epithelial cells Flashcards
what are the 5 major categories of cells?
haematopoietic cells
neural cells
contractile tissues
connective tissue cells
epithelial cells
what type of cells are haematopoietic cells?
blood cells, tissue-resident immune cells, cells of bone marrow from which they are derived
what type of cells are neural cells?
cells of the nervous system
2 main types:
- neurones - carry electrical signals
- glial cells - support cells
what type of tissues are contractile tissues?
skeletal muscle, cardiac muscle, smooth muscle
what type of cells are connective tissue cells?
fibroblasts (various tissues)
chondrocytes (cartilage)
osteocytes (bone)
what type of cells are epithelial cells?
cells forming continuous layers that line surfaces and separate tissue compartments (among other functions)
epithelial cells form layers, which requires the cells to have what properties?
well organised
make stable cell-cell junctions
why is it important that epithelial cells make stable cell-cell junctions?
stability and maintenance of junctions key to their function as a continuous layer
e.g in separating tissue compartments or lining the surface of a tissue
what are the 2 ways in which epithelial cells are categorised?
based on histological appearance
either on shape or layering
what are the 3 categories of epithelial cells by shape? give short descriptions.
squamous (flattened, plate like shapes)
columnar (arranged in columns)
cuboidal (cube like)
what are the 2 categories of epithelial cell by layering? give short descriptions.
simple epithelium (single layer)
stratified epithelium (multi layer)
what is usually found underneath the epithelium?
extracellular support matrix (basement membrane)
where is simple squamous epithelium found?
lung alveolar epithelium (thin epithelium allowing gas exchange to occur)
mesothelium (lining major body cavities)
endothelium lining blood vessels and other blood spaces
where is simple cuboidal epithelium found?
lining ducts
e..g kidney collecting ducts
where is simple columnar epithelium found?
surfaces involving absorption and secretion of molecules
e.g enterocytes lining the gut (involved in take up of breakdown products for digestion)
what are the 2 main types of epithelial cells?
keratinising and non-keratinising
what are keratinising epithelial cells? give an example of where they can be found.
cells that produce keratin
this causes them to die and become stronger structures
they lose their cellular organelles and nuclei (not visible under light microscopy)
forms thick layers that protect underlying tissues from various factors (e.g heat/cold, solvents, abrasion)
e.g in the epidermis
what are non-keratinising epithelial cells? give 5 examples of where they can be found.
cells that do not undergo keratinisation
retain nuclei and organelles
found in the mouth, oesophagus, anus, cervix and vagina (internal suraces subject to physical and chemical damage)
what is pseudo stratified epithelium and where can it be found?
epithelium appears to be multi-layered but surface cells have contact with basal lamina
e.g airway epithelium (trachea and bronchi), ducts in urinary and reproductive tracts
how is the cell membrane of epithelial cells organised into discrete domains?
by formation of junctions
describe the polarity of epithelial cells.
apical domain - at the lumenal (open) surface
basolateral domain - surface in contact with the extracellular matrix
where is the lateral membrane?
between epithelial cell surface and extracellular membrane, where membranes of adjacent cells are close to each other
what property do most epithelial functions share? give examples of these functions.
directionality
e.g secretion, fluid transport, solute transport, absorption (highly organised processes)
why is epithelial polarity important?
gives directionality needed for epithelial function (such as secretion, fluid transport, solute transport, absorption)
what is polarity in epithelial cells seen as?
different regions of the cell surface being different from each other, with discretely organised cellular contents
in transporting epithelia (which transport ions and fluids across epithelial layers) what property do the pumps and channels involved need to have?
need to be polarized
what are the consequences of the pumps and channels involved in transporting epithelia not being polarized?
they end up being present in all part of the plasma membrane
pump both apically and basal laterally
no directionality, flow is in all directions and not to one aspect of the epithelia
if channels and pumps are polarized then directionality is achieved in the direction required because pumping only occurs on one aspect of the plasma membrane
why is polarity important in secretion?
need to secrete substance either to the apical aspect (into lumen) or to the basal aspect (towards interstitial space)
what property does the machinery used in secretion require?
polarization
why does the machinery used in secretion need to be polarized?
otherwise secretion occurs both through apical and basolateral compartments - secretion in wrong aspect could be disastrous
e.g secretion of digestive enzymes to basal aspect could lead to digestion of your own tissues
what are the 4 main types of cell-cell junction?
tight junction
adherens junction
desmosomes
gap junction (channel forming junctions)
what is the function of tight junctions?
form a belt around the apical lateral membrane
involved in sealing gaps between cells
what is the function of adherens junctions?
controls formation of other junction
least conspicuous when examined in microscopy
what is the function of desmosomes?
spot junctions
form mechanically tough junctions between cells
important in tissues that have to resist mechanical stresses
what is the function of gap junctions?
form pores between cells
allow cells to exchange and share materials
what do the cell-cell junctions allow cells to do?
form communities
synchronise activities
what do the plasma membranes of transporting epithelia contain high concentrations of?
ion transporters
what feature do the basal membranes have and why is it important?
membrane has infoldings
increases SA; increased amount of membrane that can pump ions and water
which organelles are closely associated with basal membrane infoldings and why?
mitochondria
provide energy for active transport across the membranes
where are the mitochondria concentrated and why?
basal aspect (close to infoldings which contain active transporters)
provide ATP for active transport at basal membrane
what is the direction of transport in an epithelial cell (from which aspect to which aspect)?
apical to basal
where does most passive transport of water and ions occur in an epithelial cell?
apical plasma membrane
contains many ion and water channels
active transport is mainly confined to the basal membranes - this allows for what property to occur?
directionality
where are carriers transporting nutrients etc. found?
brush border membranes
e.g enterocytes (absorptive intestinal cells)
kidney proximal tubule cells
how is the surface area of the small intestine increased?
it is long
interior surface of small intestine wall folded into finger-like processes (villi) which are covered with intestinal epithelial cells
what are microvilli?
plasma membrane projections of the villi
how are secretory cells arranged in the small intestine along the villi?
goblet cells secreting mucus interspersed between absorptive cells in the villi
microvilli form a brush border in the small intestinal epithelium - what does it contain large amounts of and why?
many active transporters and channels
needed for uptake of nutrients from gut lumen
as concentration of nutrients increases in cytoplasm of absorptive cells, they diffuse down the concentration gradient into the basal interstitial space to be collected in the capillaries and distributed in the circulation
what are the 2 main types of secretion?
exocrine
endocrine
what is exocrine secretion?
secretion into a duct or lumen
what is endocrine secretion
secretion into the bloodstream
the pancreas has both exocrine and endocrine functions - describe them briefly.
endocrine - hormones (e.g insulin) secreted from cells in the islets of Langerhans
exocrine - digestive enzymes secreted
how are secretory cells arranged in epithelial tissues whose main purpose is not secretion?
individual, dispersed secretory cells present
how is the secretory apparatus of an exocrine secretory cell organised?
organelles arranged for secretion from apical plasma membrane
lots of RER in basal cytoplasm (generation of substance to be secreted)
Golgi apparatus in the middle (for packaging and processing of secretory molecules)
secretory granules in apical cytoplasm
direction of secretion: basal to apical
how is the secretory apparatus of an endocrine secretory cell organised?
organelles arranged for secretion from basal membrane into the bloodstream
lots of RER in apical cytoplasm (generation of substance to be secreted)
Golgi apparatus in the middle (for packaging and processing of secretory molecules)
secretory granules in basal cytoplasm (so that they have close access to blood circulation)
capillary surrounded by basal aspect of endocrine secretory cells
direction of secretion: apical to basal
what are the 2 ways cells can be classified based on the way they secrete?
constitutive
stimulated
what happens in constitutive endocrine secretion? give an example of when this occurs.
secretory vesicles move directly to plasma membranes as they are formed and release their contents
e.g production of plasma proteins by hepatocytes
what happens in stimulated endocrine secretion? give 2 examples of when this occurs/
secretory vesicles stored in the cytoplasm and only fuse with the plasma membrane to release their contents when stimulated
e. g release of adrenaline from cells of adrenal medulla after fight-or-flight stimulus
e. g release of digestive enzymes into ducts from pancreatic acinar cells which are stimulated when stomach contents enter the duodenum
what are the 3 types of epithelial cell specialisation?
transporting epithelia
absorptive epithelia
secretory epithelia
many epithelia are constantly ‘turning’ over due to what 2 processes?
cells lost by cell death
mechanical removal of cells (e.g abrasion)
how are cells lost by cell death or mechanical removal in the epithelia replaced?
proliferation of stem cells within the epithelium
give 2 examples of epithelia ‘turning over’.
cells in intestinal crypts replace cells lost from tips of intestinal villi
cells of basal layer of stratified squamous epithelia divide to replace cells lost from the surface
describe the process of epithelial turnover in the small intestine.
new cells produced by stem cells in Crypt of Leiberkhun
new cell migrated up the villus epithelium to replace cells constantly being lost from villus tip
why can chemotherapy produce many gastro-intestinal disturbances as side effects?
inhibition of proliferation of intestinal crypt cells
drug slows down/halts cell division
cell loss from the villus tips is normal but failure to produce new cells to replace lost cells results in loss of tissue
therefore villi are shortened
loss of villi and flattening of intestinal mucosa results in gastro-intestinal disturbances
name the layers of the dermis and the epidermis.
(deep to superficial)
dermis
(epidermis) basal lamina (membrane)
basal cell layer
prickle cell layers
granular cell layer
keratinised squamous cells (many layers)
upper layer of keratinised squamous cells; about to flake off from surface
what is the epidermis?
keratinising stratified squamous epithelium of body surface
describe the process of epithelial turnover in the epidermis.
surface cells constantly being lost
new cells form in the basal layer and migrate upwards
differentiation occurs as migration takes place so that cells flatten out and keratinise
each layer replaces the one above it as layers are lost from the surface
what is hyperproliferation of epithelial cells and why does it occur?
increased cell numbers and thickening of cell layers
happens in response to repeated or constant pressure
what can hyperproliferation of epithelial cells result in and how?
if increase in cell production due to repeated or constant pressure is greater than cell loss from the surface, cells will accumulate
accumulation creates an increased thick hard layer
local hyperproliferation can lead to hard skin or corns
how can infectious agents such as the papilloma virus induce hyperproliferation of epithelial cells?
hijack cell machinery of stratified squamous epithelia
what effect can infectious agents such as the papilloma virus have and how?
hijack cell machinery of stratified squamous epithelia to induce hyperproliferation
results in surface growth such as warts
