Degradation of basement membrane components by vascular endothelial cells: role in neovascularization

Understanding vascular development

The vasculature of an organism has the daunting task of connecting all the organ systems to nourish tissue and sustain life. This complex network of vessels and associated cells must maintain blood flow, but constantly adapt to acute and chronic changes within tissues. While the vasculature has been studied for over a century, we are just beginning to understand the processes that regulate its formation and how genetic hierarchies are influenced by mechanical and metabolic cues to refine vessel structure and optimize efficiency. As we gain insights into the developmental mechanisms, it is clear that the processes that regulate blood vessel development can also enable the adult to adapt to changes in tissues that can be elicited by exercise, aging, injury, or pathology. Thus, research in vessel development has provided tremendous insights into therapies for vascular diseases and disorders, cancer interventions, wound repair and tissue engineering, and in turn, these models have clearly impacted our understanding of development. Here we provide an overview of the development of the vascular system, highlighting several areas of active investigation and key questions that remain to be answered.

Abnormalities of basement membrane on blood vessels and endothelial sprouts in tumors

Often described as incomplete or absent, the basement membrane of blood vessels in tumors has attracted renewed attention as a source of angiogenic and anti-angiogenic molecules, site of growth factor binding, participant in angiogenesis, and potential target in cancer therapy. This study evaluated the composition, extent, and structural integrity of the basement membrane on blood vessels in three mouse tumor models: spontaneous RIP-Tag2 pancreatic islet tumors, MCa-IV mammary carcinomas, and Lewis lung carcinomas. Tumor vessels were identified by immunohistochemical staining for the endothelial cell markers CD31, endoglin (CD105), vascular endothelial growth factor receptor-2, and integrin alpha5 (CD49e). Confocal microscopic studies revealed that basement membrane identified by type IV collagen immunoreactivity covered >99.9% of the surface of blood vessels in the three tumors, just as in normal pancreatic islets. Laminin, entactin/nidogen, and fibronectin immunoreactivities were similarly ubiquitous on tumor vessels. Holes in the basement membrane, found by analyzing 1- micro m confocal optical sections, were <2.5 micro m in diameter and involved only 0.03% of the vessel surface. Despite the extensive vessel coverage, the basement membrane had conspicuous structural abnormalities, including a loose association with endothelial cells and pericytes, broad extensions away from the vessel wall, and multiple layers visible by electron microscopy. Type IV collagen-immunoreactive sleeves were also present on endothelial sprouts, supporting the idea that basement membrane is present where sprouts grow and regress. These findings indicate that basement membrane covers most tumor vessels but has profound structural abnormalities, consistent with the dynamic nature of endothelial cells and pericytes in tumors.