Epithelial Cells

Question 1

What are the 5 major cell categories?

Answer 1

1. Haematopoietic cells: red and white cells, and the cells of the bone marrow from which they are derived
2. Neural cells: cells of the nervous system - two main types; neurones (which carry electrical signals) and glial cells (support cells)
3. Contractile tissues: skeletal muscle, cardiac muscle, smooth muscle
4. Connective tissue cells: fibroblasts (many tissues), chondrocytes (cartilage), osteocytes (bone)
5. Epithelial cells: ?

Question 2

What are epithelial cells? (briefly)

Answer 2

Cohesive cells forming continuous layers, these layers line surfaces and separate tissue compartments and have a variety of other functions
Make stable cell-cell junctions and stability and maintenance are important to form one continuous layer

Question 3

What are the 2 ways epithelial cells are categorised on their histological appearance (what they look like under a light microscope)?

Answer 3

1. Shape

2. Layering

Question 4

What are the 3 types of epithelial cells based on their shape?

Answer 4

1. Squamous - flattened plate shapes
2. Columnar - arranged in columns
3. Cuboidal - cube-like

Question 5

What are the 2 types of epithelial cells based on their layering?

Answer 5

1. Simple - single layer

2. Stratified - multi-layer

Question 6

What is simple squamous and some examples?

Answer 6

Flat (wider than they are tall) and single layered

e.g. exchange - lung alveolar, mesothelium, endothelium

Question 7

What is simple columnar and some examples?

Answer 7

Taller than they are wide and single layered

e.g. absorption and secretion - enterocytes

Question 8

What is simple cuboidal and some examples?

Answer 8

As wide as they are tall and single layered

e.g. ducts - kidney collecting duct

Question 9

What is stratified squamous?

Answer 9

Flattened shape and multiple layers
Cell shapes vary in the various layers
It is squamous as the surface cells are used for description

Question 10

What are the 2 main types of squamous epithelial cells?

Answer 10

1. Keratinising

2. Non-keratinising

Question 11

What are keratinising squamous epithelial cells?

Answer 11

Nuclei not visible in the surface cells e.g. epidermis
Can form thick layers that protect underlying tissues for various physical and chemical insults e.g. heat, cold, solvents (alcohol), abrasion, etc.

Question 12

What are non-keratinising squamous epithelial cells?

Answer 12

Nuclei visible in the surface layer of cells - linings of surfaces that are subject to physical or chemical change e.g. linings of the mouth

Question 13

What is the pseudostratified and some examples?

Answer 13

Appears to be multilayered
Upon further investigation, they found all the cells are in contact with the basal lamina
e.g. upper airway (bronchi) epithelium, various ducts in the urinary and reproductive tracts

Question 14

What are the different parts of the epithelial cell and what are they in contact with?

Answer 14

The basal and later parts are group together as the basolateral domain (i.e the bottom and sides) - in contact with the extracellular matrix
The free / apical part is the apical domain (i.e the top) - in contact with the lumen or outside world

Question 15

What are some of the functions of the epithelial cell?

Answer 15

Key for secretion, transport, absorption etc.

Question 16

Why is the polarity of the epithelial plasma membrane / pumps / channels important for these functions?

Answer 16

All the processes are essentially unidirectional, hence the polarity gives the directionality to epithelial function
Without directionality, there would be an even distribution of transporters so there would be no net directional flow. Polarity is needed to ensure that the secreted products are delivered to the correct tissue compartment e.g. do not want to release enzymes into the extracellular fluid rather than the lumen

Question 17

What are the 4 cell-cell junctions in epithelia and what are their functions?

Answer 17

1. Tight junctions - belt around the apical-lateral membrane, blocks paracellular pathway by preventing materials passing between adjacent cells
2. Adherens Junction - located below tight junction, master junction which controls the formation of all of the others
3. Demosomes - scattered throughout the lateral membrane, mechanically tough junctions between cells and are important in tissues that require to resist mechanical stresses
4. Gap junction - formed from the loss of a demosome, form pores between cells to exchange / share materials, synchronize activities

Question 18

What are the 3 different types of epithelia?

Answer 18

1. Transporting epithelia
2. Absorptive epithelia (aka carriers transporting nutrients)
3. Secretory epithelia

Question 19

How are transporting epithelia adapted for their function?

Answer 19

Plasma membranes contain high concentrations of ion transporters
Typically have extensive basal membrane infoldings which contain the active transporters - increases surface area for transport
Many mitochondria at the basal membrane infoldings - provide the ATP required for active transport at these membranes

Question 20

How are absorptive epithelia adapted for their function?

Answer 20

Found on brush border membranes (such as the villi in the ileum) e.g. absorptive intestinal cells (enterocytes); kidney proximal tubule cells
Often have plasma membrane projections called microvilli to increase surface area
Secretory cells (e.g. goblet cells secreting mucus) are interspersed among the absorptive cells 
Microvillous brush-border membranes contain large amounts of active transporters and channels for the uptake of nutrients

Question 21

How do the absorptive epithelia function?

Answer 21

As the concentration of nutrients increases in the cytoplasm of the absorptive cells, it diffuses down its concentration gradient into the basal interstitial space to be collected in the capillaries and distributed in the circulation

Question 22

What are the functions of secretory epithelia and an example?

Answer 22

Main purpose is secretion and there are 2 types:

1. Exocrine - secreted from the apical side into the lumen/duct
2. Endocrine - secreted from the basal side into circulation
   e. g. the pancreas is both endocrine and exocrine

Question 23

How are the 2 types of secretory epithelia adapted for their function?

Answer 23

For the exocrine cell:
Granular ER and nucleus is usually in the basal part of the cell
There is a Golgi apparatus apical to the ER - it packages and processes the proteins produced
Secretory granules are produced in the apical part of the cell and are used for secretion
For the endocrine cell:
The reverse of the arrangement for the exocrine cell - apparatus orientated in the opposite direction
Granular ER and nucleus in the apical area
Golgi apparatus basal to the ER
Secretory granules in the basal area

Question 24

What are the two types of secretory vesicles and their functions?

Answer 24

1. Constitutive – secretory vesicles, as they are formed, move directly to the plasma membrane and release their contents, e.g. production of plasma proteins by hepatocytes (constitutive endocrine secretion)
2. Stimulated – secretory vesicles are stored in the cytoplasm and only fuse with the plasma membrane to release their contents, e.g. the release of adrenaline from the adrenal medulla (stimulated endocrine secretion)

Question 25

What is meant by epithelia ‘turning over’?

Answer 25

When epithelial cells are lost by cell death or mechanical removal (e.g. abrasion), and replaced by the proliferation of new stem cells that differentiate to perform that specific epithelial function

Question 26

What are 2 common examples of epithelia turning over?

Answer 26

1. Cells in intestinal crypts replacing cells lost from the tips of intestinal villi
2. Cells of the basal layer of stratified squamous epithelia dividing to replace cells lost from the surface

Question 27

What is the mechanism of the epithelial turnover in the small intestine?

Answer 27

At the Crypt of Leiberkhun (base of the villi), new stem cells are generated
These develop into new epithelial cells
As cells are constantly being lost at the tip of the villi, the new cells slowly migrate up to replace the lost cell by shuffling along one cell at a time

Question 28

What can affect / inhibit this crypt function?

Answer 28

Cancer chemotherapy, - results in loss of the finger-like intestinal villi and flattening of the intestinal mucosa
e.g. chemotherapy agent 5-FU
Cell loss from the villus tips continues as normal, but the failure to produce new cells to replace the lost cells results in a loss of tissue and the villi shorten

Question 29

What impact does shortened villi have on the person?

Answer 29

Creates many gastro-intestinal disturbances

Question 30

What type of epithelium is the epidermis in our skin and why is it important?

Answer 30

Keratinising stratified squamous epithelium

Acts as an important barrier to the environment and to mechanical damage

Question 31

What is the mechanism of the epithelial turnover in the epidermis?

Answer 31

New cells are generated in the basal layer
Cells are continuously lost from the surface
As the new cells move up to the replace the lost cells, they differentiate so eventually they flatten out and keratinise
Each layer replaces the one above as the layers are lost from the surface

Question 32

What is hyperproliferation?

Answer 32

Increased cell numbers and a thickening of cell layers
If increase in cell production is greater than the cell loss from the surface, cells will accumulate creating an increased thick hard layer

Question 33

Why might hyperproliferation occur?

Answer 33

In response to repeated or constant pressure
Abrasion to areas of the skin
Infectious agents such as papilloma virus

Question 34

How do infectious agents cause hyperproliferation?

Answer 34

Hijacking the cellular machinery of stratified squamous epithelia and inducing increased cell proliferation - increased surface growth