Cells do not normally live as single entities but are grouped together in specific functional and structural configurations in various tissues. Intra cellular mechanisms maintain cellular viability and provide the means necessary for their specific cellular functions. The interaction between cells is maintained by mechanisms involving extracellular signalling. Such extracellular mechanisms may include special properties of the cell surface which involve immediate cell contact, but may also represent mechanisms which act at a distance and are mediated via special secretions and/or re ceptors....