4 Cells to Tissues Flashcards
When do cells first coalesce?
During embryogenesis (5 days after fertilisation).
What is the fate of the embyroblast?
Differentiate into 3 germ celllines: ectoderm(outer), mesoderm (middle), endoderm (inner). These then differentiate into specialised cells.
Ecto eg neuronal, surface ectoderm
Meso: all tissues are lined with epithelial cells
Endo: gut tube: thyroid, liver, GI tract, pancreas…
Most tissues in the body are mesoderm.
What holds cells together?
Cell-cell adhesion molecules
Extracellular matrix proteins (fibres)
Internal-external scaffolding
Close proximity (pressure effects)
Connective tissue structure
Cell: mainly fibroblasts, fixed adipocytes, reticular cells.
Fibres: collagen, elastin, reticular fibres.
Ground substances: glucosaminoglycans.
Connective tissue functions
- Binding and supporting
- protecting
- insulating
- storage of fuel
- substance transportation
- tissue separation
Connective tissue organisation
Plentiful extracellular matrix
Cells are sparsely distributed
Matrix is rich in fibrous polymers eg collagen
Rarely have direct cell-cell attachments
What is the primary connective tissue cell?
Mesenchymal stem cell
—> has the ability to interconvert between several cell types
Epithelial tissue layer organisation
Cells are tightly bound together into sheets called epithelia
Extracellular matrix is scant, mainly consists of thin basal lamina which underlies the epithelium
Cell-cell adhesion
Strong intracellular protein filaments (cytokeratin) cross cytoplasm of each cell and attach to specialised junctions in PM to help tire adjacent cell together and to the basal laminar
Epithelial cell adherence systems:
Lateral surface
Basal surface
In the lateral surface:
- tight junctions
- adheren junctions
- desmosomes
- gap junctions
- cell adhesion molecules
In the basal surface:
- hemi-desmosome
- focal adhesions
- integrins
- proteoglycans
- cell adhesion molecules
Tight Junctions
Where?
Role?
Structure?
Always at very top of cell nearest apical surface in the lateral border
Long cell to cell fusion point
Role is to prevent movement of larger molecules through the outer layer/ lumen into deeper tissue of the organ
In the gut: can allow small molecules to cross when opened (paracellular transport)
Adhesion junctions
On lateral surface
Found in pairs
Formed from intracellular actin filaments
Linked to E-cadherin proteins that cross the intercellular space.’
Found only in epithelial and endothelial cells
Functions as tissue stabilising factor and additional transport barrier
Found a 1/3 distance from luminal surface so will form an adhesion belt through the tissue
Desosome
Strongest of all cell to cell adhesions
1/2 from top and bottom of cell
Random distribution
Found in tissues that experience mechanical stress: cardiac, bladder, gastro, epithelia
Role to provide mechanical strength and prevent tissue destruction
ONLY adhesion molecule found in the epidermal cells (skin)
Gap Junctions
Found close to base of epithelial cells
Distributed throughout cardiac and smooth muscle cells
Role to communicate quickly changes in intercellular molecular composition
Allows free flow of small molecules
Allows waves of electrical impulse in smooth muscle
ONLY spermatozoa and erythrocytes DON’T have gap junctions
Structure: made of connexins arranged hexagonally, use ATP
Hemi- desosome
ONLY found on basal surface of epithelial cells
Attach to extracellular matrix not to cells eg collagen
Role: anchor epithelial cells to the basal lamina and prevent loss to external surface
Attaches to laminin through integrins
Focal adhesions
Attach to basal lamina
Using intracellular actin filaments and integrins
Binds to fibronection
