Experimental studies on tumour angiogenesis
The aim of the studies described in this thesis was to investigate the method whereby tumours induce their vascularisation. Two experimental models were used to examine the effects of potentially angiogenic materials; an in vivo vascular bed consisting of the embryonic chick chorioallantoic membrane, and an in vitro system consisting of cultured human umbilical vein endothelial cells. When this study began neither of these models was in general use and both, therefore, had to be evaluated in some depth before being accepted as reliable models for experimental purposes. Solid tumours and a variety of normal embryonic and adult tissues of human and animal origin were transplanted onto the chorioallantoic membrane. Tumours and early embryonic tissue showed the highest percentage viability. Tumours differed from the embryonic tissue in that the former exhibited a greater increase in size with more vascular growth in the surrounding chorioallantoic membrane. Histological studies on the transplanted tumours showed them to be well vascularised, many of the vessels being of primitive sinusoidal type, often lacking a basement membrane and, indeed, sometimes lacking an endothelial lining. Tumours are thought to induce their own vascularisation, and therefore their ability to grow, by the production of an angiogenic factor - tumour angiogenesis factor (TAF). This material was successfully extracted from a number of human and animal tumours and found to be effective in inducing angiogenesis on the chorioallantoic membrane. A number of vasoactive agents - 5-hydroxytryptamine (5-HT) adenosine diphosphate (ADP), histamine and heparin - initially used as controls for TAF, were themselves found to be angiogenic on the chorioallantoic membrane. This suggested that these agents themselves might be implicated in the angiogenic response. The histological features of the angiogenesis induced by TAF and 5-HT were examined in some detail. TAF-induced angiogenesis was accompanied by significant mast cell accumulation at the site of the vascular changes: no such mast cell response occurred in the angiogenesis caused by 5-HT. This disparity suggested that mast cells could be involved in TAF-related angiogenesis. To examine this possibility and to be able to study the response in isolation from other in vivo mechanisms, these same materials - TAF, 5-HT, ADP, histamine and heparin - were tested for their ability to affect endothelial cell growth in vitro. The vasoactive chemicals all caused significant growth stimulation of human umbilical vein endothelial cells in vitro: TAF, on the other hand, had no effect on these isolated endothelial cell preparations. These studies suggest that the mast cell could well be involved in the mediation of the angiogenesis induced by TAF and, therefore, that mast cells may play a significant role in tumour angiogenesis.