Lecture 7: Integrating cells into tissues, organs and systems Flashcards
What holds cells together at the molecular level?
- Cell-cell adhesion molecules
- Extracellular matrix proteins (fibers)
- Internal-external scaffolding
- Close proximity (pressure effects)
What are the epithelial cell adherence systems in the lateral surface?
- Tight junctions
- Adherens junctions
- Desmosomes (adhesion plaque)
- Gap junctions
- Cell adhesion molecules
What are the epithelial cell adherence systems in the basal surface?
- Hemi-desmosomes
- Focal adhesions
- Integrins
- Proteoglycans
- Cell adhesion molecules
Where are tight junctions found?
Always at the very top of the cell nearest to the lumen or apical surface in the lateral border
What are the key features of tight junctions?
Relatively long cell-to-cell fusion point
What is the role of tight junctions?
Seals neighboring cells together in an epithelial sheet to prevent movement of larger molecules through the outer layer/lumen into the deeper tissue layers of the organ
Where are adhesion junctions found?
Almost always found 1/3rd distance from luminal surface
What are the features of adhesion junctions?
- In lateral surface
- Found in pairs
- Formed from intracellular actin filaments
- Linked to E-cadherin proteins that cross the intercellular space
- Also known as ‘adhesion belt’
- Found only in epithelial and endothelial cells
What is the role of adhesion junctions?
Functions as tissue stabilizing factor and additional transport barrier by joining an action bundle in one cell to a similar bundle in a neighboring cell
Where are desmosomes found?
Halfway between top and bottom of cells
What are the features of desmosomes?
- Strongest of all cell-to-cell adhesions
- Random distribution pattern
- Found in tissues that experience intense mechanical stress
- Made of cytokeratin fibers intracellularly and E-cadherins intercellularly
- Only cell-to-cell adhesion found in epidermal cells
What is the role of desmosomes?
Provide mechanical strength and prevent tissue destruction by joining the intermediate filaments in one cell to those in a neighbor
Where are gap junctions found?
Close to the base of epithelial cells
What are the features of gap junctions?
- Important in smooth muscle contraction
- Only spermatozoa, erythrocytes and other motile cells don’t have gap junctions
- Consists of cylinders of proteins arranged in hexagonal pattern that open and close
What is the role of gap junctions?
- Quickly communicate changes in intercellular molecular composition
- Allows free movement of small molecules from one cell to another
- Allows wave of electrical impulse - important in smooth muscle contraction
What are cell adhesion molecules?
Trans membrane proteins that bind to each other and to extra cellular materials
Where are hemi-desmosomes found?
Only found on basal surface of epithelial cells
What are the features of hemi-desmosomes?
- Resemble half of a desmosome
- Rather than attaching one cell to another, it attaches a cell to extracellular filaments in basement membrane
- Intracellular intermediate filaments of cytokeratin attached today laminin through integrins
What is the role of hemi-desmosomes?
Anchors intermediate filaments in a cell to the basal lamina
Where are focal adhesions found?
Basal lamina
What are the features of focal adhesions?
- Uses intracellular actin filaments (instead of cytokeratin in hema-desmosomes)
- Uses integrins (like hemidesmosomes)
- Binds to fibronectin
- When bound to fibronectin, conformational change results in binding to collagen fibers
What is the role of focal adhesions?
Similar to hemi desmosomes but anchor intermediate filaments in a cell to the basal lamina and stabilize its position
What are the features of integrins?
- Central to cohesive forces holding tissues together
- Always work as alpha-beta dimer
What are the features of the mucosal membrane?
- Lines all the moist hollow internal organs of the body
- Continuous with the skin at various body openings
- Most mucous membranes secrete mucus, a thick protective fluid
What is the function of the mucosal membrane?
- Stop pathogens and ‘dirt’ from entering the body
- Prevent bodily tissues from becoming dehydrated
- Lubricate the surface
What are some examples of mucosal membranes?
- GI tract
- Urinary tract
- Respiratory tract
What is the structure of the GI tract?
- Mucosa: epithelial cell lining + lamina propria (supporting loose connective tissue layer) + muscularis mucosae (thin discontinuous smooth muscle layer)
- Submucosa: connective tissue layer containing arteries and veins
- Muscularis externa: smooth muscle layer with muscle fibers going in 2 directions (inner circular + outer longitudinal)
- Serosa: outermost layer of loose connective tissue
What are the functions of the GI tract mucosa?
- Absorb substances from the lumen
- Prevent ingress of pathogens
- Move contents and expel waste
For the GI mucosa, how does the structure of its constituent tissues relate to its function?
- Epithelial cell specializations help to absorb substances from the lumen and prevent ingress of pathogens
- Lamina propria contains lymphatic tissue to help prevent ingress of pathogens
- Muscularis mucosae folds mucosa to increase surface area, hence aiding to absorb substances from the lumen and prevents ingress of pathogens
- Muscularis externa performs peristalsis to aid the moving of contents and expelling of waste
What is the structure of the urinary tract?
- Epithelium also known as urothelium
- Muscularis mucosa
- Lamina propria
- Inner longitudinal muscle
- Outer circular muscle
What are the functions of the urinary tract mucosa?
- Absorption of essential nutrients in the kidney
- Prevention of pathogen entry (especially lower urinary tract)
- Removal of waste products
For the urinary mucosa, how does the structure of its constituent tissues relate to its function?
- Epithelial cells produce mucus which protects the bladder from damage by acidic urine
- Tight junctions in epithelial cells prevents leakage to inner cell layers
- Mucus glands produce large amounts of sticky mucus and so prevent ingress of pathogens
What is the structure of the trachea and primary bronchi?
- Conducting portion: Nasal cavity to bronchioles
- Mucosa
- Very thin lamina propria
- No longitudinal muscularis mucosa layer
- Submucosa: connective tissue layer containing mainly collagen, elastin fibers and fibroblasts
- C-shaped Hyaline cartilage: 2 layers (perichondrium which has fibroblasts that lay down collagen fibers and chrodrogenic layer from which cartilage is formed)
- NO outer layer of smooth muscle
What is the structure of the secondary and tertiary bronchi?
- Epithelium
- Smooth muscle
- Submucosa
- Crescent shaped cartilage
- No outer layer of muscle
Similar to primary except the cartilage is not present as full circle of rings
What is the function of the conducting portion of the respiratory tract?
- Provide route for incoming and outgoing air
- Remove debris and pathogens from incoming air
- Warm and humidify incoming air
What is the structure of the alveolus?
- Specialized Epithelial cells
- Basal lamina
- Connective tissue ‘muscle’ layers
- Small amounts of collagen at the junction
What is the function of the conducting portion of the respiratory tract?
Ensure fast efficient transfer of oxygen and carbon dioxide between blood and air
For the alveolus, how does the structure of its constituent tissues relate to its function?
- Folds in basal lamina allow for expansion of air sacs when air is drawn into the lungs
- Connective tissue ‘muscle’ layers created by collagen and elastin fibers which provides elastic recoil to return the sac to empty state on exhaling gases
For the trachea and bronchi, how does the structure of its constituent tissues relate to its function?
Secretions from epithelium and Submucosa glands contain:
- Mucins and water: make sticky mucus
- Serum proteins: lubricates surfaces
- Lysozyme: destroys bacteria
- Anti-pro teases: inactivate bacterial enzymes
Mucociliary escalator in trachea
- Using a ‘cilia wave’, mucus move materials to the oral cavity where material can be swallowed
