Blood vessel maturation: vascular development comes of age

Abstract

What are the cellular and molecular mechanisms that regulate new vessel formation and maturation? This question identifies several fundamental issues of vascular biology related to the development of the vasculature via vasculogenesis and angiogenesis. Early investigations of vasculature were largely descriptive (1, 2). The advent of methods for the isolation and maintenance of vascular cells in tissue culture led to assay systems aimed at recreating the initial events of the angiogenic process, including endothelial cell protease production, migration and proliferation. However, less attention has been directed to the later stages of vascularization, such as investment of vessels with mural cells (pericytes or smooth muscle cells), production of basement membrane, induction of vessel bed specializations or the process of vascular regression. The final stages of vessel formation might also be referred to as “remodeling” and appear to define a process through which a forming vasculature becomes a stable, mature vessel bed. These steps may also be taken advantage of to develop antitumor strategies, as suggested by Benjamin et al. in this issue (3). What regulates the phenotypic changes that occur between forming and mature vessels? A number of clinical and experimental observations support the concept that the association between the vascular tube and the mural cells mediates vessel stabilization or maturation. Ultrastructural studies of vascularization during wound healing revealed a temporal correlation between pericyte investment of the capillary and the cessation of vessel growth. This observation led the authors of this study to state: “The incorporation of pericytes within the basement membrane of proliferating capillaries is proposed as the mechanism for inhibition of capillary proliferation” (4). Clinical observations are consistent with a role for mural cells in stabilizing vessels. The vessel proliferation that characterizes proliferative diabetic retinopathy is preceded by a stage characterized by the selective loss of pericytes (5). Similarly, the new vessels that form as a complication of the retinopathy of prematurity arise from immature microvessels that have yet to be invested by pericytes (6). Thus, the physical association between the nascent vascular tube and mural cells appears to signal vessel stabilization. Earlier work from Stone and Keshet indicates that vessels are dependent upon exogenous survival factors, e.g., VEGF (7) (Fig. ​(Fig.1),1), for a critical period of time during their development. The current study by Benjamin et al. in this issue (3) further suggests that the association of forming vessels with the mural cells marks the end of this period of growth factor dependence. Two aspects of vessel stabilization are addressed in this study. The first is that in the absence of associated pericytes or smooth muscle cells, the nascent endothelial cell tube is unstable and is prone to regression. The second is the notion that the nascent endothelial tube requires VEGF for survival. In normal development, the time for overlap of these events is terminated by the arrival of the mural cells. The tumor vasculature, on the other hand, appears to be trapped in a cycle where high levels of growth factors can induce and sustain immature vessels in the relative absence of mural cells. The chronic immaturity of tumor vessels has led Dvorak to characterize a tumor as a “wound that never heals” (8). At the same time, these features make tumor vessels viable targets for antitumor therapies. Benjamin et al. (3) demonstrate that removal of growth factors leads not only to the cessation of new vessel growth, but also to regression of the immature tumor vasculature (3). Figure 1 Vessel maturation: vessel development proceeds from a stage of growth-factor dependence where loss of a survival factor leads to apoptosis. Vessel stabilization is marked by investment with mural cells, local activation of TGF-β, and basement ... Although recent evidence suggests that endothelial cell–mural cell contact and VEGF-dependence are pivotal junctures during vessel maturation, questions regarding the molecular basis of vessel maturation remains unanswered. Using tissue culture models, we have previously shown that endothelial cells recruit mural cell precursors via the secretion of PDGF-BB. Consistent with this prediction, mice null for PDGF B lack pericytes in some microvascular beds (10). Furthermore, coculture studies reveal that endothelial cell–mural cell contact leads to the activation of TGF-β (9, 11), a pleiotrophic growth factor that has been shown to mediate differentiation of a variety of cell types. In the context of the microvasculature, we speculate that TGF-β functions at multiple steps leading to vessel stabilization, including inhibition of endothelial cell proliferation and migration, as well as induction of pericyte/smooth muscle cell differentiation (12) (Fig. ​(Fig.1).