Angiogenesis under normal and pathological conditions.

Angiogenesis, i.e. the generation of new blood capillaries, occurs in utero (during embryonal and fetal development) and in both physiological and pathological situations during extrauterine life. Several angiogenic factors have now been isolated, including angiogenin, acidic and basic fibroblast growth factors, and alpha and beta transforming growth factors. Their amino acid sequences have been determined and their genes cloned. Other factors await complete characterisation. An account is given of techniques used in the investigation of angiogenesis, both in vivo (transparent chambers; corneal micropockets; implantation on chick chorioallantoic membrane; employment of polymers for the sustained release of angiogenesis factors) and in vitro (cloning and long-term culture of capillary endothelial cells). The angiogenesis induced by solid tumours differs from other forms in that it is not self-limited and continues indefinitely until eradication of the tumour or death of the host. Anti-angiogenic factors have also been identified, particularly a new class of nonglucocorticoid steroids. Their employment in tumour therapy is a possibility, since neoplastic expansion is essentially dependent on angiogenesis.