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Cell Membrane > Lecture 23: Traffic Across Cells: Properties Of Epithelial Cells > Flashcards

Lecture 23: Traffic Across Cells: Properties Of Epithelial Cells Flashcards

1
Q

What is the general form of epithelial tissue?

A

Cells arranged in continuous sheets in either single or multiple layers

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2
Q

What does epithelial tissue sit on?

A

A basement membrane

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3
Q

Where does epithelial tissue form?

A

The boundary between the body’s organs or between the body and the external environment (eg pathway through body is in contact with external environment

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4
Q

Describe the renewal of epithelial tissue?

A

Epithelial tissue is subject to physical breakdown and injury so it undergoes constant and rapid renewal processes (so transports molecules but is also a barrier)

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5
Q

What 6 key things are epithelial tissue important for?

A
  1. Protection
  2. Filtration
  3. Secretion
  4. Absorption
  5. Excretion
  6. Neuroendocrine functions
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6
Q

Discuss protection function

A

Barrier on the skin etc

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7
Q

Discuss filtration function

A

Epithelial tissue lines blood vessels, capillaries, has a filtration function in the kidneys

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8
Q

Discuss secretion function

A

Such as saliva, sweat, gastric juices in stomach: requires movement of ions in water

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9
Q

Discuss the absorption process

A

Get all dietary nutrients from gut, cells absorb carbohydrates (eg glucose) and amino acids and lipids. (Depends on the position in the gut)

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10
Q

Discuss excretion

A

Epithelial tissue forms urine

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11
Q

Discuss neuroendocrine functions

A

Epithelial cells can secrete a lot of hormones which regulate other epithelial tissues (example is the secretion of insulin by the beta cells in the pancreas) (are probably more neurotransmitters in the gut than in the brain)

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12
Q

What are the two types of epithelium?

A
  1. Covering and lining epithelium

2. Glandular epithelium

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13
Q

Examples of covering and lining epithelium?

A

Epidermis of skin
Lining of blood vessels and ducts
Lining respiratory, reproductive, urinary and GI tract

(So is a layer you need to move molecules across)

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14
Q

Examples of glandular epithelium?

A

Secreting portion of glands

Thyroid, adrenal and sweat glands

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15
Q

What is the histological classification of epithelium mean?

A

Basically the two types of distinctions (which are by arrangement or shapes)

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16
Q

What are the classifications by arrangement of cells into layers?

A

Simple
Stratified
Pseudostratified

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17
Q

What is a simple arrangement?

A

One cell layer thick

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18
Q

What is stratified arrangement?

A

Many cell layers thick

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19
Q

What is Pseudostratified arrangement?

A

Single layer where all cells don’t reach the apical surface

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20
Q

What are the classifications by shape of surface cells?

A

Squamous
Cuboidal
Columnar
Transitional

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21
Q

What is the squamous shape?

A

Flat

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22
Q

What is cuboidal shape?

A

Cube-shaped

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23
Q

What is columnar shaped?

A

Tall column

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24
Q

What is a transitional shape?

A

Shape that varies with tissue stretching

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25
Q

What are the two types of glandular epithelium?

A
  1. Exocrine glands

2. Endocrine glands

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26
Q

Where are glandular derived from?

A

The epithelial cells that sink below the epithelium surface during development

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27
Q

What are exocrine glands?

A

Cells that secrete substances (sweat, ear wax, saliva, digestive enzymes) onto free surface of epithelial layer

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28
Q

How are exocrine glands connected to the surface?

A

By tubes (ducts)

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29
Q

Are exocrine glands unicellular or multicellular?

A

They can be both

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30
Q

What are endocrine glands?

A

They secrete hormones into the bloodstream (and don’t have ducts)

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31
Q

What are the structure of exocrine glands?

A

They can either be simple (single) or branched

32
Q

What do the end pieces of exocrine glands contain?

A

Secretory or acini cells

33
Q

What are the openings of exocrine glands comprised of?

A

Ductile cells that have different transport properties

34
Q

For gland secretion, what do glands consist of?

A

Two different types of epithelial cells connected in sequence

35
Q

What do acinar cells do?

A

They create a primary secretion rich in organic molecules (enzymes, regulatory molecules)

36
Q

What do duct cells do?

A

They modify the composition of the primary secretion by either absorbing or secreting specific ions

37
Q

What specific ions do duct cells secrete?

A

HCO3-, Cl-, K+, Na+

38
Q

What makes epithelial tissues different to muscle and nervous tissue?

A

Tight junctions

39
Q

What are epithelial cells separated from their neighbours by?

A

Lateral intercellular (paracellular) space

40
Q

What are epithelial cells held together by?

A

Tight junctions at their luminal edges

41
Q

What are tight junctions?

A

Proteins embedded by one cell into the other cell (like fusions)

42
Q

What are tight junctions composed of?

A

Thin bands that encircle the cell and make contact with thin bands from adjacent cells

43
Q

What do tight junctions look like in electron microscopy?

A

It appears that the membranes are fused together

44
Q

What do tight junctions look like in freeze fracture?

A

Like an interlocking network of ridges in the plasma membrane

45
Q

What do tight junctions act as?

A

A barrier and a fence

46
Q

How are tight junctions a barrier?

A

They restrict the movement of substances through the intercellular fluid

47
Q

How are tight junctions a fence?

A

They prevent membrane proteins from diffusing in the plane of the lipid bilayer (otherwise proteins would move and diffuse around the cell a la mosaic model (unless anchored by a peripheral protein) but they can’t diffuse last tight junctions

48
Q

What does the fence aspect do?

A

Seperate a epithelial cells into two distinct membrane domains

49
Q

What are the two membrane domains?

A
  1. Apical (or luminal or mucosal)

2. Basolateral

50
Q

What is the apical membrane domain?

A

The membrane that faces the lumen of the organ or body cavity

51
Q

What is the Basolateral domain?

A

The membrane that adheres to the adjacent membrane and interfaces with the blood

52
Q

Are epithelial cells symmetrical?

A

No, because of the tight junctions causing these membrane domains that can have different membrane proteins and different transport properties

53
Q

What does the distinct membrane domains mean?

A

Different transport proteins can be inserted into either the apical or Basolateral membrane

54
Q

What is transcellular transport?

A

Transport through the cell (must cross apical and Basolateral membranes)

55
Q

What is paracellular transport?

A

Transport via tight junctions

56
Q

What law governs paracellular transport?

A

Law of diffusion and tightness of junctions

57
Q

What can you measure in regards to to tight junctions?

A

The electrical resistance to ion flow through the tight junction

58
Q

What causes the higher resistance to ion flow in paracellular transport?

A

The number of tight junction strands holding the cell together

59
Q

What can epithelial cells be functionally classified into?

A

Leaky epithelium or tight epithelium

60
Q

What transport dominates leaky epithelium?

A

Paracellular

61
Q

What transport dominates tight epithelium?

A

Transcellular

62
Q

How do changes in tight junction resistance occur?

A

They change in a proximal to distal direction in the GI tract and kidney

63
Q

Describe proximal aspect of epithelium

A

Leaky epithelium
Low electrical resistance
Low number of strands
Bulk transport (paracellular)

64
Q

Examples of proximal leaky epithelium?

A

Duodenum

Proximal tubule

65
Q

Describe distal epithelium?

A

Tight junction
High electrical resistance
High number of strands
Hormonally controlled (Transcellular)

66
Q

Example of distal tight epithelium?

A

Colon

Collect duct

67
Q

What is the working definition of transcellular transport?

A

Epithelial cells use primary and secondary active transport often in combination with passive diffusion through ion channels to produce transport across epithelial cells

68
Q

What are two types of transcellular transport?

A

Absorption

Secretion

69
Q

Pathway of absorption?

A

Lumen to blood

70
Q

Pathway of secretion?

A

Blood to lumen

71
Q

4 key rules to transepithelial transport?

A
  1. Entry and exit steps
  2. Electrochemical gradients
  3. Electroneutrality
  4. Osmosis
72
Q

Discuss the entry and exit rule?

A

The entry for absorption is the apical but for secretion is the Basolateral membrane (so sometimes different proteins transport same molecule)

73
Q

Discuss electrochemical gradient?

A

Is entry or exit step active or passive?

74
Q

Discuss electroneutrality step?

A

Movement of a positive or negative ion will attract a counter ion

75
Q

Discuss osmosis step

A

Nett movement of ions will establish a difference in osmolarity that will cause water to flow by osmosis (if the water is able to move). Can’t move water without ions

76
Q

What uses different collections of transporters and channels to mediate secretion or absorption?

A

Transcellular transport

Cell Membrane (6 decks)

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