Epithelial Cells

Question 1

What are the 5 major cell types?

Answer 1

Hematopoietic cells: blood cells, tissue-resident immune cells, and the cells of the bone marrow from which they are derived.
Epithelial cells: cells forming continuous layers, these layers line surfaces, and separate tissue compartments and have a variety of other functions.
Contractile tissues: skeletal muscle, cardiac muscle, smooth muscle.
Connective tissue cells: fibroblasts (many tissues), chondrocytes (cartilage), osteocytes (bone). 
Neural cells: cells of the nervous system having two main types; neurons (which carry electrical signals) and glial cells (support cells). 

Question 2

How do epithelial cells organise in a tissue?

Answer 2

Epithelial cells form continuous, cohesive layers of cells which requires the cells to be well organised and to make stable cell-cell junctions. The stability and maintenance of these junctions is key to their function as a continuous layer, for example in separating tissue compartments or lining the surface of a tissue.

Question 3

How are epithelial cells categorised?

Answer 3

Categorised based on histological appearance. Can be in terms of shape or size. 
Shape categories:
1. Squamous - flattened, plate shaped
2. Columnar - Arranged in columns
3. Cuboidal - cube-like
Size categories:
1. Single layer - simple epithelium
2. Multiple layer - stratified epithelium

Question 4

Where are the different simple epithelia found?

Answer 4

The simple squamous epithelium are found in the lung alveolar (air sac) epithelium, mesothelium (lining major body cavities), endothelium lining blood vessels and other blood spaces). They form a thin epithelium that allows exchange to occur (e.g. gas exchange in the alveoli).
The simple cuboidal epithelium are typical of the linings found in ducts e.g. those lining the kidney collecting ducts.
The simple columnar epithelium are typical of surfaces involved in absorption and secretion of molecules. (e.g. enterocytes lining the gut, involved in the take up of the breakdown products of digestion.

Question 5

Where is the stratified squamous epithelium found?

Answer 5

In the various layers, the cell shapes vary. The squamous classification relates to the surface cells.
T​​here are two main types:
Keratinizing: Epithelial cells which produce keratin and in doing so die and become much stronger structures (e.g. epidermis (skin epithelium). Such cells lose their cellular organelles and nuclei, which are not visible under light microscopy.
Non-keratinizing: Epithelial cells which do not undergo keratinisation. They retain their nuclei and organelles. (e.g. epithelium lining the mouth, oesophagus, anus, cervix and vagina.

Question 6

What is the pseudo-stratified epithelium?

Answer 6

T​his epithelium appears to be multi-layered, but on close examination, the surface cells have contact with the basal lamina. e.g. airway (trachea and bronchi) epithelium, various ducts in the urinary and reproductive tracts.

Question 7

What is epithelial cell polarity?

Answer 7

The membrane can be seen to be organised into discrete domains by the formation of junctions. This membrane polarity is the key to generating a distinct polarity, with an apical domain at the lumenal (open) surface and a basolateral domain. The basal surface in contact with the extracellular matrix. The membrane between these two surfaces, where membranes of adjacent cells appose each other, is the lateral membrane.

Question 8

Why does epithelial cell polarity exist?

Answer 8

Most epithelial functions are directional, e.g. secretion, fluid and solute transport and absorption. These processes are not random but are highly organised. Epithelial polarity is required to give the directionality needed for epithelial function. Polarity in epithelial cells is seen as different regions of the cell surface being different from one another, with discretely organised cellular contents.

Question 9

What does secretion require in a cell.

Answer 9

Polarity. Most epithelia secrete in one direction or one way, they secrete either to the ethical aspect, intel lumen, or to the basal aspect and to the interstitial space.  So in order to do that, the secretary machinery has to be polarized.  If the secretary machinery was unpolarized, you will secrete both through the apical and basolateral compartments.

Question 10

What are the 4 types of cell junctions?

Answer 10

1. Tight junctions.
2. Adherens junction
3. Desmosomes
4. Gap Junction

Question 11

What are tight junctions?

Answer 11

There are the tight junctions which form a belt usually around the apical lateral membrane. These are involved in sealing the gaps between the cells.

Question 12

What are adherens junctions?

Answer 12

Found just below the tight junction is the adherens junction. Essentially the master junction which controls the formation of all of the others.

Question 13

What are desmosomes?

Answer 13

Scattered throughout the lateral membrane are desmosomes. These are spot junctions that form mechanically tough junctions between cells and are important in tissues that require to resist mechanical stresses.

Question 14

What are gap junctions?

Answer 14

Channel forming gap junction. These junctions actually form pores between cells and allows cells to exchange and share materials. These communicating junctions allow cells to form communities and to synchronize a number of activities.

Question 15

What are the specialised epithelia types?

Answer 15

1. Transporting Epithelium
2. Absorptive Epithelium
3. Secretory Epithelium

Question 16

What are the adaptations of the transporting epithelium?

Answer 16

In transporting epithelia, the plasma membranes contain high concentrations of ion transporters.
Typically, mitochondria are closely associated with extensive basal membrane infoldings, providing energy for active transport across the abundant membranes. The infoldings increase the amount of basal membrane that can pump ions and water. Mitochondria are concentrated in the basal aspect of the cell, close to the basal infoldings which contain the active transporters. The mitochondria provide the ATP required for active transport at these membranes. Because active transport is mainly confined to the basal membranes, ion and water. transport will have directionality.

Question 17

What are the adaptations of the absorptive epithelium?

Answer 17

Carriers transporting nutrients etc. are found on the brush-border membranes. The small intestine surface area is increased by the fact that it is long, and also by the interior surface of the wall of the small intestine being folded into numerous finger-like processes that point into the interior: the villi. The villi are covered with intestinal epithelial cells.
Microvilli are defined as plasma membrane projections of the villi. Secretory cells (goblet cells secreting mucus) interspersed among the absorptive cells of the intestinal villi.

Question 18

How does absorption in intestinal epithelial cell work?

Answer 18

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 19

What are the two main types of secretion?

Answer 19

Exocrine (into a duct or lumen) and Endocrine (into the bloodstream).

Question 20

Describe secretory apparatus of an exocrine secretory cell

Answer 20

The organelles are arranged for secretion from the apical plasma membrane. Secretory granules move from ER and Golgi Appartus to apical membrane.

Question 21

Describe secretory apparatus of an endocrine secretory cell

Answer 21

Endocrine cells secrete their contents to the basal aspect. The secretory vesicles are positioned so that when their contents are released, they have close access to the blood circulation.

Question 22

Describe constitutive and stimulated secretion

Answer 22

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
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 cells of the adrenal medulla after a fight-or-flight stimulus

Question 23

What is epithelial proliferation? Provide an example

Answer 23

Many epithelia are constantly “turning-over”, i.e. cells that are lost by cell death or by mechanical removal (e.g. abrasion) are replaced by the proliferation of stem cells within the epithelium. E.g. Cells in intestinal crypts replacing cells lost from the tips of intestinal villi.

Question 24

What are the effects of chemotherapy on proliferation?

Answer 24

Inhibition of the proliferation of intestinal crypt cells, e.g. in cancer chemotherapy, results in loss of the finger-like intestinal villi and flattening of the intestinal mucosa. This is responsible for many of the gastro-intestinal disturbances that are side-effects of chemotherapy. An example of a chemotherapy drug is 5-FU. The drug slows down or halts cell division in the crypts. 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 25

Describe process of epithelial turnover of epidermis

Answer 25

The epidermis is the keratinising stratified squamous epithelium of our body surface. Surface cells are constantly being lost, but are replaced by new cells being formed in the basal layer which migrate up while undergoing a programme of differentiation that eventually leads to them flattening out and keratinising. Each layer replaces the one above as the layers are lost from the surface.

Question 26

What is hyperproliferation?

Answer 26

Hyperproliferation of epithelial cells results in increased cell numbers and a thickening of cell layers. This can be in response to repeated or constant pressure. If the increase in cell production is greater than the cell loss from the surface, cells will accumulate creating an increased thick hard layer

Question 27

When does hyperproliferation occur?

Answer 27

Infections agents such as papilloma virus can also induce hyperproliferation. They do this by hijacking the cellular machinery of stratified squamous epithelia and inducing increased cell proliferation, which results in a surface growth.