Elastic fibbers are particularly abundant in organs such as the lung that regularly stretch. Most GAGs are linked to proteins to form proteoglycans except for hyaluronan. Adhesion proteins, a third class of EM constituents, are responsible for linking the components of the matrix both to one another and to surfaces of cells. Fibronectin is the principal adhesion protein in connective tissues. Fibronectin has binding sites for collagen, GAG and for cell surface receptors .
Basal lamina contains a distinct adhesion protein called laminin .
The major cell surface receptors responsible for the attachments of cells to the EM are the integrins . Integrins are transmembrane proteins of two subunits, alfa and beta. In addition to attaching cells to EM, integrins serve as anchors for the cytoskeleton. Selectin, integrins and members of the Ig superfamily mediate transient cell-cell adhesions, such as the interaction between leukocyte and endothelial cells during migration of leukocytes from the circulation to sites of tissue inflammation. Heterophilic cell interactions occur when an adhesion molecules on the surface of one cell recognized a different molecules on the surface of another cell.