 18

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

Question 19

basolateral surface

Answer 19

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

Question 20

cillia

Answer 20

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

Question 21

keratin

Answer 21

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)

Question 22

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

Question 23

epithelial polarisation

Answer 23

So what? • Enables transport of ions, solutes and water • Transepithelial transport • Often AGAINST their ionic (electrical) and concentration (chemical) gradient

Question 24

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

Answer 24

Paracellular • Transcellular • (Or both)

Question 25

Transepithelial transport examples - intestine - Cl- secretion into lumen, airway Cl - secretion into lumen , kidney sidum reabsorption into circulation