Cell adhesion proteins
Cell-cell adhesion is a selective process, such that cells adhere only to other cells of specific types. This is accomplished with the aid of the selectin and integrin proteins. The selectins mediate the initial adhesion this is followed by the formation of more stable adhesions, in which integrins on the surface of leukocytes bind to intercellular adhesion molecules (ICAMs), which are members of the Ig superfamily expressed on the surface of endothelial cells. The fourth group of cell adhesion molecules, are the cadherins. They are not only involved in selective adhesion between embryonic cells but are also primarily responsible for the formation of stable junctions between cells in tissues. The cell-cell interactions mediated by the selectins, integrins, and members of the Ig superfamily are transient adhesions in which the cytoskeletons of adjacent cells are not linked to one another. Stable adhesion junctions involving the cytoskeletons of adjacent cells are instead mediated by the cadherins.

Adhesion between plant cells is mediated by their cell walls rather than by transmembrane proteins. In particular, a specialized pectin-rich region of the cell wall called the middle lamella acts as a glue to hold adjacent cells together. Because of the rigidity of plant cell walls, stable associations between plant cells do not require the formation of cytoskeletal links, such as those provided by the desmosomes and adherens junctions of animal cells.

Interesting Facts

• Extracellular matrix cells have been found to cause regrowth and healing of tissue.
  
• Several diseases, including osteogenesis imperfecta, the Ehlers-Danlos Syndromes, the Marfan syndrome and the chondrodysplasias, have been attributed to mutations in collagens I, III, II or other structural glycoproteins of the Extra Cellular Matrix.