Normally, in the adult, quiescent blood vessels are covered on the ablumenal (basal) surface with a continuous basement membrane consisting primarily of laminins, collagen type IV, nidogens, and the heparan sulfate proteoglycan, perlecan (Hayashi et al. 1992
; Hallmann et al. 2005
; Bix and Iozzo 2008
). However, during the earliest stages of angiogenesis, such as in response to the angiogenic cytokine VEGF induced by wounding and ischemia, vascular basement membrane is degraded (Sundberg et al. 2001
; Rowe and Weiss 2008
; Chang et al. 2009
). Following disruption of basement membrane, and with the ensuing stage known as vascular sprouting (Nicosia and Madri 1987
), vessels become leaky and hyperpermeable to blood plasma proteins (Sundberg et al. 2001
). This vascular hyperpermeability causes leakage of the ECM proteins fibrinogen, vitronectin, and fibronectin from the blood (Senger 1996
; Sundberg et al. 2001
). Fibrinogen is subsequently converted to fibrin through enzymatic coagulation, and together with extravasated vitronectin and fibronectin instantly transform the interstitial collagen matrix to form a new, provisional ECM. Thus, the early stages of sprouting angiogenesis are generally believed to proceed in an environment rich in preexisting interstitial collagens in combination with fibrin, vitronectin, and fibronectin derived from the blood plasma. As vascular morphogenesis proceeds and vascular sprouts acquire lumens and mature, neovessels are again enshrouded in vascular basement membrane with associated pericytes and thereby achieve stability (Grant and Kleinman 1997
; Benjamin et al. 1999
). Recent studies show that pericyte recruitment to vascular tubes directly controls this basement membrane assembly step in vitro and in vivo (Stratman et al. 2009a
). Thus, in response to stimulation with angiogenic cytokines, angiogenesis in the adult is generally believed to proceed through the following basic stages: (1) degradation of vascular basement membrane and activation of quiescent endothelial cells (ECs); (2) sprouting and proliferation of ECs within provisional ECM; (3) lumen formation within the vascular sprouts, thereby creating vascular tubes; and (4) coverage of vascular tubes with mature vascular basement membrane in association with supporting pericytes.
It seems logical to expect that vascular basement membrane components associated with normal blood vessel quiescence and stability might directly function in this capacity; and observations in vivo (Risau and Lemmon 1988
) together with investigations in vitro, do indeed support a role for basement membrane in conferring vessel stabilization and vascular barrier integrity (Bonanno et al. 2000
; Liu and Senger 2004
; Stratman et al. 2009a
) (see sections Key Functions of ECM during Angiogenesis, and ECM Remodeling during Vascular Tube Formation and Stabilization). Similarly, it seems logical that components of provisional ECM would serve to support the sprouting and lumen-forming stages of angiogenesis, and there is considerable evidence that provisional ECM does indeed serve this role. Although there are distinctly different components of provisional ECM, there is abundant evidence that each of the major components (i.e., interstitial collagens, fibrin, fibronectin, and vitronectin) support EC proliferation and migration (see section Key Functions of ECM during Angiogenesis). Moreover, there is strong evidence that interstitial collagen and fibrin each support key stages of vascular morphogenesis, including cord and lumen formation (see sections Key Functions of ECM during Angiogenesis, and ECM Remodeling during Vascular Tube Formation and Stabilization).
Thus, a basic model for framing the complex roles of ECM in adult angiogenesis is to envisage the process beginning with ECs residing on a vessel-stabilizing vascular basement membrane ECM, followed by cytokine-initiated exposure of ECs to a provisional ECM that favors sprouting and lumen formation within that provisional ECM, followed by transitioning of ECs again to interactions with vessel-stabilizing vascular basement membrane. Although undoubtedly oversimplified, the fundamentals of this model are well supported by currently available evidence. Moreover, the basic complexity of ECM (i.e., the large number of distinctly different matrix proteins), combined with additional complexity provided by proteolytic remodeling that generates cell-guidance pathways (see section ECM Remodeling during Vascular Tube Formation and Stabilization) as well as matrix fragments with angiostatic functions (reviewed in Sund et al. 2004
; Bix and Iozzo 2005
; see also Lu et al. 2011
), is well matched to the biological complexity of angiogenesis.