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BDS1 - Semester 1 > Week 5: Epithelium 2 - Cell Specialisations > Flashcards

Week 5: Epithelium 2 - Cell Specialisations Flashcards

1
Q

What is the relationship between epithelial cell surfaces and extracellular environments?

A

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

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2
Q

What are the possible pathways across epithelia? What determines what pathway is taken?

A

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.

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3
Q

Give examples of apical surface specialisations of epithelial cells.

A
  • Microvilli
  • Cilia (and flagella)
  • Stereocilia
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4
Q

Describe microvilli.

A
  • 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
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5
Q

Describe cilia (and flagella)

A
  • 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)
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6
Q

Describe stereocilia.

A
  • Apical surface specialisation
  • Similar in structure to microvilli, but longer and branched
  • For bulk absorption: sensory function
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7
Q

What is the lateral surface of epithelial cells?

A

Surface between adjacent epithelial cells

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8
Q

Where do lateral surface specialisations occur?

A

They span between cells = intercellular junctions

These are not visible in light microscopy

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9
Q

Why are lateral surface specialisations important?

A

Important for barrier integrity and coordinated cellular activity

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10
Q

Epithelium is a dynamic tissue. What does this mean? What is it facilitated by?

A

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.

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11
Q

Give examples of lateral surface specialisations on epithelial cells.

A

Lateral surface specialisations are intercellular junctions. These include:
- Tight junctions
- Zonula adherens or belt desmosomes
- Macula adherens or spot desmosomes
- Gap junctions

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12
Q

What are tight junctions?

A
  • 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
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13
Q

What are the types of desmosomes?

A

Zonula adherens or belt desmosomes
Macula adherens or spot desmosome

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14
Q

What are zonula adherens/ belt desmosomes?

A

Beltlike distribution and is associated with actin filaments.

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15
Q

What are macula adherens/ spot desmosomes?

A

Spot like distribution and is associated and is associated with intermediate filaments.

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16
Q

What is the structure of tight junctions?

A
  • Consists of transmembrane proteins: junctional adhesion molecule (JAM), occluding, claudin
  • Interact with actin filaments of cytoskeleton
  • Introduce cell polarity
17
Q

What is the structure of desmosomes?

A
  • 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
18
Q

What are the functions of desmosomes?

A
  • Provide strong attachment that holds adjacent cells together
  • Numerous in epithelia subjected to abrasion and tearing stresses, e.g., epidermis, oral cavity, oesophagus
19
Q

What are gap junctions?

A
  • Aligned channel protein pores allow passage of small molecules etc. from cytoplasm of cell A to cell B
20
Q

What do gap junctions allow for?

A
  • A sheet of epithelial cells to function in unison
  • Cell-to-cell communication
  • Ionic coupling
21
Q

What is the composition of gap junctions?

A
  • Composed of many transmembrane channels (connexons) in close array
  • Connexons are the proteins that make up gap junctions
22
Q

Describe the appearance of gap junctions.

A
  • Connexons of 2 cell membranes align and bridge extracellular space
23
Q

What is the order of junction types from apical towards basal aspect of cells?

A
  • Tight junctions (zonula occludens)
  • Adherens junction (zonula adherens)
  • Desmosomes
  • Gap junctions
  • Hemidesmosomes (on basal surface)
24
Q

What do bands of tight junctions ensure?

A
  • 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
25
Q

What do zonula adherens allow for?

A
  • 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
26
Q

Function of desmosomes.

A

Attach to intermediate filaments of cytoskeleton to provide cytoskeletal support and tissue integrity

27
Q

What do gap junctions allow for?

A
  • 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
28
Q

What specialisation can occur on the basal surface of epithelial cells?

A

Hemidesmosomes

29
Q

What are hemidesmosomes responsible for?

A
  • 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
30
Q

Describe the structure of hemidesmosomes.

A
  • 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

BDS1 - Semester 1 (9 decks)

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