epithelial cells

simple

1 layer of cells

stratified

multiple layers of cells

pseudostratified

1 layer of cells, nuclei take different positions

squamous

flat, plate-like cells with central nucleus

cuboidal

isodiametric cells with central nucleus

columnal

nucleus at base of cell, height much greater than width

keratinized

nuclei no longer present, have been replaced with keratin

cilia

• microtubule-based - microtubules arranged in doublets

• larger than microvilli

• move substances across a surface by beating in time together

primary cilia

• non-motile

• sense movement and bend to initiate signalling

stereocilia

• actin based

• found in inner ear

• have fluid surrounding them - when someone speaks, there is a movement of fluid so cilia start to bend and vibrate so the membrane above cilia vibrates so stereocilia of inner ear vibrate which stimulates the nervous system so the sound is heard

microvilli

• actin-based, numerous, all parallel to eachother

• non-motile

• very small

• increase surface area - found in highly absorptive cells

• filaments have polarity - fast growing and slow growing end

cell junctions - function

• allow cell:cell communication

• attach cells to eachother via the cytoskeleton which provides structural strength

• selectively maintain a seal between epithelial cells

cell junctions - desmosomes

• composed of different molecules

• have intermediate filaments which are attached to the membrane

• involved in attaching cells together via the cytoskeleton

cell junctions - hemidesmosomes

• linked to intermediate filaments which bind to transmembrane proteins

• anchor epithelial cells to basement membrane - located on basal side of epithelia

• get blisters when intermediate filaments are broken

cell junctions - adherens junctions

• glue cells to one another - one adheren binds to another one on a separate cell

• linked to filaments which are part of the actin cytoskeleton

cell junctions - tight junctions

• form seal between cells

• consist of transmembrane proteins

• form selective barrier which allows certain molecules into/out of cells - stops lipids and proteins moving between basal and apical layer

cell junctions - gap junctions

• allow communication between cells

• cells activities coordinated with other cells - eg contraction, epithelia involved in fluid transport

• have proteins called connexins - regulate pore opening

basement membrane

• located below basal surface of epithelial cells, between epithelial cells and connective tissue

• layer of reticular fibres lie below it

function of basement membrane

• structural attachment - laminin used to attach epithelial cells to basement membrane, basement membrane attached to connective tissue

• compartmentalisation - important for cancer, prevents cancer from invading surrounding tissue and makes treatment easier, isolates epithelia, muscles and nerves from connective tissue

• filtration - allows selective movement to/from connective tissue

• tissue scaffold - basement membrane has growth factors so that cells stick to it and arrange in a specific way

• signalling - contains lots of signalling molecules, involved in regulating epithelial cell behaviour

glandular epithelia

secretory cells which are arranged into groups called glands

exocrine glands

• products secreted via ducts

• product secreted via epithelial cells

endocrine glands

• ductless

• product secreted into bloodstream and is called a hormone

secretory mechanisms of glands

• merocrine - membrane bound vesicles containing product fuse with the plasma membrane

• apocrine - release of product in membrane bound vesicles surrounded by cytoplasm and plasma membrane

• holocrine - cell containing product undergoes apoptosis and whole cell is shed

• exocytosis - product of cell diffuses into bloodstream (endocrine)