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

Current Status of Bioprinting Using Polymer Hydrogels for the Production of Vascular Grafts

Cardiovascular disease is one of the leading causes of death and serious illness in Europe and worldwide. Conventional treatment-replacing the damaged blood vessel with an autologous graft-is not always affordable for the patient, so alternative approaches are being sought. One such approach is patient-specific tissue bioprinting, which allows for precise distribution of cells, material, and biochemical signals. With further developmental support, a functional replacement tissue or vessel can be created. This review provides an overview of the current state of bioprinting for vascular graft manufacturing and summarizes the hydrogels used as bioinks, the material of carriers, and the current methods of fabrication used, especially for vessels smaller than 6 mm, which are the most challenging for cardiovascular replacements. The fabrication methods are divided into several sections-self-supporting grafts based on simple 3D bioprinting and bioprinting of bioinks on scaffolds made of decellularized or nanofibrous material.

CD146, a therapeutic target involved in cell plasticity

Since its identification as a marker for advanced melanoma in the 1980s, CD146 has been found to have multiple functions in both physiological and pathological processes, including embryonic development, tissue repair and regeneration, tumor progression, fibrosis disease, and inflammations. Subsequent research has revealed that CD146 is involved in various signaling pathways as a receptor or co-receptor in these processes. This correlation between CD146 and multiple diseases has sparked interest in its potential applications in diagnosis, prognosis, and targeted therapy. To better comprehend the versatile roles of CD146, we have summarized its research history and synthesized findings from numerous reports, proposing that cell plasticity serves as the underlying mechanism through which CD146 contributes to development, regeneration, and various diseases. Targeting CD146 would consequently halt cell state shifting during the onset and progression of these related diseases. Therefore, the development of therapy targeting CD146 holds significant practical value.

Perivascular niches: critical hubs in cancer evolution

Tumors are heterogeneous ecosystems in which cancer cells coexist within a complex tumor immune microenvironment (TIME). The malignant, stromal, and immune cell compartments establish a plethora of bidirectional cell-cell communication crosstalks that influence tumor growth and metastatic dissemination, which we are only beginning to understand. Cancer cells either co-opt or promote the formation of new blood and lymphatic vessels to cope with their need for nutrients and oxygen. Recent studies have highlighted additional key roles for the tumor vasculature and have identified the perivascular niche as a cellular hub, where intricate and dynamic cellular interactions promote cancer stemness, immune evasion, dormancy, and metastatic spreading. Here, we review these findings, and discuss how they may be exploited therapeutically.

Extracellular matrix type 0: From ancient collagen lineage to a versatile product pipeline - JellaGel™

Extracellular matrix type 0 is reported. The matrix is developed from a jellyfish collagen predating mammalian forms by over 0.5 billion years. With its ancient lineage, compositional simplicity, and resemblance to multiple collagen types, the matrix is referred to as the extracellular matrix type 0. Here we validate the matrix describing its physicochemical and biological properties and present it as a versatile, minimalist biomaterial underpinning a pipeline of commercialised products under the collective name of JellaGelTM. We describe an extensive body of evidence for folding and assembly of the matrix in comparison to mammalian matrices, such as bovine collagen, and its use to support cell growth and development in comparison to known tissue-derived products, such as Matrigel™. We apply the matrix to co-culture human astrocytes and cortical neurons derived from induced pluripotent stem cells and visualise neuron firing synchronicity with correlations indicative of a homogenous extracellular material in contrast to the performance of heterogenous commercial matrices. We prove the ability of the matrix to induce spheroid formation and support the 3D culture of human immortalised, primary, and mesenchymal stem cells. We conclude that the matrix offers an optimal solution for systemic evaluations of cell-matrix biology. It effectively combines the exploitable properties of mammalian tissue extracts or top-down matrices, such as biocompatibility, with the advantages of synthetic or bottom-up matrices, such as compositional control, while avoiding the drawbacks of the two types, such as biological and design heterogeneity, thereby providing a unique bridging capability of a stem extracellular matrix.

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.

Matrigel: history/background, uses, and future applications

Basement membranes are thin sheets of extracellular matrix with many diverse roles in the body. Those in normal tissue are also highly insoluble and resist attempts to extract and characterize their components. A mouse tumor, the EHS tumor, has provided large amounts of basement membrane material, which has facilitated the structural and functional characterization of its components. An extract of the tumor, known as Matrigel, contains components which reconstitute into a solid gel at 37°. This solid basement membrane matrix has been used in both cell culture and in vivo. Matrigel has been utilized in some 12,000-plus publications for a variety of studies with embryonic, normal, and stem or malignant cells. Evidence presented in this Commentary suggests that Matrigel isolated from tumors grown in diverse hosts could exert unique effects that could be helpful in analyzing the causes of various pathologies and for screening possible therapeutic agents.

FOXC2 Promotes Vasculogenic Mimicry in Ovarian Cancer

FOXC2 is a forkhead family transcription factor that plays a critical role in specifying mesenchymal cell fate during embryogenesis. FOXC2 expression is associated with increased metastasis and poor survival in various solid malignancies. Using in vitro and in vivo assays in mouse ovarian cancer cell lines, we confirmed the previously reported mechanisms by which FOXC2 could promote cancer growth, metastasis, and drug resistance, including epithelial-mesenchymal transition, stem cell-like differentiation, and resistance to anoikis. In addition, we showed that FOXC2 expression is associated with vasculogenic mimicry in mouse and human ovarian cancers. FOXC2 overexpression increased the ability of human ovarian cancer cells to form vascular-like structures in vitro, while inhibition of FOXC2 had the opposite effect. Thus, we present a novel mechanism by which FOXC2 might contribute to cancer aggressiveness and poor patient survival.

Ulcerated Cutaneous Melanoma: A Review of the Clinical, Histologic, and Molecular Features Associated with a Clinically Aggressive Histologic Phenotype

The presence of ulceration in melanoma is associated with poor clinical outcomes and is the third most powerful predictor of survival in the AJCC Melanoma Staging System after tumor thickness and mitotic activity. The aggressive biological behavior associated with ulceration has been hypothesized to be the result of an intrinsic biological attribute that favors dissemination and presents locally with the loss of epidermal integrity. Among the features of ulcerated melanoma, many show promise as potential prognostic tools, markers of differential immunogenicity and indicators of oncogenic drivers of invasion and metastasis. The incidence of ulcerated melanoma is greater in males, increases with age and with systemic inflammatory risk factors (diabetes, smoking, low vitamin D, elevated body mass index). Patients with ulcerated primary tumors seem to exclusively benefit from adjuvant interferon (IFN) therapy, which is likely the consequence of an altered tumor microenvironment. When ulceration is present, there is a higher density of macrophages and dendritic cells and enhanced expression of pro-inflammatory cytokines, such as IL-6. There is also an increased expression of proteins involved in tumor antigen presentation in ulcerated melanomas. Histologically, vascular density, vasculogenic mimicry and angiotropism are all significantly correlated with ulceration in melanoma. The presence of ulceration is associated with reduced protein expression of E-cadherin and PTEN and elevated levels of N-cadherin and the matrix metalloproteinases. Differential microRNA expression also holds promise as a potential prognostic biomarker of malignancy and disease spread within the setting of ulceration. However, the molecular and cellular differences associated with the ulcerated state are complex and further study will aid in determining how these differences can be harnessed to improve care for patients with melanoma.

Vessel co-option and angiotropic extravascular migratory metastasis: a continuum of tumour growth and spread?

Two fields of cancer research have emerged dealing with the biology of tumour cells localised to the abluminal vascular surface: vessel co-option (VCo), a non-angiogenic mode of tumour growth and angiotropic extravascular migratory metastasis (EVMM), a non-hematogenous mode of tumour migration and metastasis. VCo is a mechanism by which tumour cells gain access to a blood supply by spreading along existing blood vessels in order to grow locally. Angiotropic EVMM involves "pericytic mimicry" (PM), which is characterised by tumour cells continuously migrating in the place of pericytes distantly along abluminal vascular surfaces. When cancer cells are engaged in PM and EVMM, they migrate along blood vessels beyond the advancing front of the tumour to secondary sites with the formation of regional and distant metastases. In the present perspective, the authors review the current scientific literature, emphasising the analogies between embryogenesis and cancer progression, the re-activation of embryonic signals by "cancer stem cells", and the important role of laminins and epithelial-mesenchymal-transition. This perspective maintains that VCo and angiotropic EVMM constitute complementary processes and represent a continuum of cancer progression from the primary tumour to metastases and of tumour growth to EVMM, analogous to the embryonic development program.

Melanoma Plasticity: Promoter of Metastasis and Resistance to Therapy

Melanoma is the deadliest form of skin cancer. Although targeted therapies and immunotherapies have revolutionized the treatment of metastatic melanoma, most patients are not cured. Therapy resistance remains a significant clinical challenge. Melanoma comprises phenotypically distinct subpopulations of cells, exhibiting distinct gene signatures leading to tumor heterogeneity and favoring therapeutic resistance. Cellular plasticity in melanoma is referred to as phenotype switching. Regardless of their genomic classification, melanomas switch from a proliferative and differentiated phenotype to an invasive, dedifferentiated and often therapy-resistant state. In this review we discuss potential mechanisms underpinning melanoma phenotype switching, how this cellular plasticity contributes to resistance to both targeted therapies and immunotherapies. Finally, we highlight novel strategies to target plasticity and their potential clinical impact in melanoma.

TFPI2 Promotes Perivascular Migration in an Angiotropism Model of Melanoma

Purpose

Angiotropism is the process by which cancer cells attach to and migrate along blood vessels to acquire vasculature, disseminate, and metastasize. However, the molecular basis for such vessel–tumor interactions has not been fully elucidated, partly due to limited experimental models. In this study, we aimed to observe and explore the molecular mechanism underlying angiotropism in melanoma.

Methods

To monitor the interactions of human melanoma cells with the vasculature in vivo, a murine coxenograft model was employed by co-injecting highly and poorly invasive melanoma cells subcutaneously. To identify key pathways and genes involved in the angiotropic phenotype of melanoma, analysis of differentially expressed genes (DEGs) and gene set enrichment analysis (GSEA) were performed. The role of tissue factor pathway inhibitor 2 (TFPI2) in angiotropism was evaluated by immunostaining, adhesion assay, shRNA, and in vivo tumorigenicity. Angiotropism and TFPI2 expression were examined in surgical specimens of melanoma by immunohistochemical staining. Data from The Cancer Genome Atlas (TCGA) were analyzed to explore the expression and prognostic implications of TFPI2 in uveal and cutaneous melanoma.

Results

Highly invasive melanoma cells spread along the branches of intratumor blood vessels to the leading edge of invasion in the coxenograft model, resembling angiotropic migration. Mechanisms underlying angiotropism were primarily associated with molecular function regulators, regulation of cell population proliferation, developmental processes, cell differentiation, responses to cytokines and cell motility/locomotion. TFPI2 downregulation weakened the perivascular migration of highly invasive melanoma cells. High levels of TFPI2 were correlated with worse and better survival in uveal and cutaneous melanoma, respectively.

Conclusion

These results provide a straightforward in vivo model for the observation of angiotropism and suggest that TFPI2 could inhibit the angiotropic phenotype of melanoma.

Visualizing Pericyte Mimicry of Angiotropic Melanoma by Direct Labeling of the Angioarchitecture

In addition to intravascular dissemination, angiotropic melanoma cells have the propensity to spread along the external surface of blood vessels in a pericytic location, or pericytic mimicry. Such continuous migration without intravasation has been termed "extravascular migratory metastasis" or EVMM. In order to visualize this mechanism of tumor propagation, we used a murine brain melanoma model utilizing green fluorescent human melanoma cells and red fluorescent lectin-tagged murine vessels. This model allows the direct microscopic visualization and mapping of the interaction of melanoma cells with the brain vasculature. In this chapter, we describe the methodology of lectin perfusion to label the entire angioarchitecture in conjunction with confocal microscopy imaging to study the pericyte mimicry of the angiotropic GFP+ melanoma cells.

Comparison of skeletal and soft tissue pericytes identifies CXCR4+ bone forming mural cells in human tissues

Human osteogenic progenitors are not precisely defined, being primarily studied as heterogeneous multipotent cell populations and termed mesenchymal stem cells (MSCs). Notably, select human pericytes can develop into bone-forming osteoblasts. Here, we sought to define the differentiation potential of CD146⁺ human pericytes from skeletal and soft tissue sources, with the underlying goal of defining cell surface markers that typify an osteoblastogenic pericyte. CD146⁺CD31^-CD45^- pericytes were derived by fluorescence-activated cell sorting from human periosteum, adipose, or dermal tissue. Periosteal CD146⁺CD31^-CD45^- cells retained canonical features of pericytes/MSC. Periosteal pericytes demonstrated a striking tendency to undergo osteoblastogenesis in vitro and skeletogenesis in vivo, while soft tissue pericytes did not readily. Transcriptome analysis revealed higher CXCR4 signaling among periosteal pericytes in comparison to their soft tissue counterparts, and CXCR4 chemical inhibition abrogated ectopic ossification by periosteal pericytes. Conversely, enrichment of CXCR4⁺ pericytes or stromal cells identified an osteoblastic/non-adipocytic precursor cell. In sum, human skeletal and soft tissue pericytes differ in their basal abilities to form bone. Diversity exists in soft tissue pericytes, however, and CXCR4⁺ pericytes represent an osteoblastogenic, non-adipocytic cell precursor. Indeed, enrichment for CXCR4-expressing stromal cells is a potential new tactic for skeletal tissue engineering.

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.

Models and molecular mechanisms of blood vessel co-option by cancer cells

Cancer cells have diverse mechanisms for utilizing the vasculature; they can initiate the formation of new blood vessels from preexisting ones (sprouting angiogenesis) or they can form cohesive interactions with the abluminal surface of preexisting vasculature in the absence of sprouting (co-option). The later process has received renewed attention due to the suggested role of blood vessel co-option in resistance to antiangiogenic therapies and the reported perivascular positioning and migratory patterns of cancer cells during tumor dormancy and invasion, respectively. However, only a few molecular mechanisms have been identified that contribute to the process of co-option and there has not been a formal survey of cell lines and laboratory models that can be used to study co-option in different organ microenvironments; thus, we have carried out a comprehensive literature review on this topic and have identified cell lines and described the laboratory models that are used to study blood vessel co-option in cancer. Put into practice, these models may help to shed new light on the molecular mechanisms that drive blood vessel co-option during tumor dormancy, invasion, and responses to different therapies.

Angiogenic desmoplastic histopathological growth pattern as a prognostic marker of good outcome in patients with colorectal liver metastases

BACKGROUND

In patients with resectable colorectal liver metastases (CRLM), distinct histopathological growth patterns (HGPs) develop at the interface between the tumour and surrounding tissue. The desmoplastic (d) HGP is characterised by angiogenesis and a peripheral fibrotic rim, whereas non-angiogenic HGPs co-opt endogenous sinusoidal hepatic vasculature. Evidence from previous studies has suggested that patients with dHGP in their CRLM have improved prognosis as compared to patients with non-desmoplastic HGPs. However, these studies were relatively small and applied arbitrary cut-off values for the determination of the predominant HGP. We have now investigated the prognostic effect of dHGP in a large cohort of patients with CRLM resected either with or without neoadjuvant chemotherapy.

METHODS

All consecutive patients undergoing a first partial hepatectomy for CRLM between 2000 and 2015 at a tertiary referral centre were considered for inclusion. HGPs were assessed in archival H&E stained slides according to recently published international consensus guidelines. The dHGP was defined as desmoplastic growth being present in 100% of the interface between the tumour and surrounding liver.

RESULTS

In total, HGPs in CRLMs from 732 patients were assessed. In the chemo-naive patient cohort (n = 367), the dHGP was present in 19% (n = 68) and the non-dHGP was present in 81% (n = 299) of patients. This dHGP subgroup was independently associated with good overall survival (OS) (HR: 0.39, p < 0.001) and progression-free survival (PFS) (HR: 0.54, p = 0.001). All patients with any CRLM with a non-dHGP had significantly reduced OS compared to those patients with 100% dHGP, regardless of the proportion of non-dHGP (all p values ≤ 0.001). In the neoadjuvantly treated patient cohort (n = 365), more patients were found to express dHGP (n = 109, 30%) (adjusted odds ratio: 2.71, p < 0.001). On univariable analysis, dHGP was associated with better OS (HR 0.66, p = 0.009) and PFS (HR 0.67, p = 0.002). However, after correction for confounding by means of multivariable analysis no significant association of dHGP with OS (HR 0.92, p = 0.623) or PFS (HR 0.76, p = 0.065) was seen.

CONCLUSIONS

The current study demonstrates that the angiogenic dHGP in CRLM resected from chemo-naive patients acts as a strong, positive prognostic marker, unmatched by any other prognosticator. This observation warrants the evaluation of the clinical utility of HGPs in prospective clinical trials.

Pericytes in Sarcomas and Other Mesenchymal Tumors

Tumors of mesenchymal origin are a diverse group, with >130 distinct entities currently recognized by the World Health Organization. A subset of mesenchymal tumors grow or invade in a perivascular fashion, and their potential relationship to pericytes is a matter of ongoing interest. In fact, multiple intersections exist between pericytes and tumors of mesenchymal origin. First, pericytes are the likely cell of origin for a group of mesenchymal tumors with a common perivascular growth pattern. These primarily benign tumors grow in a perivascular fashion and diffusely express canonical pericyte markers such as CD146, smooth muscle actin (SMA), platelet-derived growth factor receptor beta (PDGFR-β), and RGS5. These benign tumors include glomus tumor, myopericytoma, angioleiomyoma, and myofibroma. Second and as suggested by animal models, pericytes may give rise to malignant sarcomas. This is not a suggestion that all sarcomas within a certain subtype arise from pericytes, but that genetic modifications within a pericyte cell type may give rise to sarcomas. Third, mesenchymal tumors that are likely not a pericyte derivative co-opt pericyte markers in certain contexts. These include the PEComa family of tumors and liposarcoma. Fourth and finally, as "guardians" that enwrap the microvasculature, nonneoplastic pericytes may be important in sarcoma disease progression.

Angiotropism in recurrent cutaneous squamous cell carcinoma: Implications for regional tumor recurrence and extravascular migratory spread

Extravascular migratory metastasis is a form of cancer metastasis in which tumor cells spread by tracking along the abluminal aspect of vessel walls without breaking the vascular endothelial lining or intraluminal invasion. This phenomenon has been extensively described in melanoma and is being increasingly recognized in other neoplasms. Various modalities of treatment, including radiation-, chemo-, targeted-, and immune- therapies may potentially induce angiotropic behavior in neoplastic cells. Although there is a risk for tumor recurrence and metastasis, angiotropism may be under-recognized and is rarely reported. Here, we report a case of recurrent poorly-differentiated acantholytic squamous cell carcinoma of the scalp with extensive perineural invasion, previously treated with multiple therapies. There was multifocal extravascular cuffing of neoplastic cells around and focally involving the walls of small to medium-caliber blood vessels within and surrounding the tumor, without obvious tumor intravasation. In addition, small subtle nests of neoplastic keratinocytes were noted along the abluminal aspect of a large-caliber deep dermal blood vessel in an en-face margin, away from the main tumor mass. Such involvement can be difficult to identify; and thus, may be missed particularly during intra-operative frozen section evaluation, leading to false-negative margins and is therefore, a diagnostic pitfall.

Pericyte-like spreading by disseminated cancer cells activates YAP and MRTF for metastatic colonization

Metastatic seeding by disseminated cancer cells principally occurs in perivascular niches. Here, we show that mechanotransduction signalling triggered by the pericyte-like spreading of disseminated cancer cells on host tissue capillaries is critical for metastatic colonization. Disseminated cancer cells employ L1CAM (cell adhesion molecule L1) to spread on capillaries and activate the mechanotransduction effectors YAP (Yes-associated protein) and MRTF (myocardin-related transcription factor). This spreading is robust enough to displace resident pericytes, which also use L1CAM for perivascular spreading. L1CAM activates YAP by engaging β₁ integrin and ILK (integrin-linked kinase). L1CAM and YAP signalling enables the outgrowth of metastasis-initiating cells both immediately following their infiltration of target organs and after they exit from a period of latency. Our results identify an important step in the initiation of metastatic colonization, define its molecular constituents and provide an explanation for the widespread association of L1CAM with metastatic relapse in the clinic.

Post-surgical resection prognostic value of combined OPN, MMP7, and PSG9 plasma biomarkers in hepatocellular carcinoma

Biomarkers for hepatocellular carcinoma (HCC) following curative resection are not currently sufficient for prognostic indication of overall survival (OS) and disease-free survival (DFS). The aim of this study was to investigate the prognostic performance of osteopontin (OPN), matrix metalloproteinase 7 (MMP7), and pregnancy specific glycoprotein 9 (PSG9) in patients with HCC. A total of 179 prospective patients with HCC provided plasma before hepatectomy. Plasma OPN, MMP7, and PSG9 levels were determined by enzyme-linked immunosorbent assay. Correlations between plasma levels, clinical parameters, and outcomes (OS and DFS) were overall analyzed. High OPN ( ⩾ 149.97 ng/mL), MMP7 ( ⩾ 2.28 ng/mL), and PSG9 ( ⩾ 45.59 ng/mL) were prognostic indicators of reduced OS (P < 0.001, P < 0.001, and P = 0.007, respectively). Plasma PSG9 protein level was an independent factor in predicting OS (P = 0.008) and DFS (P = 0.038). Plasma OPN + MMP7 + PSG9 elevation in combination was a prognostic factor for OS (P < 0.001). OPN was demonstrated to be a risk factorassociated OS in stage I patients with HCC and patients with low α-fetoprotein levels ( < 20 ng/mL). These findings suggested that OPN, MMP7, PSG9 and their combined panels may be useful for aiding in tumor recurrence and mortality risk prediction of patients with HCC, particularly in the early stage of HCC carcinogenesis.

Loss of E-cadherin as Part of a Migratory Phenotype in Melanoma Is Associated With Ulceration

It has been suggested that embryogenic properties of migratory cells are reactivated during wound healing and metastasis in adults. This might explain the association between wound-induced inflammation and poor survival in patients with ulcerated melanoma. Linking inflammation with a migratory phenotype, we characterize the infiltration of innate inflammatory cells, loss of cell-to-cell adhesion (E-cadherin), factors associated with extracellular matrix degradation [matrix metalloproteinase-9 (MMP-9), and neutrophil elastase (NE)], and spindle-shaped cell morphology, between ulcerated (n = 179) and nonulcerated (n = 206) melanoma. In addition, the presence of "extravascular migratory metastasis" (angiotropism) and tumor-vessel density were evaluated as important factors for tumor cell dispersal in ulcerated melanoma. We showed a correlation between expression of the granulocyte marker cd66b+ and the expression of NE and MMP-9, reflecting activated neutrophils. Ulcerated melanoma correlated with a low global E-cadherin score (P = 0.041) and weak-spot score (P = 0.0004). Thus, 28% of the nonulcerated, 42% of the minimally/moderately ulcerated melanoma, and 53% of the excessively ulcerated melanoma presented low scores as opposed to a high E-cadherin score. In addition, the presence of ulceration was correlated with angiotropism (P < 0.0001) and spindle-shaped morphology (P = 0.021). There were no differences in MMP-9 expression or intratumoral vessel density between the ulcerated and nonulcerated group. In conclusion, expression of migratory cell properties showed a highly heterogeneous pattern, which was associated with ulcerated areas and inflammatory cells, in general and with neutrophils in particular. We, therefore, suggest that wound-associated inflammation may be involved in the induction of migratory cell transition and tumor cell dispersal in ulcerated melanoma.

Direct contact with perivascular tumor cells enhances integrin αvβ3 signaling and migration of endothelial cells

The secretion of soluble pro-angiogenic factors by tumor cells and stromal cells in the perivascular niche promotes the aggressive angiogenesis that is typical of glioblastoma (GBM). Here, we show that angiogenesis also can be promoted by a direct interaction between brain tumor cells, including tumor cells with cancer stem-like properties (CSCs), and endothelial cells (ECs). As shown in vitro, this direct interaction is mediated by binding of integrin αvβ3 expressed on ECs to the RGD-peptide in L1CAM expressed on CSCs. It promotes both EC network formation and enhances directed migration toward basic fibroblast growth factor. Activation of αvβ3 and bone marrow tyrosine kinase on chromosome X (BMX) is required for migration stimulated by direct binding but not for migration stimulated by soluble factors. RGD-peptide treatment of mice with established intracerebral GBM xenografts significantly reduced the percentage of Sox2-positive tumor cells and CSCs in close proximity to ECs, decreased integrin αvβ3 and BMX activation and p130CAS phosphorylation in the ECs, and reduced the vessel surface area. These results reveal a previously unrecognized aspect of the regulation of angiogenesis in GBM that can impact therapeutic anti-angiogenic targeting.

The biological and prognostic significance of angiotropism in uveal melanoma

Angiotropism is a marker of extravascular migration of melanoma cells along vascular and other structures and a prognostic factor in cutaneous melanoma. Because of this biological and prognostic importance in cutaneous melanoma, angiotropism was studied in uveal melanoma (UM). This retrospective study performed at a single ocular oncology referral center included 89 patients from the study period 2006-2008. All patients were diagnosed with UM from the choroid and/or ciliary body. All patients underwent enucleation for prognostic purposes and definitive therapy. Clinical, histopathological, and molecular variables included patient age, gender, extraocular extension, tumor location (ciliary body or not), optic nerve invasion, angiotropism, neurotropism, melanoma cell type, BAP1 mutation, and monosomy 3. Angiotropism was defined as melanoma cells arrayed along the abluminal vascular surfaces without intravasation in the sclera and/or episcleral tissue. The study included 51 women (57.3%) and 38 men with mean and median age: 63 years (range: 25-92). Mean follow-up was 4.4 years (range: 0.2 to 11). Fifty-three (59.6%) patients developed metastases and 48 (53.9%) were dead from metastases at last follow-up. Other principal variables recorded were angiotropism in 43.8%, extraocular extension in 7.9%, epithelioid/mixed cell type in 73.1%, BAP1 mutation in 41.3%, and monosomy 3 in 53.6% of cases. On multivariate analysis, extraocular extension, angiotropism, and monosomy 3 were predictive of metastasis, whereas tumor diameter, epithelioid cell type, angiotropism, and monosomy 3 were predictive of death. Chi-square test confirmed an association between angiotropism and metastasis and death but none with BAP1 mutation and monosomy 3. In conclusion, angiotropism and monosomy 3 were independent prognostic factors for both metastases and death in UM. However, irrespective of any prognostic value, the true importance of angiotropism is its biological significance as a marker of an alternative metastatic pathway.Laboratory Investigation advance online publication, 27 February 2017; doi:10.1038/labinvest.2017.16.

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.

Pericytic mimicry in well-differentiated liposarcoma/atypical lipomatous tumor

Pericytes are modified smooth muscle cells that closely enwrap small blood vessels, regulating and supporting the microvasculature through direct endothelial contact. Pericytes demonstrate a distinct immunohistochemical profile, including expression of smooth muscle actin, CD146, platelet-derived growth factor receptor β, and regulator of G-protein signaling 5. Previously, pericyte-related antigens have been observed to be present among a group of soft tissue tumors with a perivascular growth pattern, including glomus tumor, myopericytoma, and angioleiomyoma. Similarly, malignant tumor cells have been shown to have a pericyte-like immunoprofile when present in a perivascular location, seen in malignant melanoma, glioblastoma, and adenocarcinoma. Here, we examine well-differentiated liposarcoma specimens, which showed some element of perivascular areas with the appearance of smooth muscle (n = 7 tumors). Immunohistochemical staining was performed for pericyte antigens, including smooth muscle actin, CD146, platelet-derived growth factor receptor β, and regulator of G-protein signaling 5. Results showed consistent pericytic marker expression among liposarcoma tumor cells within a perivascular distribution. MDM2 immunohistochemistry and fluorescence in situ hybridization for MDM2 revealed that these perivascular cells were of tumor origin (7/7 tumors), whereas double immunohistochemical detection for CD31/CD146 ruled out an endothelial cell contribution. These findings further support the concept of pericytic mimicry, already established in diverse malignancies, and its presence in well-differentiated liposarcoma. The extent to which pericytic mimicry has prognostic significance in liposarcoma is as yet unknown.

Imaging of Angiotropism/Vascular Co-Option in a Murine Model of Brain Melanoma: Implications for Melanoma Progression along Extravascular Pathways

Angiotropism/pericytic mimicry and vascular co-option involve tumor cell interactions with the abluminal vascular surface. These two phenomena may be closely related. However, investigations of the two processes have developed in an independent fashion and different explanations offered as to their biological nature. Angiotropism describes the propensity of tumor cells to spread distantly via continuous migration along abluminal vascular surfaces, or extravascular migratory metastasis (EVMM). Vascular co-option has been proposed as an alternative mechanism by which tumors cells may gain access to a blood supply. We have used a murine brain melanoma model to analyze the interactions of GFP human melanoma cells injected into the mouse brain with red fluorescent lectin-labeled microvascular channels. Results have shown a striking spread of melanoma cells along preexisting microvascular channels and features of both vascular co-option and angiotropism/pericytic mimicry. This study has also documented the perivascular expression of Serpin B2 by angiotropic melanoma cells in the murine brain and in human melanoma brain metastases. Our findings suggest that vascular co-option and angiotropism/pericytic mimicry are closely related if not identical processes. Further studies are needed in order to establish whether EVMM is an alternative form of cancer metastasis in addition to intravascular cancer dissemination.

Satellite in transit metastases in rapidly fatal conjunctival melanoma: implications for angiotropism and extravascular migratory metastasis (description of a murine model for conjunctival melanoma)

Little information is currently available concerning loco-regional metastases such as satellite and in transit metastases and their natural history in conjunctival melanoma as compared to cutaneous melanoma. Angiotropism, a marker of extravascular migration of melanoma cells along vascular channels, often appears responsible for microscopic satellite, satellite and in transit metastases development in cutaneous melanoma. In addition, diffuse tissue microscopic satellites are correlated with widespread melanoma dissemination and death. Herein we report rapid conjunctival melanoma progression and a fatal outcome in four of five patients following recurrence as satellite in transit metastases. Five patients aged 31, 60, 63, 56, and 67 years developed primary conjunctival melanoma, histologically characterised by tumour thicknesses of 4, 4, 1.1, 3, and 2 mm. Two or more conjunctival melanomas manifested ulceration, significant mitotic rates, necrosis, angiotropism, and intralesional transformation. The conjunctival melanoma recurred in a matter of months as one or more discrete satellite in transit lesions in the vicinity of the primary melanoma. Histological examination revealed well-defined micronodules containing atypical melanocytes in the subepithelial connective tissue stroma. All lesions were extravascular and most appeared angiotropic. Four of five patients subsequently developed parotid or other loco-regional nodal disease and rapidly ensuing widespread metastases and death. The time course from diagnosis to the demise of the patients averaged about 13 (range 7-20) months. Our findings suggest that satellite in transit metastases constitute an important new risk marker for possible rapid metastatic disease progression and death in patients with conjunctival melanoma. This finding appears to take on even greater significance if such lesions develop rapidly, i.e., in a matter of weeks or months following diagnosis of primary conjunctival melanoma, and if the primary melanoma manifests additional high-risk features. Additional studies are underway in order to further elucidate the mechanism of these metastases.

Pericyte antigens in angiomyolipoma and PEComa family tumors

Perivascular epithelioid cell tumors (PEComas) are an uncommon family of soft tissue tumors with dual myoid-melanocytic differentiation. Although PEComa family tumors commonly demonstrate a perivascular growth pattern, pericyte antigen expression has not yet been examined among this unique tumor group. Previously, we demonstrated that a subset of perivascular soft tissue tumors exhibit a striking pericytic immunophenotype, with diffuse expression of αSMA, CD146, and PDGFRβ. Here, we describe the presence of pericyte antigens across a diverse group of PEComa family tumors (n = 19 specimens). Results showed that pericyte antigens differed extensively by histological appearance. Typical angiomyolipoma (AML) specimens showed variable expression of pericyte antigens among both perivascular and myoid-appearing cells. In contrast, AML specimens with a predominant spindled morphology showed diffuse expression of pericyte markers, including αSMA, CD146, and PDGFRβ. AML samples with predominant epithelioid morphology showed a marked reduction in or the absence of immunoreactivity for pericyte markers. Lymphangiomyoma samples showed more variable and partial pericyte marker expression. In summary, pericyte antigen expression is variable among PEComa family tumors and largely varies by tumor morphology. Pericytic marker expression in PEComa may represent a true pericytic cell of origin, or alternatively aberrant pericyte marker adoption. Markers of pericytic differentiation may be of future diagnostic utility for the evaluation of mesenchymal tumors, or identify actionable signaling pathways for future therapeutic intervention.

Pericytes, microvasular dysfunction, and chronic rejection

Chronic rejection of transplanted organs remains the main obstacle in the long-term success of organ transplantation. Thus, there is a persistent quest for development of antichronic rejection therapies and identification of novel molecular and cellular targets. One of the potential targets is the pericytes, the mural cells of microvessels, which regulate microvascular permeability, development, and maturation by controlling endothelial cell functions and regulating tissue fibrosis and inflammatory response. In this review, we discuss the potential of targeting pericytes in the development of microvasular dysfunction and the molecular pathways involved in regulation of pericyte activities for antichronic rejection intervention.

The multifaceted role of CD146/MCAM in the promotion of melanoma progression

Human malignant melanoma is a common primary malignant cutaneous tumour derived from transformed epidermal melanocytes. Patients with melanoma have a high rate of mortality due to resistance to chemotherapeutic drugs, a major obstacle to a successful treatment. Several reports have suggested that CD146 plays an important role as a signalling molecule in human melanoma. This role includes CD146 as a participant in inflammation, differentiation, adhesion, tumourigenicity, metastasis, invasion and angiogenesis among other processes, which suggests that this molecule promotes the progression of human melanoma as a multifaceted regulator. In this article, we explore the effects and corresponding mechanisms with respect to the role of CD146/MUC18 in the promotion of human melanoma progression. Collectively, the studies indicated that targeting CD146, because it is a suitable marker of poor patient outcome, might be useful in the design of future strategies for the prevention and treatment of human melanoma.

Matrigel: from discovery and ECM mimicry to assays and models for cancer research

The basement membrane is an important extracellular matrix that is found in all epithelial and endothelial tissues. It maintains tissue integrity, serves as a barrier to cells and to molecules, separates different tissue types, transduces mechanical signals, and has many biological functions that help to maintain tissue specificity. A well-defined soluble basement membrane extract, termed BME/Matrigel, prepared from an epithelial tumor is similar in content to authentic basement membrane, and forms a hydrogel at 24-37°C. It is used in vitro as a substrate for 3D cell culture, in suspension for spheroid culture, and for various assays, such as angiogenesis, invasion, and dormancy. In vivo, BME/Matrigel is used for angiogenesis assays and to promote xenograft and patient-derived biopsy take and growth. Studies have shown that both the stiffness of the BME/Matrigel and its components (i.e. chemical signals) are responsible for its activity with so many different cell types. BME/Matrigel has widespread use in assays and in models that improve our understanding of tumor biology and help define therapeutic approaches.

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.

Diallyl disulfide inhibits TNFα-induced CCL2 release by MDA-MB-231 cells

Monocyte chemotactic protein-1 (MCP-1/CCL2) is released by tumor tissues, serving as a potent chemokine enabling directional homing of mononuclear cells to tumor tissue, which subsequently differentiate into tumor-associated macrophages (TAMs) via TGFβ1 signaling. TAMs readily invade tumor tissue and continue to synthesize pro-oncogenic proteins including tumor growth factors, matrix proteases (metastasis), angiogenic factors (neovascularization) and CCL2. Substances, which can attenuate or block the initial release of CCL2 have been shown to prevent cancer-associated inflammative pro-oncogenic processes. In the current study, we investigated the effects of the organosulfur compound diallyl disulfide (DADS), a natural constituent of Allium sativum (garlic) on suppression of TNFα-induced release of CCL2 from triple-negative human breast tumor (MDA-MB-231) cells. Using an initial adipokine/chemokine protein panel microarray, the data show a predominant expression profile in resting/untreated MDA-MB-231 cells for sustained release of IL6, IL8, plasminogen Activator Inhibitor 1 and TIMP1/2. Treatment with TNFα (40 ng/ml) had no effect on many of these molecules, with a single major elevation in release of CCL2 (~1,300-fold up-regulation). TNFα-induced CCL2 release was reversed by a sub-lethal concentration of DADS (100 μM), evident in antibody based assays. These findings provide evidence to support another avenue of anticancer/chemopreventative properties attributable to garlic constituents through immunomodulation.

Skin progenitor cells contribute to bleomycin-induced skin fibrosis

OBJECTIVE

The origin of the cells that contribute to skin fibrosis is unclear. We undertook the present study to assess the contribution of Sox2-expressing skin progenitor cells to bleomycin-induced scleroderma.

METHODS

Scleroderma was induced, by bleomycin administration, in wild-type mice and in mice in which CCN2 was deleted from Sox2-expressing cells. Lineage tracing analysis was performed to assess whether cells expressing Sox2 are recruited to fibrotic lesions in response to bleomycin-induced scleroderma.

RESULTS

In response to bleomycin, Sox2-positive/α-smooth muscle actin-positive cells were recruited to fibrotic tissue. CCN2-conditional knockout mice in which CCN2 was deleted from Sox2-expressing cells exhibited resistance to bleomycin-induced skin fibrosis. Collectively, these results indicate that CCN2 is required for the recruitment of progenitor cells and that CCN2-expressing progenitor cells are essential for bleomycin-induced skin fibrosis. Lineage tracing analysis using mice in which a tamoxifen-dependent Cre recombinase was expressed under the control of the Sox2 promoter confirmed that progenitor cells were recruited to the fibrotic lesion in response to bleomycin, and that this did not occur in CCN2-knockout mice. The ability of serum to induce α-smooth muscle actin expression in skin progenitor cells required the presence of CCN2.

CONCLUSION

Sox2-positive skin progenitor cells are required in order for bleomycin-induced skin fibrosis to occur, and CCN2 is required for the recruitment of these cells to the fibrotic lesion. Targeting stem cell recruitment or CCN2 may therefore represent a useful therapeutic approach in combating fibrotic skin disease.

Extravascular migratory metastasis in gynaecological carcinosarcoma

AIMS

Extravascular migratory metastasis (EVMM) is a potential mechanism of tumour spread reported most extensively in cutaneous melanoma. It has not been described previously in gynaecological malignancies. We describe EVMM in four gynaecological carcinosarcomas.

METHODS AND RESULTS

Extravascular migratory metastasis was observed in an ovarian carcinosarcoma during routine diagnostic assessment. Twenty-three additional, randomly selected gynaecological carcinosarcomas (11 tubo-ovarian and 12 endometrial) were examined retrospectively and EVMM was identified in three of these. Other than the index case, EVMM was a focal finding, identified in 12-18% of slides. The malignant cells demonstrating EVMM appeared sarcomatoid and were distributed abluminally, partly or completely surrounding the endothelium. Affected vessels often showed mural fibrin deposition. Immunohistochemistry for α-smooth muscle actin (SMA), CD31, CD34, D2-40, laminin and type IV collagen was performed on the EVMM-positive cases. The perivascular malignant cells showed more consistent SMA and laminin immunoreactivity than the non-vascular tumour elements.

CONCLUSIONS

Extravascular migratory metastasis is a hitherto unrecognized mechanism of tumour spread in gynaecological carcinosarcomas. The perivascular tumour cells appear to adopt a pericytic phenotype, and this may represent a specific pattern of epithelial-mesenchymal transition. Further studies with pericyte-specific immunohistological markers may better demonstrate the presence and possible prognostic significance of EVMM in gynaecological tumours.

Ultraviolet-radiation-induced inflammation promotes angiotropism and metastasis in melanoma

Intermittent intense ultraviolet (UV) exposure represents an important aetiological factor in the development of malignant melanoma. The ability of UV radiation to cause tumour-initiating DNA mutations in melanocytes is now firmly established, but how the microenvironmental effects of UV radiation influence melanoma pathogenesis is not fully understood. Here we report that repetitive UV exposure of primary cutaneous melanomas in a genetically engineered mouse model promotes metastatic progression, independent of its tumour-initiating effects. UV irradiation enhanced the expansion of tumour cells along abluminal blood vessel surfaces and increased the number of lung metastases. This effect depended on the recruitment and activation of neutrophils, initiated by the release of high mobility group box 1 (HMGB1) from UV-damaged epidermal keratinocytes and driven by Toll-like receptor 4 (TLR4). The UV-induced neutrophilic inflammatory response stimulated angiogenesis and promoted the ability of melanoma cells to migrate towards endothelial cells and use selective motility cues on their surfaces. Our results not only reveal how UV irradiation of epidermal keratinocytes is sensed by the innate immune system, but also show that the resulting inflammatory response catalyses reciprocal melanoma-endothelial cell interactions leading to perivascular invasion, a phenomenon originally described as angiotropism in human melanomas by histopathologists. Angiotropism represents a hitherto underappreciated mechanism of metastasis that also increases the likelihood of intravasation and haematogenous dissemination. Consistent with our findings, ulcerated primary human melanomas with abundant neutrophils and reactive angiogenesis frequently show angiotropism and a high risk for metastases. Our work indicates that targeting the inflammation-induced phenotypic plasticity of melanoma cells and their association with endothelial cells represent rational strategies to specifically interfere with metastatic progression.

From pericytes to perivascular tumours: correlation between pathology, stem cell biology, and tissue engineering

PURPOSE

Pericytes were once thought only to aid in angiogenesis and blood pressure control. Gradually, the known functions of pericytes and other perivascular stem cells (PSC) have broadly increased. The following review article will summarize the known functions and importance of pericytes across disciplines of pathology, stem cell biology, and tissue engineering.

METHODS

A literature review was performed for studies examining the importance of pericytes in pathology, stem cell biology, and tissue engineering.

RESULTS

The importance of pericytes most prominently includes the identification of the perivascular identity of mesenchymal stem cells (or MSC). Now, pericytes and other PSC are known to display surface markers and multilineage differentiation potential of MSC. Accordingly, interest in the purification and use of PSC for mesenchymal tissue formation and regeneration has increased. Significant demonstration of in vivo efficacy in bone and muscle regeneration has been made in laboratory animals. Contemporaneously with the uncovering of an MSC identity for pericytes, investigators in tumour biology have found biologically relevant roles for pericytes in tumor formation, lymphovascular invasion, and perivascular tumor spread. As well, the contribution of pericytes to perivascular tumors has been examined (and debated), including glomus tumour, myopericytoma and solitary fibrous tumour/hemangiopericytoma. In addition, an expanding recognition of pericyte mimicry and perivascular tumour invasion has occurred, encompassing common malignancies of the brain and skin.

CONCLUSIONS

In summary, pericytes have a wide range of roles in health and disease. Pericytes are being increasingly studied for their role in tumour formation, growth and invasion. Likewise, the application of pericytes/PSC for mesenchymal tissue engineering is an expanding field of interest.

Mesenchymal cells emerge as primary contributors to fibrosis in multiple tissues

A longstanding controversy exists regarding the cellular origin of myofibroblasts in tissue fibrosis. A recent study by Hung and colleagues (Am J Respir Crit Care Med 188(7):820-830, 2013) used genetic fate mapping of FoxD1 embryonic progenitor cells to show a major and direct contribution of mesenchymal cells to fibrogenesis in the lung. Future studies using FoxD1-specific inducible knockout models of pro-fibrotic genes such as CCN2 will be valuable for determining anti-fibrotic drug targets. The emergence of pericyte-like myofibroblast precursors also raises the question of whether mesenchymal stem cells in various niches contribute to fibrotic responses throughout the body.

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.