Cell junctions

apical

top of cell

basal

bottom of cell

impermeable junctions

prevent passage of molecules between cells - tight junctions - keep apical and basal membranes separate

fence function

separates apical plasma membrane from basal so they can have different compositions and polarise

gate function

regulates selective permeability and paracellular movement - between cells

transmembrane proteins

physical barrier - adhesion

cytosolic proteins

scaffolding and signalling

junction adhesion molecules eg.

occludin, tricellulin

adhesive junctions

mechanically hold cells together

adherens

connects cells to actin filaments, uses cadherins - intracellular signalling regulator

cadherin-catenin complex

E (epithelial) cadherin binds to cytosolic beta catenin on the next cells

cadherins

transmembrane proteins - nvolved in cell-cell attachment

desmosomes

connect cells to intermediate filaments, for structural integrity, can withstand mechanical stress - cadherins are desmoglein and desmocolin

focal adhesions

connect cell to substrate, integrins connect extracellular matrix to actin filaments

hemidesmosomes

connect cell base to basal lamina, integrins connect ECM to intermediate filaments

communicating junctions

passage of small molecules between cells

gap junctions

tiny pores made of connexins - 12 form junction - permeability of junctions depends on intercellular pH and intracellular calcium levels

gap junction function

sharing ions, signalling