Epithelial cells and tissues Flashcards
What are tissues?
Group of cells whose type, organisation and architecture are integral to its function
Tissues are made up of cells, ECM and fluid
What the the 5 main cell types?
- Connective tissue cells: fibroblasts, chondrocytes (cartilage), osteocytes (bone)
- Contractile tissue: skeletal muscle, cardiac muscle, smooth muscle
- Haematopoietic cells: blood cells, tissue-resident immune cells
- Neural cells: cells of nervous system- neurones and glial cells
- Epithelial cells
What are the names of some tissue-associated tumours?
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
How are epithelia organised?
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
What are the 2 main criteria of epithelial classification?
Shape and layering
How can epithelia shape be organised?
Squamous
cuboidal
columnar
How can epithelia layering be organised?
Single layer = simple epithelium
Multi-layer = stratified epithelium
How is the simple squamous epithelium structured and were can it be found ?
Found in lung alveolar epithelium, mesothelium (lines major body cavities) and endothelium lining blood vessels
Form a thin epithelium which allows exchange to occur
Where is simple cuboid epithelium found?
Typical of linings found in ducts
e.g. those lining kidney
Where is simple columnar epithelium found?
Typical of surfaces involved in absorption and section of molecules
e.g. enterocytes lining gut
What are 2 types of squamous classification?
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
What is a benefit of keratinising epithelium?
Can form thick layers that protect underlying tissues from various physical and chemical insults e.g. heat, cold
What are pseudo-stratified epithelium?
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
What is the importance of epithelial polarity?
Epithelial polarity is required to give directionality needed for epithelial function
What happens when transporting epithelia aren’t polarised?
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
What happens when transporters are polarised?
You can achieve directionality because you’re only pumping one aspect of the plasma membrane
Flow will be in desired direction
Why is polarity required for secretion?
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
What happens if secreting machinery is unpolarised?
You’ll secrete through both the apical and basolateral compartments
This could be catastrophic e.g. secreting digestive enzymes into the wrong compartment
What is a tight junction?
Form a belt usually around the apical lateral membrane
Are involved in sealing the gap between cells
What is an adheren’s junction?
Found just below the tight junction
Essentially the master junction which controls the formation of all the others
What are desmosomes?
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
What are gap junctions?
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
How are transporting epithelia specialised?
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
How is fluid transported in epithelial cells?
Mitochondria provide ATP for active transport
Because active transport is confined to basal membranes, ion and water transport will have directionality
How is absorptive epithelium specialised?
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
How is secretory epithelium specialised?
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
What are the 2 main types of secretion?
Exocrine: into duct/ lumen
Endocrine: into bloodstream
How is an endocrine secreting cell structured?
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
How are endothelial cells categorised based on the way they secrete?
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
What does “turning-over” mean?
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
What happens when proliferation of intestinal crypt cells is inhibited?
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
How is epithelial turnover achieved in the epidermis?
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
What is hyperproliferation?
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
What infections can induce hyperproliferation?
Papilla virus- hijack the cellular machinery of stratified squamous epithelia and induce increased cell proliferation
