21-09-21 - Epithelia Flashcards
What are epithelial cells?
What are they responsible for?
What types of cells are they?
What do they form?
What do they sit on top of?
- Epithelial cells are one of four primary tissue types
- They are responsible for forming tissue
- Epithelia are heterogeneous (diverse) cell types and can be specialized for distinct functions
- Epithelia form continuous sheets of cells. The sheet sits on top of a basement membrane, separating it from the underlying tissue
Where are epithelia found in the body?
- Epithelia is found anywhere in the body that comes into contact with the outside world ex. Air, food, waste.
- Found in mouth, airways, oesophagus, stomach intestines, kidney tubules etc.
What 3 things is epithelium used for in the body?
- Epithelia act as barriers:
- Between the body and the external world ex. In the airway it is on the surface.
- Between fluid compartments within the body ex. Surrounds the lumen of renal tube (where urine is formed) in the kidneys
- Epithelia act to transport molecules via absorptive processes
- Each section of kidney tubules has different epithelial cells with different functions
- Ex Sodium reabsorption back into the body from filtrate that will become urine
- Epithelia act to transport molecules via secretory processes
- Ex Chloride secretion maintains a layer of fluid in the airway. This is a critical function of epithelial cells.
What are the 3 epithelial cell types?
What is simple epithelium?
What are the 4 types of simple epithelium?
- Simple epithelium is a single cell layer.
- Every cell in this layer comes into the contact with the basement membrane, separating it from the underlying tissue.
- There are 4 types of simple epithelium:
What is a description of simple cuboid epithelium?
What are its functions?
Where can it be found?
- Single layer of cube like cells with central nuclei
- Simple cuboid epithelium is responsible for secretion and absorption.
- Simple cuboid epithelium can be found in:
- Kidney tubules
- Ducts
- Secretory portions of small glands
- Ovary surface
What is a description of simple columnar epithelium?
What are its functions?
Where can it be found?
- Cell height is greater than width
- Basal nuclei
- Ciliated/non-ciliated with microvilli
- Responsible for absorption and secretion of mucus, enzymes etc
- Non-ciliated simple columnar epithelia can be found in:
- digestive tract (e.g found in stomach and intestinal lining but NOT oesophagus)
- Gallbladder
- excretory ducts
- Ciliated simple columnar epithelia can be found in:
- Small bronchi
- Uterine tubes
What is a description of simple squamous epithelium?
What are its functions?
Where can it be found?
Why can it be given another name?
- Flat cells with disk-shaped nuclei
- Thin
- Responsible for:
- Passage of materials by diffusion and filtration
- Secretion of lubricating substances
- The simple squamous epithelium can be found in:
- Kidney glomeruli
- Alveoli of lungs (thin epithelium allows for rapid gas exchange.
- Blood vessels
- Lymphatic vessels
- Can often be given another name as it is considered specialised e.g endothelium, mesothelium, endocardium.
What is a description of simple pseudostratified epithelium?
What are its functions?
Where can it be found?
- Looks like there are multiple layers, but all cells are in contact with basement membrane, making it a simple epithelium.
- Several layers of nuclei
- Can be ciliated/non-ciliated
- Used for secretion of mucus
- Non ciliated simple pseudostratified epithelium is found in:
- Vas deferens
- Large glands
- Ciliated simple pseudostratified epithelium is found in:
- Trachea (ciliated, pseudostratified, columnar epithelium)
- Upper respiratory tract
What is stratified epithelium?
What are the different types of stratified epithelium?
- Stratified epithelium consists of multiple layers of cells, with only the bottom layer of cells connected to the basement membrane
What is a description of stratified cuboidal epithelium?
What are its functions?
Where can it be found?
- Generally, 2 layers of cuboidal cells
- Protects underlying tissue
- Found in:
- Largest ducts of sweat glands
- Mammary glands (contains tissue that can make milk)
- Salivary glands
What is a description of stratified columnar epithelium?
What are its functions?
Where can it be found?
- Several cell layers
- Basal cell layers usually cuboidal
- Superficial cells are elongated and columnar
- Responsible for protection and secretion
- They are relatively rare
- Found in:
- Urethra
- Large ducts of glands
- Conjunctiva of eyes
What is a description of stratified squamous epithelium?
What are its functions?
Where can it be found?
- Thick membrane composed of multiple cell layers
- Superficial portion of cells are squamous that are flattened
- Basal cells are cuboidal/columnar and metabolically active
- Can be Keratinised (waterproof) and non-keratinised
- Responsible for protecting underlying tissue
- Keratinized found in epidermis of skin
- Non keratinized found in:
- Oesophagus
- Mouth
- Vagina
- Urethra
- Anus
What is a description of transitional epithelium?
What are its functions?
Where can it be found?
- Several layers of cells
- Mix of everything
- Resembles stratified squamous and stratified cuboidal epithelium
- Basal cells are cuboidal/columnar
- Superficial cells are dome-shape or squamous-like depending on stretch of location they are found.
- Function – stretches readily, permitting distention of urinary organ
- Found in:
- Ureter
- Bladder
- Parts of Urethra
Describe the epithelial cell types summary diagram
What are the functions of junctional proteins in epithelial cells?
- Connect cells to each other
- Connect cells to the basement membrane
- Some allow intercellular communication e.g., transport ions
- Enable polarisation - epithelial cells quite often have a top and bottom – this is what polarisation is.
Name the 5 junctional complexes of epithelial cells
- Tight junctions
- Adhering junctions
- Desmosomes
- Gap junctions
- Hemidesmosomes
- Tight junctions
- Other name
- Location
- Structure
- Functions
- Also known as Zonula Occludens
- Most apical junction (closes to the top)
- Branching network of strands of proteins
- Zippers together neighbouring epithelial cells
- Defines apical/basolateral boundary
- Permeability barriers – gatekeeper of the paracellular pathway that runs between cells
Adhering junctions
- Other name
- Location
- Structure
- Functions
- Zonula Adherens
- Below tight junctions
- Belt that encircles epithelial cell beneath the tight junction
- Allows cell to cell adhesion via interaction between extracellular domains
- Connects intracellularly to the actin cytoskeleton
Desmosomes:
- Other name
- Location
- Structure
- Functions
- macula adherens (macula means spot)
- Spot like adhesions arranged on the lateral surfaces (sides) of cells
- Allow cell to cell adhesion via interaction between extracellular domains
- Intracellular cytoplasmic tail associates with adaptor and signalling proteins
Gap junctions:
- Location
- Structure
- Functions
- Found at lateral edges of epithelial cells, but not specific to epithelial cells, nearly all cells have gap junctions
- Consist of 2 connexons, one from each sell, each formed of 6 connexin subunits.
- Allows intercellular communication and is permeable to large molecules, such as metabolites, ions and signalling molecules
- Permeability varies between cells and can change in response to stimuli.
Hemidesmosomes:
- Location
- Structure
- Functions
- Localise to the basal surface
- Membrane spanning proteins (integrin), with a stud like structure.
- Attach epithelial cells to the basement membrane.
What are microvilli?
What are they used for?
Where can they be found
- Microvilli act as a brush border membrane in order to increase the apical surface area (top of cell)
- This can increase surface area by up to 20x
- Enables absorption
- Found in the proximal tubule of kidney and small intestine, where a high surface area is optimal for absorbing nutrients.
Where are cilia found ?
What is it used for?
What are examples of where it is used and what for?
- Longer/larger than microvilli
- Present on apical surface of epithelia
- Cilia move back and forth to move particles
- Ciliated cuboidal/columnar:
- Oviduct (non-ciliated epithelium present here too)
- Pseudostratified columnar:
- Respiratory tract (lower larynx, trachea, bronchi)
- Vas deferens (epididymis)
How can basolateral surface area be increased?
Why is this important?
Where are these methods found?
- Basolateral surface area can be increased by:
- Lateral interdigitations
- Basal infoldings
- Important for transport across the epithelial layer
- Can be used to increase the number of pumps present in areas where a lot of energy is needed for absorption
- Particularly seen in the basolateral epithelium of renal system (urinary system)
Where can keratin be found?
What properties does it bring?
What is an example of where Keratin is found?
What is a description of it when present?
- Stratified squamous cells can contain a lot of keratin (keratinized) or very little keratin (non-keratinised)
- Keratinized stratified squamous epithelium can be considered waterproof.
- An example of this is the epidermis of the skin
- A layer of keratin is at the surface of dead squamous cells. It appears anuclear.
Describe the flow chart of the Epithelial structure
Why is polarisation In epithelial cells important?
- Polarisation allows the cell to express different ion and solute transports and channels on one side compared to the other
- This often goes against their ionic concentration and chemical gradient
What are the 2 types of transepithelial transport?
- Transepithelial support is either:
- paracellular (in between the cells)
- transcellular (through the cells)
- or both.
How does paracellular transepithelial transport occur?
What determine how much transport can occur?
What can epithelia be labelled as based on its permeability?
How is it established?
- Paracellular transport occurs between cells via tight junctions
- The number and type of titchy junctions present determine how much transport can occur here.
- Epithelia are considered leaky (lots of transport) or tight (little transport) depending on tight junctions
- Established by electrochemical gradients and drag molecules
How does transcellular transepithelial support occur?
What is required for it to happen?
What does it establish?
- Transcellular transport occurs by going through the cell – crossing both the apical and basolateral membranes.
- This process requires transports/channel and is usually energised by a pump e.g Na/K ATPase
- This establishes electrochemical gradient and allows for movements of ions/solute against its electrochemical gradient.
Give 3 example of transepithelial support, the consequences if these processes are dysregulated and what it is treated with
- Cl- secretion into lumen of small intestines – Too much Cl- and water secretion causes diarrhoea – treated with oral rehydration salts
- Cl- secretion into lumen of airway – dysregulated in cystic fibrosis – treated with potentiators/collectors
- Na+ reabsorption into circulation at the kidney – Too much salt reabsorbed causes hypertension (high blood pressure) – treated with diuretics
What type of epithelium are goblet cells?
What is their function?
Where are they found?
Why are they given this name?
- Goblet cells are Simple Columnar Epithelium cells
- They are mucus synthesizing and secreting cells
- They are found in the respiratory epithelium and the large intestine epithelium
- They are given this name because they look like goblets.
