cellular organisation Flashcards
lecture 8
What are the basic types of animal tissues?
Epithelial: High cell density, less ECM (gut lining, skin)
Muscular: Enables movement
Nervous: Transmits signals
Connective: Low cell density, more ECM (e.g., bone, cartilage)
Why do animal cells form tissues?
- For structural strength and support
- To enable specialised functions
- To allow force transmission via cell-cell or cell-ECM interactions
What is the structural polarity in epithelial cells?
Apical side: Faces the lumen or external environment
Basal side: Attached to the ECM or basal lamina
What is the basal lamina?
A thin layer of ECM under epithelial cells, providing structural support and anchoring cells
How do epithelial and connective tissues differ in cell-to-ECM ratios?
Epithelial: High cell density, low ECM
Connective: Low cell density, high ECM
What are the four types of cell junctions?
Adherens junctions: Cadherin-mediated, connect to actin filaments
Desmosomes: Cadherins link to intermediate filaments
Tight junctions: Act as selective permeability barriers.
Gap junctions: Water-filled channels for cell communication
How do adherens junctions work?
Cadherins form homophilic, Ca2+- dependent bonds.
Linked to actin filaments via catenins and adapter proteins like vinculin
What are desmosomes?
Similar to adherens junctions, but connect intermediate filaments using specialised cadherins, providing mechanical strength
What is the function of tight junctions?
Create a selective barrier in epithelial tissues.
Allow a differential transport across apical and basal membranes
(e.g., glucose transport in the intestine)
What are gap junctions?
- Formed by connexins or innexins
- Enable direct communication via water-filled channels, allowing ions and small molecules to pass through.
What role do cell junctions play in tissues?
Facilitate direct cell-cell interactions
Transmit mechanical forces via cytoskeletal filaments
How do epithelial cancers affect adherens junctions?
Many epithelial cancers downregulate E-cadherin, reducing cell adhesion and enabling metastasis
What is a specialised connective tissue example?
Bone or cartilage
Why don’t animal cells have cell walls?
They rely on tissues instead to provide structural strength and allow force transmission
What are examples of epithelial tissues?
Lining of the gut and epidermal layers of the skin
How does calcium affect adherens junctions?
Calcium binding prevents cadherin flexing, stabilising homophilic cadherin-cadherin interactions
What is the function of Rac and Rho in adherens junctions?
They regulate cytoskeletal dynamics and maintain junctional integrity under mechanical force
How do tight junctions contribute to intestinal glucose transport?
Differential transporter molecules on apical (glucose uptake) and basal (glucose release) membranes mediate transport across the epithelium.
How do gap junctions regulate communication?
Gap junctions can open or close in response to signals, such as dopamine reducing communication in retinal neurons
What are the 2 main ways cells interact in tissues?
- Directly via cell junctions
- Indirectly via interactions with the ECM
How does the ECM differ between tissues?
High ECM content in connective tissues
Low ECM content in epithelial tissues
What are examples of muscular tissue?
Smooth muscle, skeletal muscle, and cardiac muscle
How does the ECM support epithelial tissues?
It anchors epithelial cells and transmits forces through the basal lamina
What structural components make desmosomes strong?
Specialised cadherins connect intermediate filaments, enabling resistance to mechanical stress
Why are gap junctions size-restrictive?
Their small pore size prevents macromolecules from passing through, allowing only ions and small molecules to exchange
How do cell junctions withstand mechanical force?
Membrane protrusions initiate cell-cell contact.
RAC: Recruits actin and cadherins to expand junctions, recruiting more cadherins and catenins.
**RHO: **Remodels actin and recruits myosin, forming contractile actin-myosin bundles that strengthen the junction.
What is the function of RAC in adherens junctions?
RAC facilitates actin polymerisation and cadherin recruitment, expanding cell junctions.
What role does RHO play in adherens junctions?
RHO supports actin remodelling and recruits myosin, creating contractile actin-myosin bundles for stronger junctions.
What are the components of gap junctions?
- Interacting plasma membranes of adjacent cells.
- Water-filled channel (~1.5 nm diameter).
- Connexon: Composed of six subunits.
- Two connexons align to form an open channel.
What is the size of a gap junction channel?
Approximately 1.5 nm in diameter.
How is a connexon structured?
A connexon is composed of six protein subunits that form a circular structure, which pairs with another connexon from an adjacent cell to create a functional channel.
What is the role of tight junctions?
Seal neighbouring cells in epithelial sheets, preventing extracellular molecule leakage.
Help polarise cells by separating apical and basal membranes.
What is the function of adherens junctions?
Join actin bundles in one cell to actin bundles in neighbouring cells, providing mechanical strength and coordination.
What is the function of desmosomes?
Connect intermediate filaments between adjacent cells, providing structural integrity and resistance to mechanical stress.
What is the function of gap junctions?
Form channels that allow the exchange of small, water-soluble molecules, inorganic ions, and metabolites between adjacent cells.
How do hemidesmosomes function?
Anchor intermediate filaments in a cell to the extracellular matrix, stabilizing the cell within tissues.
What is the function of actin-linked cell-matrix junctions?
Anchor actin filaments in a cell to the extracellular matrix, providing structural support and facilitating cell-ECM interactions.
What are the four types of direct cell-cell interactions?
Tight junctions: Prevent leakage of extracellular molecules and polarise cells.
Adherens junctions: Join actin bundles between cells.
**Desmosomes: **Join intermediate filaments between cells.
Gap junctions: Allow exchange of small molecules and ions.
What is the diameter of gap junction channels, and why is this significant?
~1.5 nm, allowing passage of small molecules while preventing the movement of macromolecules.
How do tight junctions help maintain cellular polarity?
By separating apical and basal membrane domains, ensuring selective transport and compartmentalisation.
What proteins are involved in adherens junctions?
Cadherins and actin filaments, linked via adapter proteins like catenins.
What is the key structural difference between desmosomes and adherens junctions?
Desmosomes connect intermediate filaments, while adherens junctions connect actin filaments.
How are gap junctions regulated?
Gap junctions can open or close in response to cellular signals, such as neurotransmitters like dopamine.
Why are hemidesmosomes important in tissue integrity?
They anchor cells to the ECM, ensuring mechanical stability and maintaining tissue organisation.
What is the role of the extracellular matrix in actin-linked junctions?
It serves as the anchoring site for actin filaments, mediating force transmission and structural support for cells.
