Epithelial cells Flashcards
Specialised functions of epithelial cells + examples
form a covering for body surfaces e.g. skin, gut and ducts
secretory glands e.g. salivary tissue and liver
absorption
joining of epithelial cells
joined by adhesion specialisations
anchor cytoskeleton to neighbours and underlying/surrounding extracellular materials
types of cell shape
squamous - flat, plate like cuboidal - height and width similar columnar - height 2-5x greater than width rounded polygonal
simple epithelium
single layer where all cells are in contact w/ basement membrane
stratified epithelium
several layers where only bottom layer of cells contact basement membrane
pseudostratified epithelium
epithelial cells appear to be arranged in layers, but are all in contact w/ basement membrane
transitional epithelium
special type of stratified epithelium
restricted to lining of urinary tract
varies between cuboidal and squamous, depending on degree of stretching
simple squamous epithelium
single layer of flat, plate like cells
nuclei are flattened, cytoplasm distinct
types of simple squamous epithelium
endothelium - lines blood vessels
mesothelium - lines abdominal and pleural cavities
simple cuboidal epithelium
single layer of cells w/ similar height, width and depth
centrally placed nucleus
simple columnar epithelium
single layer of cells with height 2-5x their width
basal nuclei arranged in ordered layer
stratified squamous epithelium
several layers of cells, lower layer contacts basement membrane
superficial part has squamous cells
middle and basal layers have pyramidal or polygonal cells
how is structural integrity of epithelium maintained, and how is it mediated?
adhesion of constituent cells to each other and structural extracellular matrix
cell membrane proteins act as specialised cell adhesion molecules
specialised areas of cell membrane are incorporated into cell junctions
three types of cell membrane and their functions
occluding junctions - link cells together to form an impermeable barrier
anchoring junctions - link cells to provide mechanical strength
communicating junctions - allow movement of molecules between cells
main functions of occluding junctions
prevent diffusion of molecules between adjacent cells
prevent lateral migration of specialised cell membrane proteins - delineating and maintaining specialised cell membrane domains
what is the occluding function performed by?
intramembranous proteins - mediate adhesion of adjacent cells
where are occluding junctions well developed?
small bowel:
prevent digested macromolecules from passing between cells
confine specialised cell membrane areas involved in absorption/secretion to luminal side of cell
where else are occluding junctions important?
in cells actively transporting a substance, e.g. active transport of an ion against a concentration gradient
prevent back-diffusion of substance
how are intramembranous proteins arranged?
serpiginous intertwining lines (sealing strands)
examples of intramembranous proteins
occludin and claudin
what protein is mainly responsible for diffusion barrier?
claudin
function of anchoring junctions
provide mechanical stability - allow groups of epithelial cells to function as a cohesive unit
structure of anchoring junction
intracellular link proteins attach cytoskeletal filaments of adjacent cells to transmembrane proteins
extracellular interaction is mediated by additional extracellular proteins or ions, e.g. Ca2+
example of a transmembrane protein
cadherin - uses Ca2+
structure of a specific cadherin junction
F-actin fibres are linked by alpha and beta catenins, vinculin and alpha actinin to E-cadherin - Ca2+
focal contact structure and function
link actin filament network of a cell to extracellular matrix
bundles of actin filaments interact w/ alpha actinin, vinculin and talin (actin-binding proteins) to link w/ integrin
desmosomes function
connect intermediate filament networks of adjacent cells
hemidesmosomes function
connect intermediate filament network of cells to extracellular matrix
where are adherent junctions most common?
towards apex of adjacent columnar and cuboidal epithelial cells
adhesion belt
linked submembranous actin bundles
prominent in small intestine - zone visible as eosinophilic band (terminal bar)
adherent junctions in embryogenesis
transmit motile forces generated by actin filaments across cell sheets. mediate folding of epithelial sheets to form organs in the embryo
desmosome structure - intracellular plaque, filaments inserted into plaque, cell adhesion mediation, transmembrane proteins
intracellular plaque consisting of desmoplakin, associated w/ plakoglobin and plakophilin. tonofilaments (cytokeratin intermediate filaments) are inserted into plaque
cell adhesion mediated by transmembrane proteins desmoglein and desmocollin (cadherins)
hemidesmosome structure - intracellular plaque, tonofilaments, transmembrane proteins
interacts w/ extracellular matrix
tonofilaments usually terminate end-on, don’t loop through
intracellular plaque contains proteins plectin and BPAG1e
transmembrane anchoring proteins are beta4 integrin, alpha6 integrin and BPAG2
tonofilaments
cytokeratin intermediate filaments
junctional complex
close association of several types of junction between adjacent epithelial cells - maintains structural and functional integrity
bullous pemphigoid
blistering disease
antibodies form and are directed against proteins in hemidesmosomes - bullous pemphigoid antigens 1 and 2
leads to inflammation and separation of epithelium from basal lamina, causing blistering
communication junctions - functions, proportions and site of location
allow selective diffusion of molecules between adjacent cells and direct cell-cell communication
low density in most adult epithelia
large amounts in embryogenesis - spacial organisation of developing cells
cardiac and smooth muscle cells - contraction signals
communication junctions pores
several hundred/junction
formed by 6 protein subunits traversing cell membrane - connexon
basement membrane - function, structure
attaches epithelial cells to support tissues at hemidesmosomes and focal contacts
specialised layer of extracellular matrix materials
type IV collagen - synthesised by epithelial cells
protein making up collagen + its synthesis
type IV collagen
synthesised by epithelial cells
examples of how epithelia adapt to perform functions
increased SA - microvilli, basolateral folds, membrane plaques
moving substances over their surface - cilia
microvilli - what they are, where they’re found
finger-like projections of apical cell surface
small ones found on most epithelial surfaces
most developed in absorptive cells, e.g. kidney tubule cells and small bowel epithelium
maintaining the shape of microvilli
actin filaments linked to cell membrane by spectrin, to eachother by actin binding proteins - fimbrin and fascin
lateral anchoring protein - myosin
actin links to cell membrane
spectrin
actin binding proteins
fimbrin and fascin
lateral anchoring protein
myosin
actin core in small bowel epithelium
actin core linked to actin network of adherent junctions between adjacent cells
cell membrane covering microvilli
contains cell surface glycoproteins and enzymes involved in absorptive process
detection of specific cell surface specialisations on microvilli
immunohistochemistry/enzyme histochemistry used to detect specific proteins, e.g. lactase and alkaline phosphatase
stereocilia
extremely long forms of microvilli - found on epithelial cells lining epididymis
sensors of cochlear hair cells
basolateral folds
deep invaginations of basal or lateral surface of cells
basolateral folds location and function
in cells involved in fluid or ion transport
associated w/ high concs of mitochondria
striped appearance to basal cytoplasm
renal tubular cells and ducts of secretory glands
membrane plaques function and location
rigid areas of apical cell membrane
only in epithelium lining urinary tract
fold down into cell when bladder is empty
unfolds to increase luminal area of cell when bladder is full
cilia - what they are, function
highly specialised extension of the cytoskeleton
move fluid over cell or confers cell motility
cilia core structure
orgnaised core of parallel microtubules (axoneme) bound together w/ other proteins to produce beating nexin linking protein every 86nm dynein arms every 24nm radial spoke every 29nm outer doublet tubulin microtubule central pair of microtubule central sheath projections every 14nm
where are cilia evident in?
epithelium lining respiratory tract - move mucus
epithelium lining fallobian tube - convey released ova to uterine cavity
axonemal structure
similar one to cilia is found in flagellum of spermatozoa
glycocalyx
layer of protein, glycoprotein and sugar residues
resolved ultrastructurally as amorphus fuzzy coating
stainable by PAS
surface proteins used in cell recognition and adherence mechanisms - immunity
properties of protein secreting epithelial cells
well developed RER - purple cytoplasm in H+E
polarity w/ basal RER
supranuclear Golgi
apical zone containing granules w/ protein for secretion by exocytosis
mucins
mixture of glycoproteins and proteoglycans
lubricant in mouth
barrier in stomach
characteristics of cells that produce and secrete mucin
well developed basal rough ER - faint blue basal cytoplasm
well developed supranuclear Golgi - protein glycosylation
large secretory vesicles - unstained, vacuolated apical cell cytoplasm
aggregation of mucin secreting cells
aggregated into specialised glands in genital, respiratory and intestinal tract
steroid secreting glands location and characteristics
adrenal gland, ovary and testes
well developed smooth ER - granular pink appearance of cytoplasm
free lipid in vacuoles in cell cytoplasm
prominent mitochondria w/ tubular (not flattened) cristae - biosynthesis of steroids from lipid
ion-pumping epithelial cells location, function and characteristics
kidney tubules and ducts of some secretory glands -
transport ions and water
acid producing stomach cells transport H+ ions
ion transport mediated by membrane ion pumps - use ATP
folded cell membrane - increase SA
mitochondria closely apposed to membrane folds, to provide ATP
tight junctions
substances secreted by: cells in kidney tubules and ducts of secretory glands, stomach cells
transport ions and water (sodium and water from apical surface for absorption)
move ions and fluid out of apex of cell - watery fluid secretion
transport H+ ions
mediation of ion transport
membrane ion pumps
use ATP for energy
4 mechanisms of secretion of cell product
merocrine, apocrine, holocrine and endocrine
merocrine
exocytosis from cell apex into a lumen
apocrine
pinching off of apical cell cytoplasm containing cell product
holocrine
shedding of whole cell containing cell product
endocrine
endocytosis from cell base into bloodstream
gland
organised collection of secretory epithelial cells
what happens when more secretions are required?
SA increased by invagination of the surface to form straight tubular glands or more complex coiled or branched glands
single secretory cells
secretory cells in-between surface epithelium
straight tubular gland
surface epithelium
secretory cells
coiled tubular gland
surface epithelium
ion-pumping cells
single central lumen
secretory cells in distal part of gland
branched gland
main excretory duct lined by columnar epithelial cells
minor ducts lined by ion-pumping cells + fluid + electrolytes
secretory epithelial cells around small central lumen - acinus
myoepithelial cells expel secretions
acini
secretory islands
prevents escape of secretions
supply of glands
rich vascular supply to provide metabolites
control of gland secretions
hormonal and innervatory control
barrier function specialisations
occluding junctions prevent diffusion of molecules between cells
apical cell membrane of urothelial transitional epithelium has many sphingolipids
desmosomes and hemidesmosomes provide tight linkage to resist shear and provide mechanical stability/barrier
stratified squamous epithelium - keratinisation - cytoskeleton of superficial cells becomes condensed w/ other specialised proteins into a resilient mass - cell death and tough impervious, protective layer from remaining cell membrane and cytoplasmic contents
sphingolipids
consitutes urothelial transitional epithelium
form membrane plaques
resist fluid and electrolyte movements out of cells in response to osmotic effect of concentrated urine
keratinisation
basal cells anchored to basement membrane and adjacent cells by hemidesmosomes and desmosomes and contain tonofibrils
cells differentiate and move up stratified epithelium
cytokeratin proteins change to higher molecular weight forms - cells develop lamellar bodies
lamellar bodies
membrane-bound granules containing phospholipids - secreted by exocytosis into extracellular space, form lamellar sheet between cells in upper epithelium
production of keratin
cells in upper epithelium express genes coding for specialised proteins interacting w/ tonofilaments and cell membrane to produce resilient and mechanically robust compact mass (keratin)
keratohyaline granules
small granules containing specialised proteins
protein associating w/ and thickening cell membrane
involucrin
keratin properties
non-living proteinacous material attached to underlying cells by existing anchoring junctions
mechanically strong, flexible, inert, physical barrier
intercellular phospholipid
makes epithelium impermeable to water
