their contribution to vascular development. Three papers in this issue reveal new information Medical students learning the anatomy of the human about the potential role of the TIE2 receptor and its ligand and raise interesting questions about the details cardiovascular system recognize that the blood vessels are named mainly on the basis of luminal diameter, of vessel assembly. The paper by Davis et al. (1996, this issue) reports the isolation and cloning of angiopoietin-1 branching, position, and organ supplied. Students and physicians rely upon the general constancy of vascular (a 70 kD glycoprotein), the first known ligand of the TIE2 receptor which is expressed on vascular endothelial determinants from one individual to another and take for granted that anatomy books will not go out of date. cells. Interestingly, unlike VEGF, angiopoietin-1 is neither a mitogen for endothelium, nor does it induce tube It is only when they learn that these vessels with their proper diameters and branches are formed in the em-formation in vitro. Rather, its pattern of expression in the vicinity of forming vessels suggests that it plays a bryo, mostly before the heart starts beating, that students begin to appreciate the true complexity of the role in regulating the assembly of non-endothelial vessel wall components. This supposition is supported by ob-genetic program that governs the development of the vascular system. This appreciation deepens when errors servations in the paper by Suri et al. (1996, this issue) in which mice deficient for angiopoietin-1 exhibit abnormal of the basic developmental plan are revealed as 'vascular malformations.' vascular architecture where the principal defect is a failure to recruit smooth muscle and pericyte precursors. The genetic and molecular mechanisms that control the development of the vascular system have remainedIn the heart, this defect manifests as poorly developed endocardium, characterized by incomplete association a mystery, until recently. Driven in part by the study of tumor angiogenesis in the 1970s, increased understand-of the endothelial layer with the underlying myocardial wall. ing of the growth of capillary blood vessels led to longterm in vitro culture of capillary endothelial cells and toIn the paper by Vikkula et al. (1996, this issue), venous malformations in two disparate families were mapped discovery of proteins that are mitogenic for these cells, including basic fibroblast growth factor (bFGF) and vas-to the Tie2 receptor where a missense mutation results in an arginine-to-tryptophan substitution. By overex-cular endothelial growth factor (VEGF), among others. The role of these proteins in vascular development is pressing the full-length and wild-type mutant receptors in insect cells, the authors show that the mutant receptor currently the subject of active investigation.Generation of angioblasts from mesoderm appears to has a 6 to 10-fold increase in autophosphorylation activity. Patients carrying this mutation develop vein-like require the action of members o...

We aimed to determine if and how endothelial cells (EC) recruit precursors of smooth muscle cells and pericytes and induce their differentiation during vessel formation. Multipotent embryonic 10T1/2 cells were used as presumptive mural cell precursors. In an under-agarose coculture, EC induced migration of 10T1/2 cells via platelet-derived growth factor BB. 10T1/2 cells in coculture with EC changed from polygonal to spindle-shaped, reminiscent of smooth muscle cells in culture. Immunohistochemical and Western blot analyses were used to examine the expression of smooth muscle (SM)-specific markers in 10T1/2 cells cultured in the absence and presence of EC. SM-myosin, SM22α, and calponin proteins were undetectable in 10T1/2 cells cultured alone; however, expression of all three SM-specific proteins was significantly induced in 10T1/2 cells cocultured with EC. Treatment of 10T1/2 cells with TGF-β induced phenotypic changes and changes in SM markers similar to those seen in the cocultures. Neutralization of TGF-β in the cocultures blocked expression of the SM markers and the shape change. To assess the ability of 10T1/2 cells to contribute to the developing vessel wall in vivo, prelabeled 10T1/2 cells were grown in a collagen matrix and implanted subcutaneously into mice. The fluorescently marked cells became incorporated into the medial layer of developing vessels where they expressed SM markers. These in vitro and in vivo observations shed light on the cell–cell interactions that occur during vessel development, as well as in pathologies in which developmental processes are recapitulated.

Lymphangiogenesis, an important initial step in tumor metastasis and transplant sensitization, is mediated by the action of VEGF-C and -D on VEGFR3. In contrast, VEGF-A binds VEGFR1 and VEGFR2 and is an essential hemangiogenic factor. We re-evaluated the potential role of VEGF-A in lymphangiogenesis using a novel model in which both lymphangiogenesis and hemangiogenesis are induced in the normally avascular cornea. Administration of VEGF Trap, a receptor-based fusion protein that binds and neutralizes VEGF-A but not VEGF-C or -D, completely inhibited both hemangiogenesis and the outgrowth of LYVE-1+ lymphatic vessels following injury. Furthermore, both lymphangiogenesis and hemangiogenesis were significantly reduced in mice transgenic for VEGF-A164/164 or VEGF-A188/188 (each of which expresses only one of the three principle VEGF-A isoforms). Because VEGF-A is chemotactic for macrophages and we demonstrate here that macrophages in inflamed corneas release lymphangiogenic VEGF-C/VEGF-D, we evaluated the possibility that macrophage recruitment plays a role in VEGF-A–mediated lymphangiogenesis. Either systemic depletion of all bone marrow–derived cells (by irradiation) or local depletion of macrophages in the cornea (using clodronate liposomes) prior to injury significantly inhibited both hemangiogenesis and lymphangiogenesis. We conclude that VEGF-A recruitment of monocytes/macrophages plays a crucial role in inducing inflammatory neovascularization by supplying/amplifying signals essential for pathological hemangiogenesis and lymphangiogenesis