The formation of proper vasculature during embryonic development is crucial to ensure adequate delivery of oxygen and nutrients and removal of waste for tissue growth and differentiation. Blood vessels are formed through two distinct processes: vasculogenesis, which refers to the differentiation of primitive mesodermal cells into endothelial cells; and angiogenesis, which refers to a process where endothelial cells proliferate and migrate. After the blood flow starts, the primitive blood vessels then undergo remodeling and pruning to form mature vascular networks with a characteristic and reproducible pattern in specific organs. It has been suggested that the formation of the primitive vascular network may be random; therefore, vascular remodeling or pruning by specific branch regression regulated at the level of endothelial cell survival is the major mechanism for the formation of mature vascular networks. However, remodeling is a poorly understood process that involves generation of new vessels and regression of others. In addition, it also involves change in lumen diameter as well as vessel wall thickness. Numerous growth factors are important in vascular development and the key regulator important for all stages of vasculogenesis, angiogenesis, and pruning, is vascular endothelial growth factor, or VEGF.
VEGF is a prototypic member of secreted, homodimeric glycoproteins that acts as endothelial specific mitogen and is a potent stimulator of angiogenesis in vivo. In addition, VEGF is a vasodilator and was once named vascular permeability factor. During vasculature development, VEGF is expressed by hypoxic cells and attracts the developing vasculature. When the target area receives the proper blood supply, VEGF is then down-regulated and thus slowing endothelial growth and providing a feedback mechanism. In addition to growth factors, environmental guidance cues are important in vascular development as well. Endothelial tip cells extend filopodia that explore the environment to guide the vessel growth as well as regulate branching by detecting nearby growing sprouts.
The tumor vasculature
Blood vessels are organized in a hierarchy of arterioles, capillaries and venules each having its own characteristic structure and function. Blood vessels in different organs share some common features and some organ specific characteristics. The normal brain vasculature is highly organized having a thin monolayer of endothelial cells lining the wall of the blood vessels. The endothelial cells are closely packed together via tight junctions. Tumor vessels, on the other hand are structurally and functionally very different. They are highly disorganized, tortuous, dilated, have uneven diameter and excess branches. The blood flow is chaotic which in turn leads to hypoxic and acidic regions in tumors. These conditions lower the efficacy of chemotherapeutics and also select for cancer cells that are more malignant and metastatic. The tumor vessels are also leaky due to discontinuous basement membrane and loss of tight junction proteins like CD144, ZO-1, occludin, claudin-1 and claudin-5. They are characterized by high vascular permeability. In addition, the ex
Bone marrow derived cells to tumor vascular development
Vascular development is a multifactorial process. In addition to sprouting angiogenesis, in which locally-derived factors promote nascent capillary formation by endothelial cells in the local microenvironment, a variety of proangiogenic cell types can be mobilized from the bone marrow by cytokines emanating from sites of active angiogenesis, such as an angiogenic tumor. These bone marrow derived cells (BMDCs) home to and are retained at angiogenic sites, where they contribute to the angiogenic process by various mechanisms.
Many BMDCs are CD45+/haematopoietic in nature, and promote angiogenesis in a paracrine manner, ostensibly by homing to perivascular sites and secreting proangiogenic factors. For example, CXCR4+VEGFR1+ hemangiocytes are recruited to ischemic hindlimbs of mice via elevated plasma levels of VEGF and SDF1. SDF1 contributes to the retention of the hemangiocytes at the ischemic site, where they promote revascularization. Similarly, a population of CD45+CXCR4+VEGFR1+CD11b+ BMDCs, termed recruited bone marrow-derived circulating cells (RBCCs), are recruited to sites of induced VEGF ex
An additional mechanism by which BMDCs may contribute to tumor vascular development is by direct incorporation into tumor neovasculature. For example, a population of dendritic cell precursors has been found to undergo endothelial-like differentiation ex-vivo in response to VEGF. These cells infiltrate tumors in response to beta-defensin ex
Cancer stem cells and tumor vasculature
Cancer stem cells (CSCs) may also contribute to tumor vascularization. One possibility is that these primitive, undifferentiated cells are able to differentiate into functional endothelial cells and incorporate into the vessel walls of nascent tumor capillaries. In this regard, human chronic myelogenous leukemia progenitor cells are able to generate endothelial cells and contribute to neovascularization at sites of injury in the mouse gastrointestinal tract in an in vivo assay. These results indicate that primitive tumor cells may retain endothelial cell differentiation potential possessed by stem/progenitor cells in their normal tissue counterpart. This raises the possibility that CSCs from tissues whose normal stem/progenitor cells have demonstrated plasticity toward the endothelial lineage, such as kidney, brain, skin, and muscle, may also have the ability to contribute to tumor vasculature by differentiating into vessel-incorporating endothelial cells. Another, related possibility is that, rather than differentiation into frank endothelial cells, CSCs may preferentially participate in vasculogenic mimicry – a phenomenon characterized by the acquisition of endothelial cell markers and formation of fluid-conducting, vessel-like channels by tumor cells. Vasculogenic mimicry has been observed mainly in uveal melanoma, but also in a wide variety of other malignancies, and is associated with tumor cells displaying an undifferentiated phenotype, suggesting that CSCs may be more likely than other tumor cells to participate in this process.