Lecture 23: Traffic Across Cells: Properties Of Epithelial Cells

Question 1

What is the general form of epithelial tissue?

Answer 1

Cells arranged in continuous sheets in either single or multiple layers

Question 2

What does epithelial tissue sit on?

Answer 2

A basement membrane

Question 3

Where does epithelial tissue form?

Answer 3

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

Question 4

Describe the renewal of epithelial tissue?

Answer 4

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)

Question 5

What 6 key things are epithelial tissue important for?

Answer 5

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

Question 6

Discuss protection function

Answer 6

Barrier on the skin etc

Question 7

Discuss filtration function

Answer 7

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

Question 8

Discuss secretion function

Answer 8

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

Question 9

Discuss the absorption process

Answer 9

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

Question 10

Discuss excretion

Answer 10

Epithelial tissue forms urine

Question 11

Discuss neuroendocrine functions

Answer 11

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)

Question 12

What are the two types of epithelium?

Answer 12

1. Covering and lining epithelium

2. Glandular epithelium

Question 13

Examples of covering and lining epithelium?

Answer 13

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)

Question 14

Examples of glandular epithelium?

Answer 14

Secreting portion of glands

Thyroid, adrenal and sweat glands

Question 15

What is the histological classification of epithelium mean?

Answer 15

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

Question 16

What are the classifications by arrangement of cells into layers?

Answer 16

Simple
Stratified
Pseudostratified

Question 17

What is a simple arrangement?

Answer 17

One cell layer thick

Question 18

What is stratified arrangement?

Answer 18

Many cell layers thick

Question 19

What is Pseudostratified arrangement?

Answer 19

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

Question 20

What are the classifications by shape of surface cells?

Answer 20

Squamous
Cuboidal
Columnar
Transitional

Question 21

What is the squamous shape?

Answer 21

Flat

Question 22

What is cuboidal shape?

Answer 22

Cube-shaped

Question 23

What is columnar shaped?

Answer 23

Tall column

Question 24

What is a transitional shape?

Answer 24

Shape that varies with tissue stretching

Question 25

What are the two types of glandular epithelium?

Answer 25

1. Exocrine glands | 2. Endocrine glands

Question 26

Where are glandular derived from?

Answer 26

The epithelial cells that sink below the epithelium surface during development

Question 27

What are exocrine glands?

Answer 27

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

Question 28

How are exocrine glands connected to the surface?

Answer 28

By tubes (ducts)

Question 29

Are exocrine glands unicellular or multicellular?

Answer 29

They can be both

Question 30

What are endocrine glands?

Answer 30

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

Question 31

What are the structure of exocrine glands?

Answer 31

They can either be simple (single) or branched

Question 32

What do the end pieces of exocrine glands contain?

Answer 32

Secretory or acini cells

Question 33

What are the openings of exocrine glands comprised of?

Answer 33

Ductile cells that have different transport properties

Question 34

For gland secretion, what do glands consist of?

Answer 34

Two different types of epithelial cells connected in sequence

Question 35

What do acinar cells do?

Answer 35

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

Question 36

What do duct cells do?

Answer 36

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

Question 37

What specific ions do duct cells secrete?

Answer 37

HCO3-, Cl-, K+, Na+

Question 38

What makes epithelial tissues different to muscle and nervous tissue?

Answer 38

Tight junctions

Question 39

What are epithelial cells separated from their neighbours by?

Answer 39

Lateral intercellular (paracellular) space

Question 40

What are epithelial cells held together by?

Answer 40

Tight junctions at their luminal edges

Question 41

What are tight junctions?

Answer 41

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

Question 42

What are tight junctions composed of?

Answer 42

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

Question 43

What do tight junctions look like in electron microscopy?

Answer 43

It appears that the membranes are fused together

Question 44

What do tight junctions look like in freeze fracture?

Answer 44

Like an interlocking network of ridges in the plasma membrane

Question 45

What do tight junctions act as?

Answer 45

A barrier and a fence

Question 46

How are tight junctions a barrier?

Answer 46

They restrict the movement of substances through the intercellular fluid

Question 47

How are tight junctions a fence?

Answer 47

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

Question 48

What does the fence aspect do?

Answer 48

Seperate a epithelial cells into two distinct membrane domains

Question 49

What are the two membrane domains?

Answer 49

1. Apical (or luminal or mucosal) | 2. Basolateral

Question 50

What is the apical membrane domain?

Answer 50

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

Question 51

What is the Basolateral domain?

Answer 51

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

Question 52

Are epithelial cells symmetrical?

Answer 52

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

Question 53

What does the distinct membrane domains mean?

Answer 53

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

Question 54

What is transcellular transport?

Answer 54

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

Question 55

What is paracellular transport?

Answer 55

Transport via tight junctions

Question 56

What law governs paracellular transport?

Answer 56

Law of diffusion and tightness of junctions

Question 57

What can you measure in regards to to tight junctions?

Answer 57

The electrical resistance to ion flow through the tight junction

Question 58

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

Answer 58

The number of tight junction strands holding the cell together

Question 59

What can epithelial cells be functionally classified into?

Answer 59

Leaky epithelium or tight epithelium

Question 60

What transport dominates leaky epithelium?

Answer 60

Paracellular

Question 61

What transport dominates tight epithelium?

Answer 61

Transcellular

Question 62

How do changes in tight junction resistance occur?

Answer 62

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

Question 63

Describe proximal aspect of epithelium

Answer 63

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

Question 64

Examples of proximal leaky epithelium?

Answer 64

Duodenum | Proximal tubule

Question 65

Describe distal epithelium?

Answer 65

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

Question 66

Example of distal tight epithelium?

Answer 66

Colon | Collect duct

Question 67

What is the working definition of transcellular transport?

Answer 67

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

Question 68

What are two types of transcellular transport?

Answer 68

Absorption | Secretion

Question 69

Pathway of absorption?

Answer 69

Lumen to blood

Question 70

Pathway of secretion?

Answer 70

Blood to lumen

Question 71

4 key rules to transepithelial transport?

Answer 71

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

Question 72

Discuss the entry and exit rule?

Answer 72

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

Question 73

Discuss electrochemical gradient?

Answer 73

Is entry or exit step active or passive?

Question 74

Discuss electroneutrality step?

Answer 74

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

Question 75

Discuss osmosis step

Answer 75

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

Question 76

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

Answer 76

Transcellular transport