2.3 Epithelial cells and tissues

Question 1

What are Tissues?

Answer 1

a group or groups of cells whose type, organisation and architecture are integral to its function

tissues are made up of cells, extracellular matrix and fluid

Question 2

What is the ECM (extra cellular matrix) ?

Answer 2

• material deposited by cells that forms the insoluble part of the extracellular environment
• generally composed of fibrillar proteins (e.g. elastin/collagen) embedded in hydrated gel (proteoglycans)
• May be poorly organised (e.g. loose connective tissue) or highly organised (e.g. tendon, bone, basal lamina)

Question 3

What are the 5 main cell types?

Answer 3

1. Connective tissue cells
2. Contractile tissues
3. Haematopoietic cells
4. Neural cells
5. Epithelial cells

Question 4

What is are epithelial cells?

Answer 4

cells forming continuous layers, these layers line surfaces and separate tissue compartments and have a variety of other functions.

Question 5

What tumour types are there and which tissues are they from?

Answer 5

• carcinomas- epithelial
• sarcomas- mesenchymal (connective tissue and muscle)
• leukaemias- haematopoietic cancer of bone marrow cells
• lymphomas- haematopoietic cancer of lymphocytes
• neuroblastomas- neural cell cancers of neurones
• gliomas- neural cell cancers of glial cells

Question 6

How are epithelial cells and tissues organised?

Answer 6

• epithelial cells make organised, stable cell-cell junctions to form continuous + cohesive layers
• epithelial layers line internal and external body surfaces and have a variety of functions e.g. transport, absorption, secretion, protection
• cell-cell junctions key to formation and maintenance of epithelial layers

Question 7

What are the 2 main criteria of epithelial classification?

Answer 7

their shape:
- squamous (flattened plate-shape)
- cuboidal
- columnar

their layering:
- single layer = simple epithelium
- multi-layered = stratified epithelium

Question 8

Epithelial shapes?

Answer 8

squamous (flattened plate-shape)
cuboidal
columnar

Question 9

Epithelial layering types?

Answer 9

• single layer- simple epithelium
• multi-layered- stratified epithelium

Question 10

What are the two main types of stratified squamous epithelium?

Answer 10

Keratinising

Non keratinising

Question 11

keratinising? (Epithelial cell types stratified squamous)

Answer 11

Produce keratin and die, becoming thicker and stronger protective structures (e.g. epidermis- skin epithelium) that protect underlying tissues from various physical and chemical insults e.g. heat, cold, solvents (alcohol), abrasion etc

They lose their cellular organelles and nuclei and aren’t visible under light microscope

Question 12

Non keratinising? (Epithelial cell types stratified squamous)

Answer 12

Don’t undergo keratinisation and retain nuclei and organelles

e.g. epithelium lining the mouth, oesophagus, anus, cervix and vagina

Question 13

What does pseudo-stratified mean?

Answer 13

Pseudo means false and the epithelium appears to be multi-layered

However on close examination, the surface cells have contact with the basal lamina

Question 14

Where are pseudo stratified epithelium found?

Answer 14

airway (trachea and bronchi) epithelium, various ducts in urinary and reproductive tracts

Question 15

How are transporter epithelial cells specialised?

Answer 15

• Their apical plasma membranes have high concs of ion transporters
• Mitochondria are concentrated at basal aspect of cell, close to extensive basal membrane infoldings which provides energy for active transport across the membranes
• Infoldings increase the amount of basal membrane that can pump ions and water

Question 16

How are absorptive epithelia specialised?

Answer 16

Carriers transporting nutrients etc are found on brush-border membranes e.g. absorptive intestinal cells (enterocytes), kidney proximal tubule cells

Small intestine SA is increased as it’s long and the interior wall surface is folded into numerous villi that are covered with intestinal epithelial cells

SA further increased by microvilli- plasma membrane projections of the villi

Question 17

How do nutrients cross brush border?

Answer 17

The BB contains large amounts of active transporters and channels for the uptake of nutrients from the lumen of the gut. 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 18

How are secretory epithelia specialised?

Answer 18

Secretory epithelium is often arranged in tubules and glands of varying complexity.

Alternatively in many epithelial tissues, individual, dispersed secretory cells can be present in the epithelium.

Question 19

What is exocrine secretion?

Answer 19

Secretion into a duct or lumen

Question 20

What is endocrine secretion?

Answer 20

Secretion into the bloodstream

Question 21

How are organelles in exocrine secretory cells arranged?

Answer 21

The organelles are arranged for secretion from the apical plasma membrane

Question 22

Where do endocrine cells secrete their contents?

Answer 22

Basal aspect

Secretory vesicles are positioned so that their contents have close access to the blood circulation when released.

Question 23

What are the domains of a typical epithelial cell?

Answer 23

Top apical domain at the lumenal (open) surface and a bottom basolateral domain in contact with the ECM

The membrane between these 2 surfaces where membranes of adjacent cells touch is the lateral membrane

Question 24

What is epithelial polarity and what’s it needed for?

Answer 24

different regions of the cell surface are different from one another with organised cellular contents

Needed to give the directionality needed for epithelial function

Question 25

Epithelial polarity in transporter epithelia
- What would happen if there was no polarity?

Answer 25

There would be no net directional flow as ions and fluid are transported in all directions

Question 26

How is polarisation important for secretion?

Answer 26

If the secretory machinery wasn’t polarised, you would secrete through both apical and basolateral compartments

Question 27

Why might unpolarised secretory machinery be dangerous?

Answer 27

if secreting digestive enzymes into a duct, you wouldn’t want it to be secreted through basal aspect and digest your own tissues

Question 28

What are the 4 main types of cell-cell junctions in epithelia?

Answer 28

Tight junctions
Adherens junction
Desmosomes
Gap junctions

Question 29

What does it mean that many epithelia are constantly turning over?

Answer 29

Cells that are lost by cell death/mechanical removal e.g. abrasion, are replaced by proliferation of stem cells within epithelium

Question 30

What is epithelial turnover like in the small intestine?

Answer 30

Cells are produced by stem cells in the crypts of Leiberkhun next to villi

They migrate up the villus epithelium and new cells are constantly produced to replace the ones constantly being lost from the villus tip
(Animation on slides)

Question 31

What does inhibition of proliferation of intestinal crypt cells (e.g. in cancer chemo) do?

Answer 31

Results in loss of intestinal villi and flattening of intestinal mucosa

This is responsible for many gastrointestinal disturbances that are side effects of chemo

Question 32

What is epithelial turnover like in the epidermis?

Answer 32

Epidermis is the keratinising stratified squamous epithelium of body surface

Surface cells continually lost and replaced by new cells formed in basal layer which migrate up while undergoing programmed differentiation that leads to them flattening out and keratinising

Each layer replaces the one above as layers are lost from surface

Question 33

What does papilloma virus do?

Answer 33

Induces hyperproliferation by hijacking cellular machinery of stratified squamous epithelia and inducing increased cell proliferation which results in a surface growth e.g. wart