Chapter 18

• Cell Junctions, Cell Adhesion, and the Extracellular Matrix

Rich in ECM

Cell junctions to adjacent cells or Basal lamina

A cross-sectional view of part of the wall of the intestine

Cell junctions:

1. Occluding junctions--seal cells together, prevent even smallmolecules from leaking from one side of the sheet to the other;

2. Anchoring junctions--mechanically attach cells and theircytoskeleton to their neighbors or to the extracellular matrix;

3. Communicating junctions--mediate the passage of chemicalor electrical signals from one interacting cell to its partner

The role of tight junctions in transcellular transport

Diffusion barrier Sealing strands:a long row of transmembrane adhesionproteins

ClaudinsOccludins

ZO (zonula occludens):anchor the strands to actincytoskeleton

The structure of a tight junction

Septate junctions: occluding junctions in invertebrates

More regular a structureA continuous band around each epithelial cellParallele rows of junctional proteins:regular periodicity

Discs-large in fly in related to ZO

Anchoring junction:connecting cytoskeletonto a neighbor or to the ECM

Adherens junctions and desmosomes: among cells-cadherinsFocal adhesions and hemidesmosomes-integrins

Actin filamentsIntermediate filaments

Adherens junctions

Actin bundles are linked via the cadherins and anchor proteinsinto an extensive transcellular network

Adhesion belt

contractile

Desmosomes connect intermediate filaments from cell to cell

Pemphigus: autoimmume disease-antibodies bind to and disruptthe desmosomes-blistering of the skin with leakage of body fluidinto the loosened epithelium

Buttonlike

Focal adhesionactin vinculin

Muscle cells attach to their tendons this way

Desmosomes and hemidesmosomes

integrins

Gap junction

Electrical and chemical coupling

Gap!

Summary of cell juctions in a vertebrate epithelial cell

Plasmodesmata are the plant gap junctions

Plant don’t need anchoring junctions

Cells assemble to form a tissue

The structure and function of cadherins

Calcium-dependentHomophilic

E-cadherin

N-cadherin

Cadherin-7 in ganglion cells from neural crest cells

Three ways cell-surface molecules can mediate cell-cell adhesion

Cadherin-dependentcell sorting

Selectins mediate transient cell-cell adhesions in the bloodstream

Calcium-dependentCarbohydate-binding proteins (lectins)

Binding of white blood cell to endothelial cells

Immonoglobulin superfamily proteinsmediate Calcium-independentcell-cell adhesion

Summary of the junctional and nonjunctional adhesive mechanisms

The repeating disaccharide sequence of dermatansulfate glycosaminoglycan (GAG)

Negatively charged

ECM: polysaccharide chains GAGs linked to protein(proteoglycans)And fibrous proteins (collagen, elastin, fibronectin, laminin)

A model of the molecular structure of a basal lamina

Dog aorta

Covalent bonds

Stretch and recoil like a rubber band

Five times more extensible

Plant cell walls are made of polysaccharide cellulose,the most abundant organic macromolecule on earth

Long unbranched chainsOf 1,4-linked glucoseuints

Summary

1. Three types of cell junctions: structure and function2. Cell adhesion has important role in development and

function3. Two main types of ECM molecules:

proteoglycans and fibrous proteins