Collateral growth: cells arrive at the construction site.

Coronary artery disease (CAD) and peripheral artery occlusion disease are the most common diseases in the Western world which are treated by pharmacological and surgical therapies. However, patients in the endstage of the disease are not suitable candidates for bypass surgery. Alternative therapies that boost the endogenous collateralization are required. Two mechanisms are naturally activated after onset of ischemia: 1. angiogenesis, sprouting of capillaries, and 2. arteriogenesis, enlargement of small preexisting arterioles. In the first part of this review, we describe the sequence of events during the development of collateral vessels. The second part focuses on two types of cells which are crucial for the development of collateral circulation, and which migrate to the site of vessel growth via peripheral blood: monocytes/macrophages and endothelial progenitor cells. The role of these cells and the implications for their use in treating ischemic diseases of cardiac and sceletal muscle are discussed.