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. Recent studies on cell-cell contact have tended to stress cell sur face components directly mediating cellular interactions; the extracellular medium as a metabolically active compartment has been rather neglected. However, it represents a vital medium through which cells communicate, being important in, for example, chemotaxis in primitive organisms, and in devel~ment and in the coordination of multiple functions in multi cellular 0 ganisms. It is not surprising, therefore, that a number of mole cular me anisms have developed together with increasing biological complexi during evolution. Two aspects of the extracellular space have received increasing attention in the last few years. First, several macro molecules such as collagen, laminin and fibronectin have been identified as components of an extracellular matrix giving a structural dimension to the extracellular compartment.

Inhalt
Perspectives.- Ectoenzymes: Their Modulation and Similarity to Certain Enzymes of Intracellular Membranes.- Ectonucleotidases: General Aspects.- Ectonucleotidases of Vascular Endothelial Cells: Characterisation and Possible Physiological Roles.- Regulation of the Hydrolysis of Adenine Nucleotides at the Surface of Cultured Vascular Cells.- Hydrolysis of ATP and Formation of Adenosine at the Surface of Cholinergic Nerve Endings.- Enzymatic Characteristics and Possible Role of Synaptosomal Ecto-Adenosine Triphosphatase from Mammalian Brain.- Properties and Expression of an ATP-Degrading Ectoenzyme Cascade on Human Lymphoid Cells.- Study of Ecto-5?Nucleotidase, Ecto-ATPase, and Adenosine Deaminase Activities in Relation to Lymphoid Cell Differentiation.- Ectonucleotidases: 5?Nucleotidase.- The Properties, Structure, Function, Intracellular Localisation and Movement of Hepatic 5?Nucleotidase.- Ecto-5?-Nucleotidase as a Probe for the Analysis of Membrane Flow During Fluid Phase Pinocytosis.- Isolation and Characterization of 5?Nucleotidase from Avian Muscle Sources and its Interaction with Filamentous Actin.- 5?Nucleotidase in the Nervous System.- The Role of 5?Nucleotidases in the Heart.- Ecto-Protein Kinase.- Detection and Biochemistry of Cell Surface Protein Kinase.- Physiological Aspects of Surface Protein Kinase Activity.- Ectoglycosylation.- Ectoglycosyltransferase Activities During Neuronal Differentiation in Cell Cultures.- Modulation Theory of Adhesion: Role of the Ectoenzymes Glycosidase and Glycosyltransferase.- Ectoacetylcholinesterase.- Molecular Forms of Acetylcholinesterase.- An Ectocellular Form of Acetylcholinesterase on the Plasma Membrane of Nerve Terminals in Torpedo Electric Organ.- Release of Acetylcholinesterase from Nigro-StriatalNeurons.- Ectopeptidases.- Endopeptidase-24.11: an Ectoenzyme Capable of Hydrolysing Regulatory Peptides at the Surface of Many Different Cell Types.- Ectopeptidases Responsible for the Inactivation of Enkephalins.- Cell Surface-Bound Leucine Aminopeptidase: Target of the Immunomodulator Bestatin.- The Intramolecular Turnover Heterogeneity, a New Property of Plasma Membrane Glycoproteins. Dipeptidylaminopeptidase IV as a Model.- Appendix 1 Approaches to Establishing Ecto Orientation of Enzymes.- Appendix 2 Summary Table.