Epithelial cells and tissues

Question 1

What are tissues?

Answer 1

Group of cells whose type, organisation and architecture are integral to its function
Tissues are made up of cells, ECM and fluid

Question 2

What the the 5 main cell types?

Answer 2

1. Connective tissue cells: fibroblasts, chondrocytes (cartilage), osteocytes (bone)
2. Contractile tissue: skeletal muscle, cardiac muscle, smooth muscle
3. Haematopoietic cells: blood cells, tissue-resident immune cells
4. Neural cells: cells of nervous system- neurones and glial cells
5. Epithelial cells

Question 3

What are the names of some tissue-associated tumours?

Answer 3

Carcinoma: epithelial cancer
Sarcoma: mesenchymal (connective tissue and muscle) cancer
Leukaemia: haematopoietic cancer from bone marrow cells
Lymphoma: haematopoietic cancer from lymphocytes
Neuroblastoma: neural cell cancer from neural cells
Gliomas: neural cell cancer from glial cells

Question 4

How are epithelia organised?

Answer 4

Form continuous, cohesive layers
They line internal and external body surfaces and have variety of function
Cell- Cell junctions key to formation and maintenance of epithelial layers

Question 5

What are the 2 main criteria of epithelial classification?

Answer 5

Shape and layering

Question 6

How can epithelia shape be organised?

Answer 6

Squamous
cuboidal
columnar

Question 7

How can epithelia layering be organised?

Answer 7

Single layer = simple epithelium

Multi-layer = stratified epithelium

Question 8

How is the simple squamous epithelium structured and were can it be found ?

Answer 8

Found in lung alveolar epithelium, mesothelium (lines major body cavities) and endothelium lining blood vessels
Form a thin epithelium which allows exchange to occur

Question 9

Where is simple cuboid epithelium found?

Answer 9

Typical of linings found in ducts

e.g. those lining kidney

Question 10

Where is simple columnar epithelium found?

Answer 10

Typical of surfaces involved in absorption and section of molecules
e.g. enterocytes lining gut

Question 11

What are 2 types of squamous classification?

Answer 11

Keratinising: epithelial cells produce keratin and die in doing so- become strong structures e.g. epidermis. They loose their organelles and nuclei

Non-keratinising: don’t undergo keratinisation. Retain their organelles and nuclei e.g. oesophagus, anus, vagina

Question 12

What is a benefit of keratinising epithelium?

Answer 12

Can form thick layers that protect underlying tissues from various physical and chemical insults e.g. heat, cold

Question 13

What are pseudo-stratified epithelium?

Answer 13

Epithelium appears multi-layered but on close examination the surface cells have contact with the basal laminae
e.g. airway epithelium, various ducts in urinary and reproductive tracts

Question 14

What is the importance of epithelial polarity?

Answer 14

Epithelial polarity is required to give directionality needed for epithelial function

Question 15

What happens when transporting epithelia aren’t polarised?

Answer 15

In transporting epithelia (transport of ions and fluids across epithelial layers), pumps and channels need to be polarised
When they’re not polarised they’re present in all parts of the plasma membrane- this means they’re pumping apically and basolaterally
The consequence is that direction of flow is in every direction

Question 16

What happens when transporters are polarised?

Answer 16

You can achieve directionality because you’re only pumping one aspect of the plasma membrane
Flow will be in desired direction

Question 17

Why is polarity required for secretion?

Answer 17

Most epithelia secrete in one direction or one way

They secrete either to the ethical aspect, intel lumen or basal aspect and to the interstitial space

Question 18

What happens if secreting machinery is unpolarised?

Answer 18

You’ll secrete through both the apical and basolateral compartments
This could be catastrophic e.g. secreting digestive enzymes into the wrong compartment

Question 19

What is a tight junction?

Answer 19

Form a belt usually around the apical lateral membrane

Are involved in sealing the gap between cells

Question 20

What is an adheren’s junction?

Answer 20

Found just below the tight junction

Essentially the master junction which controls the formation of all the others

Question 21

What are desmosomes?

Answer 21

Scattered throughout the lateral membrane
These are spot junction that form mechanically tough junctions between cells and are important in tissues that need to resist mechanical force

Question 22

What are gap junctions?

Answer 22

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

Question 23

How are transporting epithelia specialised?

Answer 23

Mitochondria are closely associated with extensive basal membrane infoldings providing energy for active transport across abundant membranes
Infoldings increase amount of basal membrane that can pump ions and water

Question 24

How is fluid transported in epithelial cells?

Answer 24

Mitochondria provide ATP for active transport

Because active transport is confined to basal membranes, ion and water transport will have directionality

Question 25

How is absorptive epithelium specialised?

Answer 25

Carriers transporting nutrients are found on brush border membranes
Small intestine surface area increased by the fact that its long and there are many plasma membrane projections (microvilli)
Secretory cells are interspersed among absorptive cells of intestinal villi

Question 26

How is secretory epithelium specialised?

Answer 26

Epithelium is often arranged unto tubules and glands of varying complexity in secretory function
However in many epithelial tissues, individual dispersed secretory cells can be present in epithelium

Question 27

What are the 2 main types of secretion?

Answer 27

Exocrine: into duct/ lumen
Endocrine: into bloodstream

Question 28

How is an endocrine secreting cell structured?

Answer 28

Secrete their contents into the basal aspect

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

Question 29

How are endothelial cells categorised based on the way they secrete?

Answer 29

Constitutive: secreting vesicles move directly to plasma membrane and release their contents
Stimulated: secretory vesicles are stored in cytoplasm and only fuse with plasma membrane to release their contents

Question 30

What does “turning-over” mean?

Answer 30

Epithelial cells that are lost by cell death or mechanically removed are replaced by proliferation of stem cells in epithelium
e.g. cells in intestine crypts replacing cells lost from tips of intestinal villi

Question 31

What happens when proliferation of intestinal crypt cells is inhibited?

Answer 31

There is loss of intestinal villi and flattening of intestinal mucosa
This leads to many gastro-intestinal disturbances- can be a side effect of chemotherapy

Question 32

How is epithelial turnover achieved in the epidermis?

Answer 32

surface cells are constantly lost but are replaced by new cells being formed in basal layer which migrate up while undergoing differentiation that leads to flattening and keratinising

Question 33

What is hyperproliferation?

Answer 33

Results in an increase in cell number and thickening of cell layers
This can be in response to repeated or constant pressure
If cell production is greater than cell loss then cells will accumulate leading to thick hard layer - “hard-skin” or “corns” due to pressure on skin

Question 34

What infections can induce hyperproliferation?

Answer 34

Papilla virus- hijack the cellular machinery of stratified squamous epithelia and induce increased cell proliferation