Capillary growth: a two-cell system

Angiogenesis is central to a number of normal and pathologic processes, including tumor growth. The identification of several angiogenic factors and the isolation and culture of capillary endothelial cells (EC) have led to a greater understanding of the cellular and biochemical bases of new vessel growth. Until recently EC have been the focus of most studies of microvascular growth. However, capillaries are not simply tubes of EC but have also a second cellular component, the mural cell or pericyte. Little is known about the later stages of vessel growth, including the addition of the pericyte to the capillary and its influence on EC growth and function. Historically the pericyte was defined by its abluminal association with the EC in the capillary. Though the pericyte's function was largely unknown, ultrastructural studies led to speculation regarding a role for the pericyte in contraction, as a stem cell and in the control of microvascular growth. Establishment of methods for the isolation, culture and identification of pericytes has permitted investigation into the role of the pericyte. EC and pericytes make frequent contact in vivo and co-culture studies of EC and pericytes reveal that the two cell types interact in a variety of ways including diffusible growth regulators, heterotypic contacts, and gap junctions. This intercellular communication is likely to be an important component of the complex mechanism(s) controlling microvascular growth and function.