Cell-cell and tissue-tissue interactions initiate and regulate genomic expression in the initiation and maintenance of cell differentiation and tissue morphogenesis for virtually every vertebrate organ system. They operate as the major signaling system between microenvironment, extracellular milieu, cells, and tissues. Signaling can bedirect, requiring establishment of intimate cell-to-cell contact between signaling and responding cells, or indirect, involving either release and recognition of signal molecules, or an interaction between responding cells and extracellular matrices of signaling cells.
