I am very grateful to the Committee of the Physiological Society for the invitation to present the results of my and my collaborator's work in this year's review lecture. To talk about growth of vessels in Oxford is particularly pertinent because of the important contributions made to this field by G. Sanders and G. Schoefl. Sanders greatly improved techniques that enabled direct observation of microcirculation under pathological condition and thus helped to elucidate the problems in development of collateral circulation (North & Sanders, 1958), neovascularization in ocular diseases (Sanders, 1961) and vascularization of tumours (Sanders, 1963). Schoefl (1963; Schoefl & Majno, 1964) studied mostly growth of capillaries during regeneration and inflammation using electron microscopy. Her work represents a classic description of capillary growth by sprouting. Until relatively recently, growth of vessels was studied mainly under pathological conditions and during development. The basic pattern of growth was established during the last century and the beginning of this century (see Hudlicka, 1984): mesenchymal cells develop into angioblasts which differentiateinto both blood islands and erythroblasts and into endothelial cells. The latter form capillary networks which grow by sprouting, and larger vessels are formed from these networks by apposition of fibroblasts that later develop into smooth muscle cells (Clark & Clark, 1940). It is thus important to establish factors involved in capillary. growth in the firstplace.

Capillary growth under physiological circumstances Capillary proliferation is very rare in adultorganisms under physiological circumstances. It can be best demonstrated by incorporation of labelled [3H] thy-midine into DNA of endothelial cell nuclei using autoradiography. The labelling index, ie the number of labelled nuclei expressed as a percentage of all counted nuclei, is extremely low (001-1%) in normal tissues (Denekamp, 1984). The turnover time of endothelial cells calculated on the basis of labelling index is around 10000 days in capillaries in the brain and skin, 1000 in skeletal muscle and bladder