Rather there is a complex infiltrating stroma, frequently induced by the malignant cells themselves, containing many host cells.This stroma is composed of blood vessels, interstitial colla gens, proteoglycans, noncollagenous proteins and Nebivolol hydrochloride cellular infiltrates containing, amongst other cell types, lymphocytes, macrophages, mast cells, fibroblasts and his tiocytes.Dvorak has suggested thatthe deposition of a fibrin and fibronectin matrix containing enmeshed platelets and leukocytes is anearly event in tumour devel opment which helpsto produce a complex tumour growth from a simple aggregate of neoplastic cells.It is believed thatthe normally vascularlycontained fibrinogen and fibronectin are able to atta in an extravascular position because tumour cells are capable of secreting a factor, termed vascular permeability factor, which causes normal microvasculature to become extra permeable to macro molecular proteins. Itis interesting to speculate whether metastatic tumours of unknown origin, where metastasis is evident but where no primary can be found, secrete large amounts of this factor which could permit the early migration of tumour cells in to the permeabilised vessels.Itis conceivable that heavy infiltrates of such cells may contribute markedly to the angiogenesis of certain tumours.Equally, since Rifaximin heparin is known to be a potent enhancer of angiogenic activity,it may be of interest thatcollections of mast cells often are found atthe invasive edge of certain tumours. It need not only be the cellular component of the tumour stroma which has an effect on angiogenesis.The extracellular matrix components of the stroma, which include collagens, proteoglycans, glycosaminoglycans and the noncollage nous glycoprotein adhesionpromoting molecules like laminin and fibronectin, may have a profound effect on microvascular endothelial cells. These molecules exert a strong chemotactic effect upon endothelial cells and, since in uivo endothelial cell migration precedes prolifera tion in the process of capillary formation,it is possible that such a response plays an important part in determining vessel ingrowth.The ability of endothelial cells to respond to a variety of molecules has been shown to be influenced by the nature of the substratvm to which they adhere.Thus, endothelial cells cultured on collagen substrata adopt a differentiated phenotype and form capillarylike structures whereas on a laminin sub stratum they proliferate rapidly.Recently we have shown thatpolyurethane sponge discs presoaked in laminin or fibrinogen solutions, prior to implantation in the subcutis of rats, induced a more rapid onset of capillary formation in the sponge than did sponges presoaked in type IV collagen solution. Itseems likely, therefore, thatnot only the na ture and degree of host infiltrating cells but also the composition of the extracellular matrix can have a profound effect on the rate and extent of the angiogenic response.Since this composition varies considerably from tumour type to tumour type, it is conceivable thatit will influence the biological behaviour of specificor individual neoplasms.Furthermore, tumour cells grown on a laminin substrate have been shown to exhibit an upregulation of type IV collagenase synthesis. Deposition of laminin by endothelial cells in the vicinity of new vessel growth might therefore confer increased invasive, and subsequently metastatic, potential on adjacent tumour cells.Inhibition of angiogenesis as an antimetastatic strategy A number of studies have, as indicated earlier, shown that growth of a tumour beyond a critical mass is only possible if there is a parallel increase in the number of new, functional capillary units to ensure adequate nutrition of the tumour and efficient removal of potentially toxic metabolites.