Human melanoma cell migration along capillary-like structures in vitro: a new dynamic model for studying extravascular migratory metastasis

Monitoring Angiotropic Extravascular Migratory Metastasis In Vitro

The mechanism of cancer cell migration from the primary tumor toward secondary sites is not fully understood. In addition to intravascular cellular migration, angiotropic extravascular migratory metastasis (EVMM) has been recognized as a metastatic pathway involving tumor cells crawling along the abluminal vascular surface to distant sites. A very simple in vitro 3D assay is described here, which is based on a previous in vitro angiogenesis assay. The assay involves monitoring single fluorescence-tagged migrating cancer cells in the presence of vascular structures in real time. This coculture assay represents a quantitative approach for monitoring the migration processes of cancer cells along vessels, demonstrating phenotypic switching and migration dynamics. This protocol can be used for molecular analyses and can also be adapted for screening of therapeutic agents to block cancer metastasis.

Angiotropism, pericytic mimicry and extravascular migratory metastasis: an embryogenesis-derived program of tumor spread

Intravascular dissemination of tumor cells is the accepted mechanism of cancer metastasis. However, the phenomenon of angiotropism, pericyte mimicry (PM), and extravascular migratory metastasis (EVMM) has questioned the concept that tumor cells metastasize exclusively via circulation within vascular channels. This new paradigm of cancer spread and metastasis suggests that metastatic cells employ embryonic mechanisms for attachment to the abluminal surfaces of blood vessels (angiotropism) and spread via continuous migration, competing with and replacing pericytes, i.e., pericyte mimicry (PM). This is an entirely extravascular phenomenon (i.e., extravascular migratory metastasis or EVMM) without entry (intravasation) into vascular channels. PM and EVMM have mainly been studied in melanoma but also occur in other cancer types. PM and EVMM appear to be a reversion to an embryogenesis-derived program. There are many analogies between embryogenesis and cancer progression, including the important role of laminins, epithelial-mesenchymal transition, and the re-activation of embryonic signals by cancer cells. Furthermore, there is no circulation of blood during the first trimester of embryogenesis, despite the fact that there is extensive migration of cells to distant sites and formation of organs and tissues during this period. Embryonic migration therefore is a continuous extravascular migration as are PM and EVMM, supporting the concept that these embryonic migratory events appear to recur abnormally during the metastatic process. Finally, the perivascular location of tumor cells intrinsically links PM to vascular co-option. Taken together, these two new paradigms may greatly influence the development of new effective therapeutics for metastasis. In particular, targeting embryonic factors linked to migration that are detected during cancer metastasis may be particularly relevant to PM/EVMM.

Angiotropism in primary cutaneous melanoma is associated with disease progression and distant metastases: A retrospective study of 179 cases

BACKGROUND

Angiotropism is the histopathological correlate of pericytic mimicry and extravascular migratory metastasis (EVMM), a mechanism of melanoma spread by migration along the external surface of blood and lymphatic vessels. The frequency of angiotropism in primary cutaneous melanoma and the clinical utility of its detection remain unclear.

METHODS

We investigated angiotropism in 179 primary cutaneous melanomas by hematoxylin and eosin (H&E), CD31, and S100/D240 stains.

RESULTS

We detected angiotropism in 31 cases (17%) by H&E. CD31 immunohistochemistry increased detection to 59 cases (33%). When lymphatic vessels were included by using S100/D240 stains, 67 cases (37%) cases were positive. Angiotropism was associated with lymphatic invasion and mitotic rate with all detection methods. There was an association with increased tumor thickness when detected by H&E and CD31. No association with sentinel lymph node status was seen. By H&E and CD31 staining, angiotropism was associated with disease progression and distant metastases by univariate, but not multivariate analysis. Overall survival was not affected by the presence of angiotropism.

CONCLUSIONS

Angiotropism is relatively common in primary melanoma when immunohistochemical stains are used for detection and associated with mitotic rate and intravascular lymphatic invasion. The association with disease progression and distant metastasis suggests that it represents an alternative pathway of metastasis, that is, EVMM/pericytic mimicry vs intravascular spread.

Lymphatic invasion and angiotropism in primary cutaneous melanoma

Access of melanoma cells to the cutaneous vasculature either via lymphatic invasion or angiotropism is a proposed mechanism for metastasis. Lymphatic invasion is believed to be a mechanism by which melanoma cells can disseminate to regional lymph nodes and to distant sites and may be predictive of adverse outcomes. Although it can be detected on hematoxylin- and eosin-stained sections, sensitivity is markedly improved by immunohistochemistry for lymphatic endothelial cells. Multiple studies have reported a significant association between the presence of lymphatic invasion and sentinel lymph node metastasis and survival. More recently, extravascular migratory metastasis has been suggested as another means by which melanoma cells can spread. Angiotropism, the histopathologic correlate of extravascular migratory metastasis, has also been associated with melanoma metastasis and disease recurrence. Although lymphatic invasion and angiotropism are not currently part of routine melanoma reporting, the detection of these attributes using ancillary immunohistochemical stains may be useful in therapeutic planning for patients with melanoma.

The Role of Neutrophilic Inflammation, Angiotropism, and Pericytic Mimicry in Melanoma Progression and Metastasis

Angiotropism in melanoma correlates with ulceration and poor prognosis. It has been shown to be a marker of pericytic mimicry, that is, the spreading of tumor cells in a pericyte location along abluminal vascular surfaces. Such extravascular tumor spread may represent another form of tumor plasticity with reversion to a neural crest cell migratory phenotype. In a murine melanoma model, it has recently been demonstrated that neutrophilic skin inflammation promotes angiotropism and metastatic spread of primary melanomas. This review discusses the role of neutrophilic inflammation in angiotropism and pericytic mimicry in melanoma progression, metastasis, tumor cell plasticity, and tumor therapeutic resistance.

Angiotropism, pericytic mimicry and extravascular migratory metastasis in melanoma: an alternative to intravascular cancer dissemination

For more than 15 years, angiotropism in melanoma has been emphasized as a marker of extravascular migration of tumor cells along the abluminal vascular surface, unveiling an alternative mechanism of tumor spread distinct from intravascular dissemination. This mechanism has been termed extravascular migratory metastasis (EVMM). During EVMM, angiotropic tumor cells migrate in a 'pericytic-like' manner (pericytic mimicry) along the external surfaces of vascular channels, without intravasation. Through this pathway, melanoma cells may spread to nearby or more distant sites. Angiotropism is a prognostic factor predicting risk for metastasis in human melanoma, and a marker of EVMM in several experimental models. Importantly, analogies of EVMM and pericytic mimicry include neural crest cell migration, vasculogenesis and angiogenesis, and recent studies have suggested that the interaction between melanoma cells and the abluminal vascular surface induce differential expression of genes reminiscent of cancer migration and embryonic/stem cell state transitions. A recent work revealed that repetitive UV exposure of primary cutaneous melanomas in a genetically engineered mouse model promotes metastatic progression via angiotropism and migration along the abluminal vascular surface. Finally, recent data using imaging of melanoma cells in a murine model have shown the progression of tumor cells along the vascular surfaces. Taken together, these data provide support for the biological phenomenon of angiotropism and EVMM, which may open promising new strategies for reducing or preventing melanoma metastasis.

Could pericytic mimicry represent another type of melanoma cell plasticity with embryonic properties?

We hypothesize that the interaction between angiotropic melanoma cells and the abluminal vascular surface can induce or sustain embryonic and/or stem cell migratory properties in these tumor cells. As a result, such angiotropic melanoma cells may migrate along the abluminal vascular surface, demonstrating pericytic mimicry. Through these cellular interactions, melanoma cells may migrate toward secondary sites.

Pilot study on "pericytic mimicry" and potential embryonic/stem cell properties of angiotropic melanoma cells interacting with the abluminal vascular surface

The interaction of tumor cells with the tumor vasculature is mainly studied for its role in tumor angiogenesis and intravascular metastasis of circulating tumor cells. In addition, a specific interaction of tumor cells with the abluminal surfaces of vessels, or angiotropism, may promote the migration of angiotropic tumor cells along the abluminal vascular surfaces in a pericytic location. This process has been termed extravascular migratory metastasis. The abluminal vascular surface may also provide a vascular niche inducing or sustaining stemness to angiotropic tumor cells. This pilot study investigated if angiotropic melanoma cells might represent a subset population with pericytic and embryonic or stem cell properties. Through microarray analysis, we showed that the interaction between melanoma cells and the abluminal surface of endothelial cells triggers significant differential expression of several genes. The most significantly differentially expressed genes have demonstrated properties linked to cancer cell migration (CCL2, ICAM1 and IL6), cancer progression (CCL2, ICAM1, SELE, TRAF1, IL6, SERPINB2 and CXCL6), epithelial to mesenchymal transition (CCL2 and IL6), embryonic/stem cell properties (CCL2, PDGFB, EVX1 and CFDP1) and pericytic recruitment (PDGFB). In addition, bioinformatics-based analysis of the differentially expressed genes has shown that the most significantly enriched functional groups included development, cell movement, cancer, and embryonic development. Finally, the investigation of pericyte/mesenchymal stem cells markers via immunostaining of human melanoma samples revealed expression of PDGFRB, NG2 and CD146 by angiotropic melanoma cells. Taken together, these preliminary data are supportive of the "pericytic mimicry" by angiotropic melanoma cells, and suggest that the interaction between melanoma cells and the abluminal vascular surface induce differential expression of genes linked to cancer migration and embryonic/stem cell properties.

Atypical spitzoid melanocytic neoplasms with angiotropism: a potential mechanism of locoregional involvement

Atypical spitzoid melanocytic neoplasms (ASMN) may prove difficult to distinguish microscopically from melanoma, and their biological behavior may be unpredictable. ASMN may result in regional lymph node (LN) metastases and frequent sentinel lymph node (SLN) deposits. Angiotropism and extravascular migratory metastasis may account for locoregional metastases in melanoma and thus may potentially explain such locoregional involvement in ASMN. Nine ASMN with angiotropism from 2006 to 2010 were studied. Angiotropism was defined as melanocytes closely opposed to the external surfaces of microvascular channels without intravasation. There were 5 women and 4 men aged 6-40 (mean 18.7) years with ASMN involving the head and neck (5), the extremities (3), and the trunk (1), and the lesions ranged in diameters from 3.5 to 10 (mean 6.2) mm. Breslow thicknesses ranged from 0.66 to 5.35 (mean 3.21) mm, 5 lesions Clark level IV and 4 level V, and dermal mitotic rates varied from 1 to 5 (mean 2.4) per square millimeter. Despite follow-up of 6 months or less in 4 subjects, 5 patients showed regional tumor spread based on detection of SLN deposits, local recurrence, or clinical satellite and LN metastases. Four of 5 patients (80%) undergoing SLN biopsy showed nodal positivity with 2 SLN deposits of >6 mm. Among 4 patients not having SLN biopsy, 1 patient developed local LN metastases after 2 years. We report for the first time angiotropism in ASMN and suggest that such angiotropism seems to correlate with and may explain regional tumor spread in this neoplastic system.

Metastasis suppressor genes at the interface between the environment and tumor cell growth

The molecular mechanisms and genetic programs required for cancer metastasis are sometimes overlapping, but components are clearly distinct from those promoting growth of a primary tumor. Every sequential, rate-limiting step in the sequence of events leading to metastasis requires coordinated expression of multiple genes, necessary signaling events, and favorable environmental conditions or the ability to escape negative selection pressures. Metastasis suppressors are molecules that inhibit the process of metastasis without preventing growth of the primary tumor. The cellular processes regulated by metastasis suppressors are diverse and function at every step in the metastatic cascade. As we gain knowledge into the molecular mechanisms of metastasis suppressors and cofactors with which they interact, we learn more about the process, including appreciation that some are potential targets for therapy of metastasis, the most lethal aspect of cancer. Until now, metastasis suppressors have been described largely by their function. With greater appreciation of their biochemical mechanisms of action, the importance of context is increasingly recognized especially since tumor cells exist in myriad microenvironments. In this chapter, we assemble the evidence that selected molecules are indeed suppressors of metastasis, collate the data defining the biochemical mechanisms of action, and glean insights regarding how metastasis suppressors regulate tumor cell communication to-from microenvironments.

Melanoma prognostic factors found in the dermatopathology report

Significant prognostic information is available in a routine melanoma dermatopathology report. Features that are enumerated in the pathology report and that portend a potentially poorer prognosis are older age, site (acral, head, neck), male sex, increasing Breslow tumor thickness, increasing Clark's level, ulceration, increasing number of mitoses, vertical growth phase, regression, absence of a host inflammatory response, increased tumor vascularity, angiotropism, vascular invasion, neurotropism, marked atypia, and satellite metastasis.

C16 laminin peptide increases angiotropic extravascular migration of human melanoma cells in a shell-less chick chorioallantoic membrane assay

BACKGROUND

As distinct from intravascular dissemination, extravascular migratory metastasis (EVMM) has been described as a potential additional mechanism of melanoma spread in which tumour cells migrate along the external surfaces of vessels. Recent experimental studies strongly suggest a correlation of angiotropism of melanoma cells with EVMM. Angiotropic melanoma cells are linked to the endothelium by an amorphous matrix confirmed to contain laminin.

OBJECTIVES

To investigate whether laminin plays a role in this extravascular mechanism of tumour spread.

METHODS

We tested the effect of the C16 laminin peptide on melanoma spread in a shell-less chick chorioallantoic membrane model.

RESULTS

After 3 days, green fluorescent protein-expressing melanoma cells were observed spreading along or in the immediate proximity of vessels. The C16 laminin peptide significantly lengthened the distance of extravascular, angiotropic migration of melanoma cells. Histopathology confirmed the angiotropism of melanoma cells without intravasation, compatible with that observed with human angiotropic melanoma.

CONCLUSIONS

The results of this study suggest that the C16 laminin gamma1 chain peptide has angiotropic, extravascular migration-promoting activity on human melanoma cells, and might be a molecular target for preventing melanoma metastasis.

Angiotropic melanoma and extravascular migratory metastasis: a review

How metastases develop is poorly understood. The concept of intravascular dissemination of cancer cells has been widely accepted as a central paradigm. In addition to this explanation, however, other mechanisms may be operable. Ultrastructural studies have identified in malignant melanoma an angio-tumoral complex, in which tumor cells are linked to endothelium by a matrix containing laminin without evidence of intravasation. This observation has suggested that melanoma cells may migrate along the external surface of vessels and other anatomic structures, a mechanism termed "extravascular migratory metastasis" (EVMM). Angiotropism (melanoma cells cuffing the external surface of vessels) is the histopathologic counterpart of the angio-tumoral complex. The authors have recently drawn attention to the importance of angiotropism as a biologic phenomenom and prognostic factor in melanoma and as a likely correlate of EVMM. In addition, recent experimental studies strongly suggest a correlation of angiotropism of melanoma cells with EVMM. These studies, including cocultures of melanoma cells with capillarylike structures in vitro and the growth of green fluorescent protein-labeled melanoma cells in the shell-less chick chorioallantoic membrane model, have demonstrated the migration of angiotropic melanoma cells along the vascular channels, supporting the concept of EVMM. The new field of EVMM reviewed in this paper may prove useful in elucidating the molecular interactions involved in melanoma metastasis.

Malignant melanoma in the 21st century, part 2: staging, prognosis, and treatment

Critical to the clinical management of a patient with malignant melanoma is an understanding of its natural history. As with most malignant disorders, prognosis is highly dependent on the clinical stage (extent of tumor burden) at the time of diagnosis. The patient's clinical stage at diagnosis dictates selection of therapy. We review the state of the art in melanoma staging, prognosis, and therapy. Substantial progress has been made in this regard during the past 2 decades. This progress is primarily reflected in the development of sentinel lymph node biopsies as a means of reducing the morbidity associated with regional lymph node dissection, increased understanding of the role of neoangiogenesis in the natural history of melanoma and its potential as a treatment target, and emergence of innovative multimodal therapeutic strategies, resulting in significant objective response rates in a disease commonly believed to be drug resistant. Although much work remains to be done to improve the survival of patients with melanoma, clinically meaningful results seem within reach.

Angiotropism of Human Melanoma: Studies Involving In Transit and Other Cutaneous Metastases and the Chicken Chorioallantoic Membrane

Melanoma cell migration along the outside of vessels has been termed "extravascular migratory metastasis" (EVMM), as distinct from intravascular dissemination. Previous studies in both human and experimental melanoma models have shown angiotropism of melanoma cells, suggesting EVMM. Our objectives are to study the mechanism of dissemination of human melanoma cells in the chick chorioallantoic membrane (CAM) and to compare the histopathology in the CAM with that of patients with in transit and other cutaneous melanoma metastases. Human and murine melanoma cells were inoculated onto the CAM and observed over a 10-day period for tumor dissemination. Both human melanoma specimens from 26 patients and melanoma cells growing on the CAM showed the presence of tumor cell angiotropism at the invasive front of the tumor and at some distance from the tumor mass. In addition, a clear progression of melanoma cells spreading on the CAM was observed along the abluminal surface of vessels, where they occupied a perivascular location. By day 10 after injection, small micrometastases had developed along vessels, in a pattern similar to that in transit and other cutaneous melanoma metastases. In addition, the results suggested that the number of micrometastases directly correlated with increasing tumor volume. Taken together, these data suggest that the CAM is a relevant model for studying tumor cell dissemination, and that EVMM may be a mechanism by which some melanoma cells spread to nearby and even distant sites.

Angiotropism of human prostate cancer cells: implications for extravascular migratory metastasis

OBJECTIVES

To report several samples of invasive human prostate cancer showing angiotropism, and to use human prostate cancer cells stably expressing green fluorescence protein (GFP) in in vitro and in vivo models to assess the dissemination pathway of prostate cancer cells.

MATERIALS AND METHODS

Malignant melanoma and prostate carcinoma cells can migrate along anatomical structures such as nerves; previous studies showed that melanoma cells can be perivascular, on the outside of the endothelium, i.e. they are angiotropic, which suggests the hypothesis that melanoma cells also may migrate along vascular channels, termed 'extravascular migratory metastasis' (EVMM). Thus we examined histologically 10 human prostatic carcinoma specimens for the presence of angiotropism. In vitro, the PC-3 prostate cancer cells were co-cultures with capillary-like structures. In vivo, PC-3 cells were implanted on the chick chorio-allantoic membrane (CAM).

RESULTS

Histologically, in all 10 cases, angiotropism was detected at least focally within the tumour or at the advancing front of the tumour. In vitro, the PC-3 cells spread along the external surface of the vascular tubules; in vivo, PC-3 cells formed a cuff around some vessels a few millimetres beyond the tumour, showing angiotropism. Histopathology of the CAM confirmed the perivascular location of tumour cells and the absence of tumour cells within the vessel lumina.

CONCLUSION

The presence of angiotropic tumour cells in human invasive prostate cancers, associated with the angiotropism of GFP prostate cancer cells cultivated in vitro and in vivo in angiogenic models, raises the possibility that some prostate tumour cells may migrate along the external surface of vessels as a mechanism of spread, i.e. EVMM.

Angiotropic malignant melanoma and extravascular migratory metastasis: description of 36 cases with emphasis on a new mechanism of tumour spread

AIMS

We have identified in malignant melanoma an angio-tumoural complex in which tumour cells occupy a pericytic location along the endothelium of microvessels without evidence of intravasation. This pericytic angiotropism of melanoma cells, without any sign of intravasation, suggests that melanoma cells may migrate along the external surface of vessels, a mechanism we have termed 'extravascular migratory metastasis' (EVMM), as distinct from intravascular dissemination.

METHODS

The present study describes, for the first time, a series of 36 invasive melanoma cases ascertained for the presence of the histopathological characteristic of angiotropism.

RESULTS

All cutaneous melanomas (31/35) were level IV with the exception that two melanomas were level II and two level V. In all cases, angiotropism was easily observed at the advancing front of the tumour or in nearby tissue.

CONCLUSIONS

The study demonstrates that angiotropism of melanoma cells can be easily detected microscopically in routine tissue sections, i.e., in close proximity to microvessels (in a pericytic location). This phenomenon may prove to be important both biologically and prognostically in the routine histopathological assessment of melanoma, since we have recently shown that angiotropism could be a prognostic factor predicting risk for metastasis of melanoma. Our continued investigations to elucidate the significance of angiotropism in melanoma may help in understanding the molecular basis of metastasis and EVMM.

Pericyte-like location of GFP-tagged melanoma cells: ex vivo and in vivo studies of extravascular migratory metastasis

Previous studies have demonstrated that some tumor cells occupy a pericyte-like location in melanoma, forming angio-tumoral complexes. We hypothesized that these tumor cells are migrating along the abluminal surface of the endothelium, a mechanism termed "extravascular migratory metastasis." In the present study, we have used human and murine melanoma cells that stably express enhanced green fluorescence protein (GFP) to examine, in an ex vivo co-culture model, melanoma cell interactions with vessels that have sprouted from rat aortic rings. We also used in vivo tumor growth on the chick chorioallantoic membrane (CAM) to observe the dissemination pathway of melanoma cells. In the ex vivo rat aorta system, we observed a pericyte-like location of tumor cells that were spreading along the vascular channels. For examination of the CAM in vivo, we have used the Lugassy preparation, allowing one to obtain striking images of the relationship between fluorescent GFP cells and microvessels. Melanoma cells were found cuffing the outside of vessels around the tumor. Tumor cells were observed along the vessels several centimeters from the tumor. Confocal microscopy and histopathology confirmed the pericyte-like location of tumor cells, without any observable intravasation. The results indicate that melanoma cells can migrate along the abluminal surface of vessels. This study also demonstrates that these models can provide quantitation analysis that may prove useful in elucidating the molecular interactions involved in extravascular migratory metastasis.

Pathology of melanocytic lesions: New, controversial, and clinically important issues

Patients with primary cutaneous melanocytic lesions rely not only on the knowledge, skills, and experience of their treating clinician but also on the fundamentally important input of their pathologist for accurate diagnosis and appropriate management. Free and precise communication between pathologists and surgeons is important and undoubtedly improves patient care, particularly when managing difficult or complicated cases. To provide both patient and surgeon with the necessary information they require to make the most appropriate decisions, the pathology report should include all pathologic factors that are important in determining the patient's prognosis and management. Use of a synoptic format for pathology reporting of melanomas can facilitate this. Recent studies have established that the dermal mitotic rate of a primary cutaneous melanoma is a major prognostic determinant, and have shown that its assessment and that of other important histopathologic prognostic variables are reproducible between pathologists. Sentinel node (SN) biopsy has provided a minimally invasive procedure that can accurately predict the regional node status of melanoma patients. It is well demonstrated that the use of immunohistochemical stains assists in the detection of melanoma micrometastases in SNs, although it remains unclear which is the optimal pathologic protocol for SN evaluation and whether there is a role for reverse transcriptase polymerase chain reaction (RT-PCR) in SN assessment. False negative SN biopsies may occur as a result of errors in lymphatic mapping or sentinel lymphadenectomy, or because of a deficiency in the process of histopathologic evaluation. Recent studies have shown that the likelihood of non-SN involvement when the SN is positive correlates mostly with the extent of SN involvement, in particular the tumor penetrative depth (defined as the maximum distance of melanoma cells from the inner margin of the SN capsule). It appears that assessment of the micromorphometric features of positive SNs may be useful in predicting which patients have a low probability of having metastatic tumor in non-SNs, and therefore in selecting patients who potentially may be spared a completion lymph node dissection. It is likely that future advances in our understanding of the molecular biology of melanoma will provide new insights into tumor classification, improve diagnostic accuracy and prognostic ability, and lead to the development of more precisely targeted therapies.

The basement membrane matrix in malignancy

Cancer is the second most common cause of death among Americans, although for several age groups it ranks first. Most of these deaths are not due to the primary tumour but rather to tumour cell metastases to distant organs. There are many steps that lead to metastasis, all of which are being studied with the goal of preventing these fatalities. Normally, cells attach to the extracellular matrix to maintain tissue integrity. During cancer progression, cells become more motile and acquire invasive qualities. Tumour cells move along blood and lymph vessels or invade into them to travel to distant sites. Then, the tumour cells must attach to the vessel wall, extravasate from the vessel, invade the new tissue, proliferate, and form a secondary tumour. Angiogenesis, the formation of new blood vessels, is critical to survival of these cells at the new site and is also important for primary tumour growth and spread. Tumour cell metastasis is a complex cascade of sequential steps, each of which is not yet fully understood. Progress has been made in identifying several key activators, one of which is the extracellular matrix. A major tumour promoter is the glycoprotein laminin, which is predominantly found in the extracellular matrix produced by endothelial and epithelial cells. This review will follow the metastatic process with particular attention to the effect of laminin on tumour cells.

Pericytic-like angiotropism of glioma and melanoma cells

We have identified in malignant melanoma an angiotumoral complex in which tumor cells occupy a pericytic location along the endothelium of microvessels without evidence of intravasation. We have suggested that this pericytic-like angiotropism could be a marker of an extravascular migration of tumor cells along the abluminal surface of vessels. The extravascular migratory metastasis proposed for melanoma has close analogies with glioma migration. To compare our hypothesis of extravascular migration by melanoma with the migration of glioma cells, we have used the B16 murine melanoma cell line and the GL26 murine glioma cell line in an in vivo murine brain tumor model and in vitro using endothelial cells that have formed capillary-like structures and have been cocultivated with tumor cells. In the brain tumors, a clear progression of glioma and melanoma cells was observed along the abluminal surface of vessels, where they occupied a pericytic location along the periendothelial laminin. In vitro, time-lapse videomicroscopy recorded the migration of tumor cells toward endothelial tubules. After 24 hours, both the melanoma cells and the glioma cells were localized along the external surfaces of the vascular tubules, occupying a pericytic-like location. These similarities between glioma and melanoma support the hypothesis of an extravascular migration of melanoma cells, particularly along the abluminal surface of vessels.