Vascular Channel Formation Research Articles

Vasculogenic Mimicry: How Convincing, How Novel, and How Significant?

In a recent publication, Maniotis et al 1 report that blood vessels of malignant eye tumors known as uveal melanomas are formed by tumor cells instead of endothelial cells. The authors use the term vasculogenic mimicry to describe this phenomenon and consider it a novel concept in the biology of tumor vascularization. The paper has received widespread attention and apparent validation through two commentaries, one published along with the paper in The American Journal of Pathology 2 and another published concurrently in Science. 3 Despite the paper’s impact the evidence is, in our view, unconvincing. The problems are, however, not easily detected by readers unfamiliar with the background or pitfalls of this specialized topic. Although it is intriguing and worthy of further study, the evidence presented in Maniotis et al for a functionally significant contribution of tumor cell-lined blood vessels to vascularization and blood flow in uveal melanomas is neither persuasive nor novel. The purpose of this commentary is to examine the evidence for vasculogenic mimicry and the reasons for our assessment. (Note: This commentary does not address the in vitro or microarray data presented by Maniotis et al, because the interpretation of these results is dependent on the histological, immunohistochemical, and electron microscopic evidence that is the focus of our remarks. Also, this commentary does not question the validity of the relationship between the periodic acid-Schiff (PAS) staining pattern of uveal melanomas and clinical outcome, as reported by Folberg et al in several publications. 4,5 This correlation may be clinically useful even if the PAS staining pattern does not faithfully represent the microvascular architecture. Neither does our commentary question the usefulness of microvascular density as a prognostic factor for survival in uveal melanomas. 6,7 Indeed, PAS staining pattern and microvascular density may offer complementary indices of the lethality of these tumors. 6-9 )

Divergent effects of extracellular oxygen on the growth, morphology, and function of human skin microvascular endothelial cells

Partial pressure of extracellular oxygen influences a number of major cellular functions. The purpose of this study was to determine if the proliferation, morphology, and synthesis of proteins important in the function of skin microvascular endothelial cells were significantly altered by an extracellular oxygen tension used to culture endothelial cells. Microvascular endothelial cells were isolated from the dermis of neonatal foreskins and were studied at a venous capillary oxygen level (5% O2, 38 mm Hg) and at an atmospheric oxygen level (20.8% O2, 158 mm Hg). At all time points studied and at all passage numbers, a significant inhibition of proliferation was observed at 20.8% O2 compared to identical cultures grown and subcultured at 5% O2. Two morphologically distinct endothelial cell populations were observed at 5% O2. When mediators of angiogenesis and inflammation—such as basic fibroblast growth factor (bFGF), phorbol myristate acetate (PMA), and interleukin-1β (IL-1β)—were studied, additional differences in proliferation were observed. Atmospheric O2 inhibited the synthesis of a major basement membrane protein (Type IV collagen), a major surface protein (PECAM-1), and increased the synthesis of von Willebrand factor (vWf). The rate of vascular channel formation induced by collagen gels was decreased at 5% O2. These results demonstrate that an increase in extracellular oxygen tension from 5 to 20.8% can significantly alter the cellular physiology of human skin microvascular endothelial cells. J. Cell. Physiol. 182:134–140, 2000. © 2000 Wiley-Liss, Inc.

Divergent effects of extracellular oxygen on the growth, morphology, and function of human skin microvascular endothelial cells.

Partial pressure of extracellular oxygen influences a number of major cellular functions. The purpose of this study was to determine if the proliferation, morphology, and synthesis of proteins important in the function of skin microvascular endothelial cells were significantly altered by an extracellular oxygen tension used to culture endothelial cells. Microvascular endothelial cells were isolated from the dermis of neonatal foreskins and were studied at a venous capillary oxygen level (5% O(2), 38 mm Hg) and at an atmospheric oxygen level (20.8% O(2,) 158 mm Hg). At all time points studied and at all passage numbers, a significant inhibition of proliferation was observed at 20.8% O(2) compared to identical cultures grown and subcultured at 5% O(2). Two morphologically distinct endothelial cell populations were observed at 5% O(2). When mediators of angiogenesis and inflammation-such as basic fibroblast growth factor (bFGF), phorbol myristate acetate (PMA), and interleukin-1beta (IL-1beta)-were studied, additional differences in proliferation were observed. Atmospheric O(2) inhibited the synthesis of a major basement membrane protein (Type IV collagen), a major surface protein (PECAM-1), and increased the synthesis of von Willebrand factor (vWf). The rate of vascular channel formation induced by collagen gels was decreased at 5% O(2). These results demonstrate that an increase in extracellular oxygen tension from 5 to 20.8% can significantly alter the cellular physiology of human skin microvascular endothelial cells.

Vascular Channel Formation by Human Melanoma Cells in Vivo and in Vitro: Vasculogenic Mimicry

Tissue sections from aggressive human intraocular (uveal) and metastatic cutaneous melanomas generally lack evidence of significant necrosis and contain patterned networks of interconnected loops of extracellular matrix. The matrix that forms these loops or networks may be solid or hollow. Red blood cells have been detected within the hollow channel components of this patterned matrix histologically, and these vascular channel networks have been detected in human tumors angiographically. Endothelial cells were not identified within these matrix-embedded channels by light microscopy, by transmission electron microscopy, or by using an immunohistochemical panel of endothelial cell markers (Factor VIII-related antigen, Ulex, CD31, CD34, and KDR[Flk-1]). Highly invasive primary and metastatic human melanoma cells formed patterned solid and hollow matrix channels (seen in tissue sections of aggressive primary and metastatic human melanomas) in three-dimensional cultures containing Matrigel or dilute Type I collagen, without endothelial cells or fibroblasts. These tumor cell-generated patterned channels conducted dye, highlighting looping patterns visualized angiographically in human tumors. Neither normal melanocytes nor poorly invasive melanoma cells generated these patterned channels in vitro under identical culture conditions, even after the addition of conditioned medium from metastatic pattern-forming melanoma cells, soluble growth factors, or regimes of hypoxia. Highly invasive and metastatic human melanoma cells, but not poorly invasive melanoma cells, contracted and remodeled floating hydrated gels, providing a biomechanical explanation for the generation of microvessels in vitro. cDNA microarray analysis of highly invasive versus poorly invasive melanoma tumor cells confirmed a genetic reversion to a pluripotent embryonic-like genotype in the highly aggressive melanoma cells. These observations strongly suggest that aggressive melanoma cells may generate vascular channels that facilitate tumor perfusion independent of tumor angiogenesis.

Differential Activation of Protein Kinase C Isozymes by Phorbol Ester and Collagen in Human Skin Microvascular Endothelial Cells

Human dermal microvascular endothelial cells participate in activities including inflammation, wound healing, and angiogenesis (neovascularization). Two stages of angiogenesis can be mimicked in vitro by two models of cultured foreskin human dermal microvascular endothelial cells: the differentiation of epithelioid endothelial cells to spindle-shaped mesenchymal-like cells induced by phorbol ester treatment; and the formation of vascular channels induced by exposing the luminal surface of endothelial cell monolayers to type I collagen gels. The mechanisms underlying these two processes, however, are largely unknown. Protein kinase C isozymes, which are activated by phorbol esters, are important mediators in the angiogenic process. In the current work, we identified the protein kinase C isozymes present in human dermal microvascular endothelial cells and determined which of the isozymes are activated in response to phorbol ester or to collagen treatments. Using western blot analysis, we found that microvascular endothelial cells contain at least six protein kinase C isozymes (alpha, beta, delta, epsilon, zeta, eta). Immunocytochemical studies demonstrated that the isozymes are located in distinct cellular compartments and that following treatment with phorbol 12-myristate 13-acetate or with a collagen gel overlay, most isozymes (protein kinase C alpha, beta1, betaII, delta, epsilon, eta) translocated to different parts of the cell. Moreover, for two of these isozymes (betaII and eta), the localization differs after phorbol 12-myristate 13-acetate treatment as compared with collagen treatment. These results demonstrate that agents that mimic two stages in the angiogenic process in vitro initiate diverse changes in the subcellular localization of specific protein kinase C isozymes and suggest a role for different isozymes in this process.

Hepatic haemodynamics and microvascular architecture after portal venular embolization in the rat

To clarify the relationship between hepatic damage and alterations of hepatic microvascular architecture and haemodynamics following portal venule occlusion, the present study was performed in rats which had been injected into the portal vein with 700,000 plastic beads (30-35 microns in diameter). When the occluded segment of the portal venule was short, infarction did not occur because of the prompt development of vascular channels bypassing the obstructed venules. On the other hand, when the occluded segment was long, infarction usually occurred. In this case, formation of collateral vascular channels began as dilatation of the sinusoids in the vicinity of the occluded portal venules and increased in diameter and developed into new portal venules in 5 days. Increased portal vascular resistance after embolization was normalized after 5 days in parallel with the progression of reconstruction of vascular channels. Reduced bile production returned to a normal level 3 days after embolization. Necrotic tissue was resorbed completely and replaced by fibroblasts 5 days after embolization. These findings indicate that the development of infarct following portal venule occlusion depends on the length of the occluded segment, and that hepatic microvasculature has a great capacity for adaptation and rapid reconstruction of vascular channels which leads to speedy recovery from hepatic circulatory disturbances and resulting hepatic damage.

Evolution of schistosomal hepatic vascular lesions after specific chemotherapy.

Mice infected with 30 cercariae of Schistosoma mansoni showed extensive destruction, obstruction, and distortion of the intrahepatic portal vein system 10 weeks after exposure. One month after curative treatment for schistosomiasis, these lesions had considerably regressed. The vascular pattern was almost normal by 2 months and completely normal by 6 months following treatment. The techniques used to study vascular changes included plastic casting, injection of colored gelatin, and conventional histopathologic methods. Main transformations observed after treatment were shrinkage of periovular granulomas and formation of new vascular channels around them, which restored the continuity of the portal vein system. Shrunken granulomas remained avascular.

Formation of new vascular channels in the lungs of a patient with secondary pulmonary hypertension.

Formation of new vascular channels in the lungs of a patient with secondary pulmonary hypertension.