* Adherent junctions, which hold epithelial cells, as well as cardiac muscle cells, together. This is achieved by connecting cytoskeletal elements of the cells.
* Tight (occluding) junctions, which segregate the apical and basolateral domains of the cell membrane by sealing the lateral intercellular junctions. They prevent pericellular diffusion of water and ions, thereby performing a barrier function.
* Gap (communicating) junctions, which allow intercellular diffusion of ions and signalling molecules. These are composed of hexagonal arrays of identical and tightly packed connexins or gap junction channel proteins, each of which shows a central pore of an approximate diameter of 1.5 nm. They form a connexon. Gap junctions permit electrical coupling between cells.
Types of adherent junctions
Actin filament (microfilament) attachments
Cell-to-cell: adherens junctions: Cadherins Cell-to-extracellular matrix: focal adhesions: integrins
Intermediate filament attachment sites
Cell-to-cell: desmosomes (spot and belt): cadherins Cell-to-extracellular matrix: hemi-desmosomes: integrins
Cell-cell signalling mechanisms may be:
Endocrine: inter-glandular or inter-structure, i.e. hormones produced by an endocrine gland act on target cells at a distant body site. Paracrine: local intercellular, i.e. act on neighbouring target cells. Autocrine: intracellular, i.e. act on the cell responsible for production.
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