Lecture 7 - Structure and Function of Epithelia Flashcards

1
Q

What does epithelia consist of?

A

Epithelia consists of 1 or more layers of epithelial cells, a basement membrane, as well as supporting tissues (such as extracellular matrix and cells).

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2
Q

What are the two sides of epithelia?

A

apical and basolateral

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3
Q

What is the apical membrane also referred to as?

A

luminal or mucosal membrane

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4
Q

What does the apical membrane face?

A

Lumen

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5
Q

What does the basolateral membrane face?

A

the interstitial fluid/internal body

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6
Q

What are features found at the apical membrane?

A

microvilli and the tight junction

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7
Q

What features are found at the basolateral membrane?

A

It is the membrane that contains the basement membrane and the basal membrane which lines the lateral intercellular space

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8
Q

What are common characteristics of the basolateral membrane?

A
  • High K+ permeability
  • High water permeability
  • Housekeeping transporters
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9
Q

Why do we have housekeeping transporters in the basolateral membrane?

A

As Na+ concentration inside the cell is low, Na+ can enter the cell via passive transport. However, in order to move Na+ out of the cell, active transport is required, hence the need for housekeeping transporters

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10
Q

What are some examples of housekeeping transporters?

A

Na+,K+-ATPase (Always in basolateral membrane)
Ca2+-ATPase and Na+/Ca2+ exchanger
Na+/H+ exchanger (NHE-1)

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11
Q

What are common characteristics of the apical membrane?

A

Selective permeability to ions
Absence of housekeeping transporters
Variable water permeability

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12
Q

What is the overall function of absorptive epithelia?

A

transport of solutes from the mucosal membrane to the serosal (outside to inside)

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13
Q

What is absorption driven by?

A

Absorption is driven by the active transport of Na+ (Na/K-ATPase)

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14
Q

What are the two types of absorptive epithelia?

A

Leaky and tight

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15
Q

What seperates leaky and tight absorptive epithelia?

A

a) Transport properties derived from electrical characteristics
b) Apical membrane transport pathways
c) Hydraulic conductivity (relative water flow)
d) Responsiveness to hormones
e) Location within organs

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16
Q

What are the eletrical characteristics of absorptive epithelia?

A

Voltage and resistance

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17
Q

What is transepithelial voltage?

A

Transepithelial voltage describes the electrical potential difference that is developed across an epithelium due to the active transport of ions.

18
Q

What is transepithelial resistance?

A

The electrical resistance to flow of ions

19
Q

What provides the difference in transepithelial resistance in leaky and tight absorptive epithelia?

A

This difference in transepithelial resistance and consequent voltage is due to the properties of the “tight” junctions.

20
Q

What are the electrical characteristics of absorptive epithelia defined by?

A

Tight junctions

21
Q

What is the eletrical characteristics of tight absorptive epithelia?

A

Low P in TJ
High Resistance (300 - 10,000 ohms · cm2)
Low Flow
High Voltage (20 - 100 mV)

22
Q

What is the eletrical characteristics of leaky absorptive epithelia?

A

High P in TJ
Low Resistance (20 - 100 ohms · cm2)
High Flow
Low Voltage (0 - 5 mV)

23
Q

What are apical membrane transport pathways defined by?

A

apical transporters vs channels

24
Q

What apical membrane transport pathway is found in tight absorptive epithelia?

A

Transporters
- Na+ selective channels (e.g. ENaC)

25
Q

What apical membrane transport pathway is found in leaky absorptive epithelia?

A

Channels:
- Na+ dependent cotransporters.
- NHE

26
Q

What is Hydraulic conductivity defined by?

A

Aquaporins and tight junction

27
Q

What is the HC of tight absorptive epithelia?

A

Absorb variable amounts of hyperosmotic fluid – low HC basal conditions, higher under hormonal control

28
Q

What is the HC of leaky absorptive epithelia?

A

Absorbs large amounts isosmotic fluid – high HC

29
Q

What is the tight absorptive epithelia’s response to hormones?

A

High response (more receptors)
Aldosterone - regulates Na+ transport (ENaC)
Vasopressin - regulates water transport (AQP2)

30
Q

What is the leaky absorptive epithelia’s response to hormones?

A

Low, little control.

31
Q

Where is tight absorptive epithelia found and why?

A

Distal regions (late distal tubule and collecting duct of nephron, distal colon of the large intestine)
- For fine tuning

32
Q

Where is leaky absorptive epithelia found and why?

A

Proximal regions (proximal tubule of the nephron and the duodenum, jejunum and ileum of small intestine)
- For bulk transport

33
Q

What solute is important for absorptive epithelia?

A

Sodium

34
Q

What solute is important for secretory epithelia?

A

Cl-

35
Q

Leaky absorptive epithelia
A. have a high transepithelial resistance.
B. can mainly be found in distal parts of organs.
C. do not have tight junctions.
D. have a low hydraulic conductivity.
E. possess mainly Na+ dependent co-transporters/exchangers in their apical membrane.

A

E. possess mainly Na+ dependent co-transporters/exchangers in their apical membrane.

36
Q

What type of transporter is NHE?

A

Counter-transport, Na+ coupled
Secondary Active

37
Q

What type of transporter is SGLT?

A

Cotransport, Na+ coupled glucose cotransport
Secondary Active

38
Q

What does amiloride inhibit?

A

ENaC

39
Q

What is ENaC?

A

Passive Epithelial Na+ Channel

40
Q

What is the Na+ transport in the apical membrane of tight absorptive epithelia?

A

Na+ channel: ENaC