The process of capillary growth, angiogenesis, is an integral part of wound healing and repair mechanisms. When it occurs during these conditions, it is tightly controlled and strictly delimited. However, in tumor growth and in a variety of vascular diseases, unrestrained angiogenesis can contribute significantly to the pathology and persistence of these manifestations (1 ). Research on angiogenesis was initiated with the development of several bioassays that have permitted direct observations of the microvasculature in the living animal. The bioassays have been used for a variety of purposes; for example, to detect angiogenesis activity in malignant and normal cells and tissue, to screen purified test substances for angiogenic activity, and to elucidate the cellular events that accompany vessel growth. The response observed after the introduction of an appropriate stimulus such as mechanical injury or injection of neoplastic tissue implants has allowed the cataloging of the main events of the angiogenic cascade as well as the characterization of pro- and antiangiogenic factors.
