Week 5: Epithelium 2 - Cell Specialisations Flashcards
(30 cards)
What is the relationship between epithelial cell surfaces and extracellular environments?
Different cell surfaces are in contact with differing extracellular environments.
- Apical surface abuts lumen
- Lateral surfaces abut other epithelial cells
- Basal surface abuts basement membrane and underlying CT
What are the possible pathways across epithelia? What determines what pathway is taken?
Paracellular pathway = between cells
Transcellular pathway = through cell cytoplasm
Which pathway is taken is influenced by membrane domain (proteins and channels present) and presence/ absence of surface specialisations.
Give examples of apical surface specialisations of epithelial cells.
- Microvilli
- Cilia (and flagella)
- Stereocilia
Describe microvilli.
- Apical surface specialisation
- Central dense core of actin filaments
- Finger-like protrusions found on apical surface of cells
- Increase SA of membrane and number of enzymes, carrier proteins etc., to maximise absorption
Describe cilia (and flagella)
- Apical surface specialisations
- Motile processes on cell surface, covered with cell membrane
- Taller than microvilli
- Beat in unison to create a unidirectional current along the surface of cells –> associated with movement
- Contain a core of 9+2 microtubular structures (axoneme) anchored to a basal body (similar to centriole)
Describe stereocilia.
- Apical surface specialisation
- Similar in structure to microvilli, but longer and branched
- For bulk absorption: sensory function
What is the lateral surface of epithelial cells?
Surface between adjacent epithelial cells
Where do lateral surface specialisations occur?
They span between cells = intercellular junctions
These are not visible in light microscopy
Why are lateral surface specialisations important?
Important for barrier integrity and coordinated cellular activity
Epithelium is a dynamic tissue. What does this mean? What is it facilitated by?
Epithelium is a dynamic tissue that has large regenerative abilities and a capacity to adapt to changes in its environment. This adaptability is largely facilitated by a process called metaplasia - a reversible change in which one mature cell type within another epithelial tissue is replaced by another cell type in response to a stressor. This is due to the high turnover times of the tissue.
Give examples of lateral surface specialisations on epithelial cells.
Lateral surface specialisations are intercellular junctions. These include:
- Tight junctions
- Zonula adherens or belt desmosomes
- Macula adherens or spot desmosomes
- Gap junctions
What are tight junctions?
- Seal intercellular space/ gap
- Adjacent cell membranes fuse together; belt-like distribution, like a ribbon internally bracing the cell
- Intercellular passage is blocked
- Creates a barrier between two environments
What are the types of desmosomes?
Zonula adherens or belt desmosomes
Macula adherens or spot desmosome
What are zonula adherens/ belt desmosomes?
Beltlike distribution and is associated with actin filaments.
What are macula adherens/ spot desmosomes?
Spot like distribution and is associated and is associated with intermediate filaments.
What is the structure of tight junctions?
- Consists of transmembrane proteins: junctional adhesion molecule (JAM), occluding, claudin
- Interact with actin filaments of cytoskeleton
- Introduce cell polarity
What is the structure of desmosomes?
- Localised spot-like junction
- Attachment plaque on cytoplasmic side of cell (desmoplakins and plakoglobins)
- Anchorage site for intermediate filaments
- Transmembrane proteins are cadherins
- Wide intercellular space between adjacent cell membranes
What are the functions of desmosomes?
- Provide strong attachment that holds adjacent cells together
- Numerous in epithelia subjected to abrasion and tearing stresses, e.g., epidermis, oral cavity, oesophagus
What are gap junctions?
- Aligned channel protein pores allow passage of small molecules etc. from cytoplasm of cell A to cell B
What do gap junctions allow for?
- A sheet of epithelial cells to function in unison
- Cell-to-cell communication
- Ionic coupling
What is the composition of gap junctions?
- Composed of many transmembrane channels (connexons) in close array
- Connexons are the proteins that make up gap junctions
Describe the appearance of gap junctions.
- Connexons of 2 cell membranes align and bridge extracellular space
What is the order of junction types from apical towards basal aspect of cells?
- Tight junctions (zonula occludens)
- Adherens junction (zonula adherens)
- Desmosomes
- Gap junctions
- Hemidesmosomes (on basal surface)
What do bands of tight junctions ensure?
- All passage across epithelium must occur through cytoplasm of cells (selectivity of passage)
- No intercellular seepage (seals also protective with respect to external environment)
- Ions can be transported against a concentration gradient
- Membrane proteins remain localised to correct domain