Week 5: Epithelium 2 - Cell Specialisations Flashcards
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
What do zonula adherens allow for?
- Enable adhesion of cellular sheets to maintain barrier integrity
- Attach to actin filaments that can change cell shape and bring about contour of epithelial surface
Function of desmosomes.
Attach to intermediate filaments of cytoskeleton to provide cytoskeletal support and tissue integrity
What do gap junctions allow for?
- Communicating junctions
- Allow direct passage of small molecules from one cell to the next = cell-to-cell transport
- Chemical and electrical coupling of cellular activities –> syncytium
What specialisation can occur on the basal surface of epithelial cells?
Hemidesmosomes
What are hemidesmosomes responsible for?
- Responsible for cell-matrix adhesion at the basal surfaces of epithelia
- Very important in providing immobility to gingival epithelium and enabling its strong attachment to underling CT
Describe the structure of hemidesmosomes.
- Similar to half a desmosomes
- Plaques attache to intermediate filaments (keratins) in cytoplasm
- Link to transmembrane proteins that are integrins
- Integrins bind to extracellular laminin molecules of basal lamina
