2. Epithelial tissues Flashcards
Epithelia are classified by
Shape
Layering
What shapes can Epithelia take?
Squamous (flattened plate-shape)
Cuboidal
Columnar
What are the types of layering in Epithelia?
Single layer = simple epithelium
Multi-layered = stratified epithelium
Where is simple squamous epithelium found?
Lung alveolar
Mesothelium (lining major body cavities)
Endothelium lining blood vessels and other blood spaces
Where is simple cuboidal epithelium found?
Lining kidney collecting duct
Many other ducts
Where is simple columnar epithelium found?
Enterocytes (intestinal absorptive)
Many other absorptive and secretory epithelia
Where is keratinising stratified squamous epithelium found?
Epidermis (skin)
nuclei not visible in surface layer cells
Where is non-keratinising stratified squamous epithelium found?
Linings of mouth, oesophagus, anus, cervix and vagina (nuclei are visible in surface layer cells)
Where is pseudostratified epithelia found?
Airway (trachea & bronchi)
Various ducts in the urinary and reproductive tracts
Epithelia functions require …
Polarity
Must be directional
Epithelial layer surfaces
Apical surface at lumens (open) surface
Basal surface in contact with ECM
Basal & Lateral membrane of Epithelia are usually grouped
Basolateral membrane
What can segregate the epithelial plasma membrane into apical & basolateral domains?
Belt junctions
2 Types of cell-cell junctions between Epithelial cells
- Zonulae (continuous belts)
* Maculae (discrete spots)
Usually, cell-cell junctions are arranged as an apical junctional complex containing a…
Tight Junction nearest the apex and an Adherens junction just below it
Where are desmosomes (spot adhering junctions) often found?
Scattered throughout lateral membrane
What do Gap Junctions do?
Act as regions of direct communication between adjacent cells
What do cell-cell junctions do?
Give epithelia mechanical integrity
Act to seal intercellular pathways of the layer
Tight junctions seal the paracellular pathway
Prevents things passing between membrane of adjacent cell
Paracellular
Passing/ situated beside/between cells
Transcellular
Passing through cells
Tight Junctions allow cells to establish and maintain apical- basolateral polarity by…
Preventing the mixing of proteins and lipids between different plasma membrane compartments
Zonula occludens are also known as
Tight junctions
What is the consequence of paracellular pathways (between cells) being sealed by tight junctions?
Concentration differences across cell layers can be maintained
The more elaborate the network of contacts in a tight junction…
The tighter the seal
Which pathways do tight junctions seal?
Paracellular pathways (Gate function)
What is the fence function of tight junctions?
TJ’s segregate apical and basolateral membrane polarity
Describe the membrane in transporting epithelia e.g. distal convoluted tubules
Apical membrane contains many ion/ water channels
Basal membrane has many infoldings and mitochondria for active transport
What increases efficiency of absorption? How can this be achieved?
Increasing surface area
Make it longer
Fold the lining to form structures e.g. villi
Create tubular protrusions e.g. microvilli
Secretory tissues: The pancreas has both … and … functions
Exocrine
Endocrine
Exocrine
Secretion into lumen or duct
Endocrine
Secretion into bloodstream
Example of exocrine secretors
Goblet cells
Pancreateic acinar cells
Direction of movement in exocrine secretions
Basal to Apical
Direction of movement in endocrine secretions
Apical to Basal
Other than endocrine and exocrine, how else can secretory cells be classified?
Constitutive
Stimulated
Constitutive secretory cells
secretory vesicles, as they are formed, move directly to plasma membrane and release their contents
Stimulated secretory cells
secretory vesicles stored in cytoplasm and only after signal fuse with plasma membrane to release their contents
Example of constitutive endocrine secretion
production of plasma proteins by hepatocytes
Example of stimulated endocrine secretion
release of adrenaline from cells of adrenal medulla after fight-or-flight stimulus
Example of stimulated exocrine secretion
when stomach contents enter duodenum, pancreatic acinar cells are stimulated to release their digestive enzymes into ducts
What type of epithelia do protective epithelia usually constitute of?
Stratified squamous
Give 2 examples of protective epithelia
Skin epidermis
Lining of oesophagus
How does protective epithelia perform its function?
Can form thick layers that protect underlying tissues from various physical and chemical insults
e.g. heat, cold, solvents (alcohol), abrasion
What are the 2 forms of stratified squamous epithelia?
Keratinising
Non-keratinising
What is keratinising stratified squamous epithelia?
Cells of upper (surface) layers are “dry”
because the cells have hardened and died
Their nuclei are not visible
What is non-keratinising stratified squamous epithelia?
upper (surface) layer is “wet”
because cells are alive
Their nuclei are visible.
What are the 3 main layers in skin?
Epidermis
Dermis
Hypodermis
What type of epithelial cells does the epidermis have?
Keratinising stratified squamous epithelial layer
What is the main function of the epidermis?
Act as a barrier to environment
What do defects in cytokeratins or cell junctions lead to?
Blistering diseases as a result of damage to fragile epidermal cells
What do desmosomes mechanically link?
Cells to each other and the ECM (via hemidesmosomes)
What happens if desmosome function is compromised?
Epithelia become fragile and easily damaged
What do cervical smear tests sample? What is looked for?
Cells of the non-keratinising stratified squamous epithelium of the uterine cervix
Looking for abnormal nuclei, size and shape
Cell turnover in tissues
A balance between cell proliferation and cell death is key to maintaining architecture of tissues.
How often is the lining of the gut replaced?
Every 3-10 days
How often is fat tissue, heart muscle and bone replaced?
8-10 years
How often are most of the neuronal cells of the CNS replaced?
Most are not replaced
If they die they are lost
Cell turnover in epithelia
Consistent
Whole population replaced over relatively short periods
What is responsible for the renewal of surface epithelial cells in the intestine?
Cell division by stem cells in the intestinal crypts
Epithelial-turnover in the small intestine
New cells are constantly being produced by the crypt stem cells, to replace the cells constantly being lost from villus tip.
How does disturbing the balance of cell proliferation/loss affects tissue dynamics?
Reduce cell proliferation
Increase cell proliferation
How does Antimetabolite chemotherapy drug, 5-FU effect cell turnover in the intestines?
Slows/ stops cell division in crypts
Cell loss from villus tips continues as normal
Failure to produce new cells to replace loss means villi shorten
What occurs when there is too much cell proliferation?
Rate of cell loss is not sufficient to maintain normal tissue volume
What happens if a cell mutates so proliferation is dysregulated in the colon?
Too much tissue is formed: a benign tumour
= Adenoma (polyp)
Although adenomas are not malignant…
They have a high risk of acquiring more mutations that switch to cancer
Cell turnover in epidermis
Surface cells are constantly being lost
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.
Examples of infectious agents that affect the turnover of the epidermis
Warts result from infection of epidermal keratinocytes by 1 of the family of HPV
Virus hijacks cell proliferation machinery and increases cell proliferation.
If cell loss = cell production
Tissue is in steady state
If cell loss > cell production
Reduction in tissue mass results
If cell loss < cell production
Increase in tissue mass
Changes to steady state of cell turnover can be…
Physiological
Pathological
Examples of physiological change in cell turnover are seen in…
Cyclic production and loss of endometrial epithelial lining of uterus in menstrual cycle.
The large increase, during pregnancy, in number and size of epithelial glands of breast and their loss after weaning
