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H.V. Wilson Sponge Experiment 

• First demonstrated the ability of cells to recognize and adhere to one another 

• Used the cells of 2 sponge species 

• Their indiv cells were seperated using a fine mesh 

• The cells were then mixed together 

• Overtime, the cells from the same species were able to recognize and associate back together 

  • Cells from diff species didn’t associate 

Johannes Holtfreter: Frog Embryo Experiment

• Showed cell recognition and adhesion using frog embryos

• Took cells from 2 different developmental germ layers and seperated indiv cells 

• Similar tissue recognized eachother and associated

• The associations mimicked original embryo organization

What are the three developmental germ layers of an early embryo

1. Endoderm 

2. Ectoderm

3. Mesoderm 

During embryogenesis, how do cells recognize and stay together?

• Requires transmembrane proteins called CAMs (cell adhesion molecules)

• After aggregation, they form specialized junctions stabilizing the cell interactions

• Facilitated communication between adjacent cells 

How are epithelial cells organized, and what do they form in the body?

• Epithelial cells connect along their lateral surfaces to form epithelial sheets.

• These sheets line body cavities and cover surfaces like the digestive tract and skin.

• Each epithelial cell has distinct surfaces:

  • Apical surface: faces the lumen or outside (e.g., with microvilli in the intestine).

  • Basal surface: faces inward and is attached to the basal lamina / basement membrane.

What connects epithelial cells to the underlying extracellular matrix?

• The basal surface anchors to the basal lamina (basement membrane).

• Hemidesmosomes are adhesion complexes that connect the cell’s basal side to the extracellular matrix (ECM), providing structural support.

Which types of adhesion complexes connect the lateral surfaces of epithelial cells?

1. Tight junctions

2. Adherens junctions

3. Desmosomes

4. Gap junctions

Tight Junctions

• zonula occludens 

• Connect adjacent cells below the apical surface 

• It completely seals the space between the cells 

  • Prevents fluid from moving across the layer

  • Restricts diffusion of small molecules in gastrointestinal track to prevent enzyme leakage 

• Done by linear arrays of occludin and claudin 

  • ‘Pinches’ cells together 

Gap Junction Function

• Link the cytosol of one cell to the other

• Allows for integration of metabolic activities of all cells in a tissue by allowing ion/small molecule exchange 

  • Ex. cAMP and Ca++

Diameter of Gap Junction Channels

• 1.5-2 nm

• Allows for free diffusion of molecules up to 1 kDa in size

Gap Junction Structure

• 6 connexin proteins make a hexagonal connexon hemichannel

• One hemichannel will sit in the cell membrane of each connected cell

• Two lined-up hemichannels form a gap junction

• These hemichannels are found in groups to form gap junction rich regions

Gap Junction Applications

• Allows for diffusion convenient for interconnected cells

• rapid coordination of cardiac muscle contraction 

• rapid uterine muscle contraction 

• Stimulation of one cell leads to a response shared by many cells through diffusion of secondary messangers 

Gap Junctions in Plant Cells: Plasmodesmata

• Important to the structure and function of phloem 

• Phloem is a system of tubes formed by cells connecting linearly 

• It carries nutrients to the rest of the plant

• Sieve-tube elements are connected by plasmodesmata that form the seive tube plate 

  • They’re metabolically inactive 

  • Companion cells provide ATP and substances to these cells 

  • They’re also ocnnected by plasmodesmata 

• Plasmodesmata also helps with communication through informational molecules

  • Gene transcripts, small RNA, etc.

  • Pathogens also exploit this though

Anchoring Junctions

• Includes:

  • Adherens junctions

  • desmosomes: Link 2 cells together

  • hemidesmosomes: Attach cells to extracellular matrix

• Distinguished by their association with actin filaments

• Through the connections, adherens junctions indirectly connect the actin cytoskeleton between neighbouring cells

Four Families of Cell Adhesion Molecules that Make up Adherens Junctions

1. Cadherins

2. Ig-superfamily

3. Integrins

4. Selectins

Which cell adhesion families form homophilic interactions?

• This means association of similar cells 

• Cadherins 

• Ig-superfamily CAMs

Which cell adhesion families form heterophilic interactions?

• This binds non-similar cells

• Integrins 

• Selectins 

Cadherins Function

• Cell adhesion molecules of adherens junctions

• They’re calcium dependent CAMs mediating homophilic interactions

• They mediate epithelial cell adhesion near the apical surface

What are the three major classes of cadherins

1. E-cadherin (epithelial)

2. N-cadherin (neural)

3. P cadherin (placental)

Cadherins Mechanism

• Adhesion involves

  • transmembrane cadherins

  • cytosolic cofactors 

  • catenins (anchors cadherin to actin)

• Cells do not aggregate into sheets under standard cell conditions

  • E-cadherin must be expressed

  • It’s calcium-dependent

  • Without calcium, E-cadherin cannot function, and cells remain separate.

Extravasation

• The movement of WBC from blood stream to surrounding tissue

• 5-step process initiated by a signal created by infection

Why are transient (temporary) cell adhesions important, and when do they occur?

• Not all cell adhesions are permanent — some are temporary to allow movement.

• Transient adhesions are essential for:

  • Cell migration across extracellular surfaces.

  • Cell movement during embryogenesis.

• These connections form and break repeatedly, enabling cells to travel where needed.

How do leukocytes use transient adhesion during an immune response?

• Leukocytes must exit blood vessels to reach sites of infection or injury.

• This extravasation relies on a sequence of temporary adhesive interactions with endothelial cells

• Normally, adhesion between endothelial cells prevents blood leakage

• During an immune response, leukocytes temporarily attach and cross the vessel wall to enter tissues.

What are the 3 families of WBCs / Leukocytes

1. Granulocytes: Neutrophils 

2. Monocytes: Macrophages

3. Lymphocytes: T and B cells 

Granulocytes

• Target pathogens 

• Include neutrophils, eosinophils, and basophils 

Neutrophils

• Most common granulocyte

• Primarily targets bacteria infections 

• One of the first cells to respond to trauma 

• Capable of extravasation

Monocytes 

• They differentiate into microphages 

• They engulf invading bacteria or dead cells through phagocytosis

• Capable of extravasation

Lymphocytes

• Include NK (natural killer) cells

  • Lyse virally infected cells and tumour cells 

• Include T and B cells 

  • Produce antibodies as immune response 

• Can undergo extravasation

What are the five steps of extravasation

1. capture

2. rolling

3. slow-rolling 

4. firm adhesion 

5. transmigration

Extravasation: Step 1

• Capture (Using Neutrophil Ex) 

• This is the transient association between the neutrophil and the apical surface of endothelial cell 

• They’re still being pushed by bloodflow but slower 

• The cells roll along the surface of endothelial cells

Extravasation: Step 2 / 3

• Rolling / Slow Rolling

• Since the transient associations are slowing the neutrophil, it rolls along the surface

• The rate slows down as # of associations increase

• This leads to firm adhesion

Extravasation: Step 4

• Firm adhesion 

• Occurs with stronger attachment of neutrophil with endothelial cells 

• This is accompanied by changes allowing the WBC to break connections b/w endothelial cells 

• This allows migration along the cell surface to outside the blood vessel 

Extravasation: Step 5

• Transmigration

• The seperation of endothelial cells allow the neutrophil to migrate out of the blood vessel

• Causes swelling as transmigration occurs

Extravasation Capture Mechanism

• Cytokines (e.g., TNF-α) are released at the infection site

• They signal endothelial cells of blood vessels.

• This signal (received at the basal surface) triggers endothelial cells to move P-selectins from secretory vesicles to their apical surface.

• P-selectins on the endothelial surface then bind to selectin-specific glycoprotein ligands on neutrophils

• This captures them from the bloodstream and initiates the immune response.

Extravasation Rolling Mechanism

• Adhesion of neutrophil to endothelial cells slow movement 

• Eventually, they start rolling along the walls

• This involves them being pushed over the surface while establishing and losing transient connections

Extravasation Slow-Rolling Mechanism

• Density of selectins on endothelial cells inc closer to site of infection 

  • Many endothelial cells are displayed P and E selectin here 

• The inc associations between selectins and the ligands on neutrophils flows their movement 

• They are no undergoing slow-rolling 

Extravasation Firm Adhesion Mechanism

• Slow rolling lets new interactions form between neutrophils and endothelial cells.

• PAF (platelet activating factor) on endothelial cells binds to the PAF receptor on neutrophils (a

  • Ex. receptors CXCR1 and CXCR2

• This interaction occurs only during slow rolling and activates a signal transduction pathway inside the neutrophil.

• The signal activates integrin adhesion molecules on the neutrophil, enabling them to bind ICAMs on endothelial cells.

• This binding slows the neutrophil further, leading to firm adhesion (tight binding) to the vessel wall.

Integrin Protein Structure (Extravasation Firm Adhesion)

• Inactive integrin (dimeric) has its propeller and β-A domains folded down, preventing ligand binding.

• PAF signaling triggers a conformational change, activating the integrin so it can bind ICAMs on endothelial cells.

• Integrin–ICAM binding is much stronger than selectin interactions, resulting in firm adhesion of the neutrophil.

• Activation also initiates actin cytoskeleton reorganization, preparing the neutrophil for cell migration out of the blood vessel.

Extravasation Transmigration Mechanism

• The neutrophil has stopped at the site of infection

• It can migrate b/w the endothelial cells 

• The connections b/w them are broken by enzymes produced by transmigrating neutrophil

Progressive Activation of Extravasation

• Selectins are activated first

  • Mediates capture, rolling, and slow-rolling 

• Signalling pathways activate integrins 

  • Mediates firm adhesion 

  • Allows transmigration