ABSTRACT
Research into the role of mechanical factors in vitro in the growth of vascular endothelial and smooth muscle cells has expanded enormously during the last decade, manifested by the proliferation of knowledge of signal transduction mechanisms and molecular basis of these processes described in this book. The role of mechanical factors in vivo, however, is currently acknowledged widely in only a few situations, such as development or hypertension. Growth factors rather than mechanical factors are considered to be much more important in vivo regulators of growth. Yet increased or decreased blood flow or blood pressure are linked with growth of vessels or their regression under many circumstances. High blood flow occurs concomitantly with capillary growth in physiological conditions such as exercise or exposure to high altitude, and is also a feature of tumors, where its restriction has been used successfully to cause their regression (Denekamp et al., 1983). In wounds, vasodilation precedes capillary growth (Hughes and Dann, 1941)
and it is an accompanying sign of other inflammatory diseases linked with angiogenesis such as arthritis or psoriasis. Folkman and Shing (1992) commented, nevertheless, that the role of vasodilation has been overlooked as an important initial step in the angiogenic cascade. Other mechanical factors such as deformation of the extracellular matrix or cell membrane occurring when tissues are stretched or when shear stress is elevated by increased velocity of blood flow may be necessary for the activation of various cytokines involved in angiogenesis (see Folkman, 1997). One reason why the role of mechanical factors in vascular remodeling in vivo has been rather neglected is the fact that the evidence is somewhat circumstantial, putative transduction mechanisms being difficult to study. What is known, however, can hopefully contribute to an understanding of their role in vessel growth or involution under both physiological and pathological circumstances.
