Epithelia

Question 1

What is epithelia?

Answer 1

Boundary between controlled internal environment and uncontrolled external environment
Present in every organ
- Covers surfaces
- Lines cavities e.g. mouth
- Forms glands

Question 2

What are the functions of epithelia?

Answer 2

Protection
Barrier
Diffusion
Absorption
Secretion

Question 3

What are the common properties of epithelia?

Answer 3

Epithelial cells form dynamic barrier - able to import or expel substances
- Connection via tight junctions
- Distinct apical and basolateral domains (polarised)
- Lacks blood vessels (avascular)
- Lacks extracellular fibres
- Minimal EC space

Question 4

What is polarity of epithelia?

Answer 4

Apical-Basal polarity
Apical membranes - in contact with external environment e.g. GI lumen
BLM - in contact with ECF compartment (BM)
There is specialisation on both surfaces

Question 5

What is the basement membrane?

Answer 5

• Separates cells from underlying connective tissue
• Made from collagens, laminins, proteoglycans
• Provides structural support and organisation
• Anchors epithelial cells to underlying connective tissue

Question 6

Why are junctional complexes important?

Answer 6

For cell adhesion and communication

Question 7

What are tight junctions?

Answer 7

• Joins adjacent cells together
• Impede paracellular movement

Question 8

What are claudins?

Answer 8

Protein strands that determine the tightness of tight junctions
Very tight = high barrier function e.g. renal TAC
or Leaky e.g. renal PT
The combination of claudin genes present determines permeability

Question 9

What are adhering junctions?

Answer 9

• Form a belt around the cell under the TJ
• Linked with actin filaments and cadherins
• Disruption decreases the organisation - can lead to metastasis in epithelial tumours

Question 10

What are gap junctions?

Answer 10

• Permit small molecule diffusion between cytoplasm of neighbouring cells
• Provides lateral communication
• Therefore cells electrically coupled due to ion movement

Question 11

What are desmosomes?

Answer 11

• Form strong adhesion points between cells
• Have EC cadherin domains
• Have anchor proteins that link cadherin to intermediate filaments
• Myosin filament interactions for contraction

Question 12

How are cells anchored to the BM?

Answer 12

Actin-linked cell-matrix junctions
Hemidesmosomes

Question 13

Why is there continuous replacement of cells?

Answer 13

One side of epithelia in contact with hostile external environment
(Also high amounts of abrasion)
- Replacement by stem cells
Intestine self renewal = 5 days
Lung epithelium = 6 months

Question 14

What are the two main types of epithelia?

Answer 14

Simple - single cell layer e.g. lung
Stratified - many cell layers e.g. skin

Question 15

What is pseudo-stratified epithelia?

Answer 15

One tall cell layer - two nuclei layers
e.g. upper respiratory tract

Question 16

What is simple squamous epithelia?

Answer 16

• Appears as thin scales
• Facilitates rapid molecule passage
• Secretion/absorption via AT

Question 17

What is simple cuboidal epithelia?

Answer 17

• Secretion/absorption via AT
• Many have single, non-motile cilium on AM - deformation senses fluid flow

Question 18

What are the types of columnar epithelia

Answer 18

Simple columnar - majority of GI tract
- Can have cilia or microvilli
- AT

Pseudostratified columnar
- Single layer
- All touching BM

Question 19

What are the types of stratified epithelia?

Answer 19

Stratified squamous
- Most common type of stratified e.g. skin
- Apical cells appear squamous
- Basal cells appear cuboidal or columnar

Stratified cuboidal
- Glands

Question 20

Which type of epithelia appears in areas of high abrasion?

Answer 20

Stratified squamous

Question 21

What is stratified columnar epithelia?

Answer 21

Allows tissue stretch / contraction
e.g. conjunctiva, pharynx, anus male urethra

Question 22

What is transitional epithelia?

Answer 22

Cells are rounded when relaxed
Facilitates shape change in distension without damage to epithelial lining
e.g. Bladder empty (rounded cells) bladder full (distended cells)

Question 23

What are exocrine glands

Answer 23

• Multicellular glands
• Secrete directly onto epithelial cells
• Can have complex ducts

Question 24

What are endocrine glands?

Answer 24

• Have single cell types or multiple cell types
• Secrete directly into blood
• Ductless

Question 25

What are the different types of glands?

Answer 25

Mucus glands
Serous glands - secrete fluid containing proteins e.g. salivary
Sebaceous glands - oily compounds, sebum