2.3 Epithelial Cells And Tissues Flashcards
What are epithelial cells?
Cells which form continuous layers which line surfaces and separate tissue compartments
What is the importance of having well organized and stable cell-cell junctions?
The cells are not leaky and thus fully cohesive
What is the importance of epithelial cells?
Separating tissue compartments or lining the surfaces of a tissue
What two features of epithelial cells are used to classify them?
Shape and size
What do squamous cells look like?
Flattened, plate shaped
What do columnar epithelium cells look like?
Arranged in columns
What do cuboidal epithelium look like?
Cube-like
What are the three classifications of shape in epithelial cells?
Squamous, Columnar and Cuboidal
How many layers do simple epithelia have?
one layer
What is the name given to describe the multilayered appearance of epithelium?
Stratified
Where are simple squamous epithelium found?
Alveoli, mesothelium and endothelium of blood spaces
What is the purpose of simple squamous epithelium?
Form a thin epithelium that allows gas exchange to occur
Which surfaces are simple columnar epithelium found in?
Surfaces involved in absorption and secretion of molecules (e.g. enterocytes lining the gut) or surfaces involved in the uptake or breakdown of product of digestion
What are the two types of stratified squamous epithelium?
Keratinizing and non-keratinizing
What is meant by non-keratinizing epithelium?
Epithelial cells which do no undergo keratinisation and so retain their nuclei and organelles
Where are non-keratinizing stratified squamous epithelium found?
Epithelium lining the mouth, oesophagus, anus, cervix and vagina
What is meant by keratinizing epihelium?
Epithelial cells which produce keratin
What happens to cells after they produce keratin?
The cells produce keratin and then die
They become a stronger surface (e.g. epithelium) to protect tissues
They lose their organelles and nuclei (not visible on light microscope)
What is the benefit of the keratinizing ability of some stratified squamous epithelium?
Can form thick layers which are useful for protecting underlying tissues from physical and chemical insults
What is meant by pseudo-stratified epithelium?
This epithelium appears to be multi-layered but all surface cells have contact with the basal laminae
Where are pseudo-stratified epithelium found?
Airway (trachea and bronchi) epithelium
Various ducts in the urinary and reproductive tracts
What are tissues made up from?
Cells, extracellular matrix and fluid
What organises tissues into discrete domains?
Junctions
Where is the apical domain found?
At the lumenal surface
What is the basolateral domain in contact with?
The basal laminae
Why is polarisation of secretory epithelia so essential?
To ensure that contents are secreted in the right direction
Otherwise could result in the secretion of substances which would harm our own tissues
What is meant by polarity in epithelial cells?
Different regions of the cell surface have their own discretely organised cellular compartments
What is epithelial polarity so essential for?
Ensuring that in specialised epithelia, transport or secretion occurs in a specific and intended direction
What would happen in transporting epithelium if there was no polarisation?
Lack of directional flow, and ions would be pumped across both sides
What are the four types of junctions seen between epithelial cells?
What do tight junctions do?
Seal the gap between cells
What do adherens junctions do?
Controls the formation of the other three junction types
Regulates actin cytoskeleton
What go gap junctions do?
Form pores between cells to allow communication and the exchange of materials
What do desmosomes do?
Form mechanically tough junctions between cells that are important in tissues to allow them to resist mechanical stress
What are the three types of specialised epithelium which can be found?
- Absorptive
- Secretory
- Transporting
Describe how transporting epithelia are adpated to their function?
The plasma membrane contains high concentrations of ion transporters
Basal membrane has extensive folding in order to provide a lot of energy for active transport across the membranes
Where are infoldings found in transporting epithelium?
In the basal membrane
Why are there a lot of mitochondria near the basal membrane in transporting epithelium?
In order to provide energy for the active transport of substances
How is directionality achieved in transporting epithelium?
The mitochondria provide the ATP required for active transport at these membranes. Because active transport is mainly confined to the basal membranes, ion and water transport will have directionality.
What increases the surface area for absoprtion in the small intstine?
Villi
What are microvilli?
Plasma membrane projections of the villi
What can be found to be interspersed among the absorptive cells of the intestinal villi?
Secretory cells (goblet cells secreting mucus)
Where are carriers transporting nutrients found?
Carriers transporting nutrients are typically found on microvillous brush-border membranes
This includes absorptive intestinal cells (enterocytes) and kidney proximal tubule cells
What does the brush border contain?
Large amounts of active transporters and channels for the uptake of nutrients from the lumen of the gut
Describe how substances are absorbed into circulation?
As the concentration of nutrients increases in the cytoplasm of the absorptive cells, it diffuses down its concentration gradient into the basal interstitial space to be collected in the capillaries and distributed in the circulation
What are the two main types of secretion?
Exocrine and endocrine secretion
How does the pancreas display both endocrine and exocrine functions?
Exocrine - secretes pancreatic enzymes and pancreatic juice into ducts
Endocrine - secretes hormones from the islet cells into blood
Describe the organisation of an exocrine secretory cell
Extensive RER in the basal cytoplasm
Secretory granules in the apical cytoplasm
Golgi apparatus between the two
Why do endocrine cells have rough ER in the basal cytoplasm and secretory granules in the apical cytoplasm?
Positioned so that when their contents are released, they have close access to the blood circulation
What is meant by constitutive secretion?
Secretory vesicles, as they are formed, move directly to the plasma membrane and release their contents
E.g. production of plasma proteins by hepatocytes (constitutive endocrine secretion)
What is meant by stimulated secretion?
Secretory vesicles are stored in the cytoplasm and only fuse with the plasma membrane to release their contents when stimulated
What is an example of stimulated endocrine secretion?
The release of adrenaline from cells of the adrenal medulla after a fight-or-flight stimulus
What is an example of stimulated exocrine secretion?
When stomach contents enter the duodenum, pancreatic acinar cells are stimulated to release their digestive enzymes into ducts
Describe the process of epithelial proliferation?
Many epithelia are constantly “turning-over”
Cells that are lost by cell death or by mechanical removal (e.g. abrasion) are replaced by the proliferation of stem cells within the epithelium
What is epithelial turnover like in the small intestine?
Cells are produced by stem cells in the crypts of Leiberkhun next to villi
They migrate up the villus epithelium and new cells in the basal layer are constantly produced to replace ones being lost from the villus tip
How are cells lost from the surface of stratified squamous epithelium replaced?
Cells of the basal layer of stratified squamous epithelia divide to replace cells lost from the surface
Where are new cells produced in the small intestine?
In the crypts of Leiberkhun
What are the effects of chemotherapy on epithelial cells?
Inhibits the proliferation of intestinal crypt cells
E.g. cancer chemotherapy results in loss of the finger-like intestinal villi and flattening of the intestinal mucosa
Which virus can induce hyperproliferation of epithelial cells?
Papilloma virus
What forms if the increase in cell production is greater than cell loss from the surface of the eidermis?
Cells accumulate and produce “hard skin” or a corn
