A novel imaging-based high-throughput screening approach to anti-angiogenic drug discovery

Imaging Techniques: Essential Tools for the Study of SARS-CoV-2 Infection

The world has seen the emergence of a new virus in 2019, SARS-CoV-2, causing the COVID-19 pandemic and millions of deaths worldwide. Microscopy can be much more informative than conventional detection methods such as RT-PCR. This review aims to present the up-to-date microscopy observations in patients, the in vitro studies of the virus and viral proteins and their interaction with their host, discuss the microscopy techniques for detection and study of SARS-CoV-2, and summarize the reagents used for SARS-CoV-2 detection. From basic fluorescence microscopy to high resolution techniques and combined technologies, this article shows the power and the potential of microscopy techniques, especially in the field of virology.

Integrated Multiparametric High-Content Profiling of Endothelial Cells

Endothelial cells (ECs) are widely heterogeneous at the cell level and serve different functions at the vessel and tissue levels. EC-forming colonies derived from induced pluripotent stem cells (iPSC-ECFCs) alongside models such as primary human umbilical vein ECs (HUVECs) are slowly becoming available for research with future applications in cell therapies, disease modeling, and drug discovery. We and others previously described high-content analysis approaches capturing unbiased morphology-based measurements coupled with immunofluorescence and used these for multidimensional reduction and population analysis. Here, we report a tailored workflow to characterize ECs. We acquire images at high resolution with high-magnification water-immersion objectives with Hoechst, vascular endothelial cadherin (VEC), and activated NOTCH staining. We hypothesize that via these key markers alone we would be able to distinguish and assess different EC populations. We used cell population software analysis to phenotype HUVECs and iPSC-ECFCs in the absence or presence of vascular endothelial growth factor (VEGF). To our knowledge, this study presents the first parallel quantitative high-content multiparametric profiling of EC models. Importantly, it highlights a simple strategy to benchmark ECs in different conditions and develop new approaches for biological research and translational applications for regenerative medicine.

Customizable biomaterials as tools for advanced anti-angiogenic drug discovery

The inhibition of angiogenesis is a critical element of cancer therapy, as cancer vasculature contributes to tumor expansion. While numerous drugs have proven to be effective at disrupting cancer vasculature, patient survival has not significantly improved as a result of anti-angiogenic drug treatment. Emerging evidence suggests that this is due to a combination of unintended side effects resulting from the application of anti-angiogenic compounds, including angiogenic rebound after treatment and the activation of metastasis in the tumor. There is currently a need to better understand the far-reaching effects of anti-angiogenic drug treatments in the context of cancer. Numerous innovations and discoveries in biomaterials design and tissue engineering techniques are providing investigators with tools to develop physiologically relevant vascular models and gain insights into the holistic impact of drug treatments on tumors. This review examines recent advances in the design of pro-angiogenic biomaterials, specifically in controlling integrin-mediated cell adhesion, growth factor signaling, mechanical properties and oxygen tension, as well as the implementation of pro-angiogenic materials into sophisticated co-culture models of cancer vasculature.

Identification of Angiogenesis Inhibitors Using a Co-culture Cell Model in a High-Content and High-Throughput Screening Platform

Angiogenesis is an important hallmark of cancer, contributing to tumor formation and metastasis. In vitro angiogenesis models for analyzing tube formation serve as useful tools to study these processes. However, current in vitro co-culture models using primary cells have limitations in usefulness and consistency. Therefore, in the present study, an in vitro co-culture assay system was optimized in a 1536-well format for high-throughput screening using human telomerase reverse transcriptase (hTERT)–immortalized mesenchymal stem cells and aortic endothelial cells. The National Center for Advancing Translational Sciences (NCATS) Pharmaceutical Collection (NPC) library containing 2816 drugs was evaluated using the in vitro co-culture assay. From the screen, 35 potent inhibitors (IC₅₀ ≤1 µM) were identified, followed by 15 weaker inhibitors (IC₅₀ 1–50 µM). Moreover, many known angiogenesis inhibitors were identified, such as topotecan, docetaxel, and bortezomib. Several potential novel angiogenesis inhibitors were also identified from this study, including thimerosal and podofilox. Among the inhibitors, some compounds were proved to be involved in the hypoxia-inducible factor-1α (HIF-1α) and the nuclear factor-kappa B (NF-κB) pathways. The co-culture model developed by using hTERT-immortalized cell lines described in this report provides a consistent and robust in vitro system for antiangiogenic drug screening.

High-Content Assay Multiplexing for Vascular Toxicity Screening in Induced Pluripotent Stem Cell-Derived Endothelial Cells and Human Umbilical Vein Endothelial Cells

Endothelial cells (ECs) play a major role in blood vessel formation and function. While there is longstanding evidence for the potential of chemical exposures to adversely affect EC function and vascular development, the hazard potential of chemicals with respect to vascular effects is not routinely evaluated in safety assessments. Induced pluripotent stem cell (iPSC)-derived ECs promise to provide a physiologically relevant, organotypic culture model that is amenable for high-throughput (HT) EC toxicant screening and may represent a viable alternative to traditional in vitro models, including human umbilical vein endothelial cells (HUVECs). To evaluate the utility of iPSC-ECs for multidimensional HT toxicity profiling of chemicals, both iPSC-ECs and HUVECs were exposed to selected positive (angiogenesis inhibitors, cytotoxic agents) and negative compounds in concentration response for either 16 or 24 h in a 384-well plate format. Furthermore, chemical effects on vascularization were quantified using EC angiogenesis on biological (Geltrex™) and synthetic (SP-105 angiogenesis hydrogel) extracellular matrices. Cellular toxicity was assessed using high-content live cell imaging and the CellTiter-Glo^® assay. Assay performance indicated good to excellent assay sensitivity and reproducibility for both cell types investigated. Both iPSC-derived ECs and HUVECs formed tube-like structures on Geltrex™ and hydrogel, an effect that was inhibited by angiogenesis inhibitors and cytotoxic agents in a concentration-dependent manner. The quality of HT assays in HUVECs was generally higher than that in iPSC-ECs. Altogether, this study demonstrates the capability of ECs for comprehensive assessment of the biological effects of chemicals on vasculature in a HT compatible format.

Organotypic brain slice cultures as a model to study angiogenesis of brain vessels

Brain vessels are the most important structures in the brain to deliver energy and substrates to neurons. Brain vessels are composed of a complex interaction between endothelial cells, pericytes, and astrocytes, controlling the entry of substrates into the brain. Damage of brain vessels and vascular impairment are general pathologies observed in different neurodegenerative disorders including e.g., Alzheimer's disease. In order to study remodeling of brain vessels, simple 3-dimensional in vitro systems need to be developed. Organotypic brain slices of mice provide a potent tool to explore angiogenic effects of brain vessels in a complex 3-dimensional structure. Here we show that organotypic brain slices can be cultured from 110 μm thick sections of postnatal and adult mice brains. The vessels are immunohistochemically stained for laminin and collagen IV. Co-stainings are an appropriate method to visualize interaction of brain endothelial cells with pericytes and astrocytes in these vessels. Different exogenous stimuli such as fibroblast growth factor-2 or vascular endothelial growth factor induce angiogenesis or re-growth, respectively. Hyperthermia or acidosis reduces the vessel density in organotypic slices. In conclusion, organotypic brain slices exhibit a strong vascular network which can be used to study remodeling and angiogenesis of brain vessels in a 3-dimensional in vitro system.

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: focus on the cancer hallmark of tumor angiogenesis

One of the important 'hallmarks' of cancer is angiogenesis, which is the process of formation of new blood vessels that are necessary for tumor expansion, invasion and metastasis. Under normal physiological conditions, angiogenesis is well balanced and controlled by endogenous proangiogenic factors and antiangiogenic factors. However, factors produced by cancer cells, cancer stem cells and other cell types in the tumor stroma can disrupt the balance so that the tumor microenvironment favors tumor angiogenesis. These factors include vascular endothelial growth factor, endothelial tissue factor and other membrane bound receptors that mediate multiple intracellular signaling pathways that contribute to tumor angiogenesis. Though environmental exposures to certain chemicals have been found to initiate and promote tumor development, the role of these exposures (particularly to low doses of multiple substances), is largely unknown in relation to tumor angiogenesis. This review summarizes the evidence of the role of environmental chemical bioactivity and exposure in tumor angiogenesis and carcinogenesis. We identify a number of ubiquitous (prototypical) chemicals with disruptive potential that may warrant further investigation given their selectivity for high-throughput screening assay targets associated with proangiogenic pathways. We also consider the cross-hallmark relationships of a number of important angiogenic pathway targets with other cancer hallmarks and we make recommendations for future research. Understanding of the role of low-dose exposure of chemicals with disruptive potential could help us refine our approach to cancer risk assessment, and may ultimately aid in preventing cancer by reducing or eliminating exposures to synergistic mixtures of chemicals with carcinogenic potential.

Cellular context-mediated Akt dynamics regulates MAP kinase signaling thresholds during angiogenesis

This study examines the temporal regulation of Akt in endothelial cells during formation of capillary-like networks induced by cell–cell contact with vascular smooth muscle cells (vSMCs) and vSMC-associated VEGF. Heterotypic cell–cell interaction between mural and endothelial cells alters Akt kinase protein dynamics, which regulates angiogenesis.

The formation of new blood vessels by sprouting angiogenesis is tightly regulated by contextual cues that affect angiogeneic growth factor signaling. Both constitutive activation and loss of Akt kinase activity in endothelial cells impair angiogenesis, suggesting that Akt dynamics mediates contextual microenvironmental regulation. We explored the temporal regulation of Akt in endothelial cells during formation of capillary-like networks induced by cell–cell contact with vascular smooth muscle cells (vSMCs) and vSMC-associated VEGF. Expression of constitutively active Akt1 strongly inhibited network formation, whereas hemiphosphorylated Akt1 epi-alleles with reduced kinase activity had an intermediate inhibitory effect. Conversely, inhibition of Akt signaling did not affect endothelial cell migration or morphogenesis in vSMC cocultures that generate capillary-like structures. We found that endothelial Akt activity is transiently blocked by proteasomal degradation in the presence of SMCs during the initial phase of capillary-like structure formation. Suppressed Akt activity corresponded to the increased endothelial MAP kinase signaling that was required for angiogenic endothelial morphogenesis. These results reveal a regulatory principle by which cellular context regulates Akt protein dynamics, which determines MAP kinase signaling thresholds necessary drive a morphogenetic program during angiogenesis.

Generation, expansion and functional analysis of endothelial cells and pericytes derived from human pluripotent stem cells

Human endothelial cells (ECs) and pericytes are of great interest for research on vascular development and disease, as well as for future therapy. This protocol describes the efficient generation of ECs and pericytes from human pluripotent stem cells (hPSCs) under defined conditions. Essential steps for hPSC culture, differentiation, isolation and functional characterization of ECs and pericytes are described. Substantial numbers of both cell types can be derived in only 2-3 weeks: this involves differentiation (10 d), isolation (1 d) and 4 or 10 d of expansion of ECs and pericytes, respectively. We also describe two assays for functional evaluation of hPSC-derived ECs: (i) primary vascular plexus formation upon coculture with hPSC-derived pericytes and (ii) incorporation in the vasculature of zebrafish xenografts in vivo. These assays can be used to test the quality and drug sensitivity of hPSC-derived ECs and model vascular diseases with patient-derived hPSCs.

In Vitro Characterization of Valproic Acid, ATRA, and Cytarabine Used for Disease-Stabilization in Human Acute Myeloid Leukemia: Antiproliferative Effects of Drugs on Endothelial and Osteoblastic Cells and Altered Release of Angioregulatory Mediators by Endothelial Cells

The combined use of the histone deacetylase inhibitor valproic acid (VPA), the retinoic acid receptor- α agonist all-trans retinoic acid (ATRA), and the deoxyribonucleic acid polymerase- α inhibitor cytarabine (Ara-C) is now considered for disease-stabilizing treatment of acute myeloid leukemia (AML). Leukemogenesis and leukemia cell chemoresistance seem to be supported by neighbouring stromal cells in the bone marrow, and we have therefore investigated the effects of these drugs on primary human endothelial cells and the osteoblastic Cal72 cell line. The results show that VPA and Ara-C have antiproliferative effects, and the antiproliferative/cytotoxic effect of Ara-C was seen at low concentrations corresponding to serum levels found during low-dose in vivo treatment. Furthermore, in functional assays of endothelial migration and tube formation VPA elicited an antiangiogenic effect, whereas ATRA elicited a proangiogenic effect. Finally, VPA and ATRA altered the endothelial cell release of angiogenic mediators; ATRA increased levels of CXCL8, PDGF-AA, and VEGF-D, while VPA decreased VEGF-D and PDGF-AA/BB levels and both drugs reduced MMP-2 levels. Several of these mediators can enhance AML cell proliferation and/or are involved in AML-induced bone marrow angiogenesis, and direct pharmacological effects on stromal cells may thus indirectly contribute to the overall antileukemic activity of this triple drug combination.

A systems biology view of blood vessel growth and remodelling

Blood travels throughout the body in an extensive network of vessels – arteries, veins and capillaries. This vascular network is not static, but instead dynamically remodels in response to stimuli from cells in the nearby tissue. In particular, the smallest vessels – arterioles, venules and capillaries – can be extended, expanded or pruned, in response to exercise, ischaemic events, pharmacological interventions, or other physiological and pathophysiological events. In this review, we describe the multi-step morphogenic process of angiogenesis – the sprouting of new blood vessels – and the stability of vascular networks in vivo. In particular, we review the known interactions between endothelial cells and the various blood cells and plasma components they convey. We describe progress that has been made in applying computational modelling, quantitative biology and high-throughput experimentation to the angiogenesis process.

Development and characterization of a high-throughput in vitro cord formation model insensitive to VEGF inhibition

Background

Anti-VEGF therapy reduces tumor blood vessels, however, some vessels always remain. These VEGF insensitive vessels may help support continued tumor growth and metastases. Many in vitro assays examining multiple steps of the angiogenic process have been described, but the majority of these assays are sensitive to VEGF inhibition. There has been little focus on the development of high-throughput, in vitro assays to model the vessels that are insensitive to VEGF inhibition.

Methods

Here, we describe a fixed end-point and kinetic, high-throughput stem cell co-culture model of cord formation.

Results

In this system, cords develop within 24 hours, at which point they begin to lose sensitivity to VEGF inhibitors, bevacizumab, and ramucirumab. Consistent with the hypothesis that other angiogenic factors maintain VEGF-independent vessels, pharmacologic intervention with a broad spectrum anti-angiogenic antagonist (suramin), a vascular disrupting agent (combretastatin), or a combination of VEGF and Notch pathway inhibitors reduced the established networks. In addition, we used our in vitro approach to develop an in vivo co-implant vasculogenesis model that connects with the endogenous vasculature to form functional blood vessels. Similar to the in vitro system, over time these vessels become insensitive to VEGF inhibition.

Conclusion

Together, these models may be used to identify novel drugs targeting tumor vessels that are not sensitive to VEGF inhibition.

Domains I and IV of annexin A2 affect the formation and integrity of in vitro capillary-like networks

Annexin A2 (AnxA2) is a widely expressed multifunctional protein found in different cellular compartments. In spite of lacking a hydrophobic signal peptide, AnxA2 is found at the cell surface of endothelial cells, indicative of a role in angiogenesis. Increased extracellular levels of AnxA2 in tumours correlate with neoangiogenesis, metastasis and poor prognosis. We hypothesised that extracellular AnxA2 may contribute to angiogenesis by affecting endothelial cell-cell interactions and motility. To address this question, we studied the effect of heterotetrameric and monomeric forms of AnxA2, as well as its two soluble domains on the formation and maintenance of capillary-like structures by using an in vitro co-culture system consisting of endothelial and smooth muscle cells. In particular, addition of purified domains I and IV of AnxA2 potently inhibited the vascular endothelial growth factor (VEGF)-dependent formation of the capillary-like networks in a dose-dependent manner. In addition, these AnxA2 domains disrupted endothelial cell-cell contacts in preformed capillary-like networks, resulting in the internalisation of vascular endothelial (VE)-cadherin and the formation of VE-cadherin-containing filopodia-like structures between the endothelial cells, suggesting increased cell motility. Addition of monoclonal AnxA2 antibodies, in particular against Tyr23 phosphorylated AnxA2, also strongly inhibited network formation in the co-culture system. These results suggest that extracellular AnxA2, most likely in its Tyr phosphorylated form, plays a pivotal role in angiogenesis. The exogenously added AnxA2 domains most likely mediate their effects by competing with endogenous AnxA2 for extracellular factors necessary for the initiation and maintenance of angiogenesis, such as those involved in the formation/integrity of cell-cell contacts.

Cell-based screening identifies paroxetine as an inhibitor of diabetic endothelial dysfunction

We have conducted a phenotypic screening in endothelial cells exposed to elevated extracellular glucose (an in vitro model of hyperglycemia) to identify compounds that prevent hyperglycemia-induced reactive oxygen species (ROS) formation without adversely affecting cell viability. From a focused library of >6,000 clinically used drug-like and pharmacologically active compounds, several classes of active compounds emerged, with a confirmed hit rate of <0.5%. Follow-up studies focused on paroxetine, a clinically used antidepressant compound that has not been previously implicated in the context of hyperglycemia or diabetes. Paroxetine reduced hyperglycemia-induced mitochondrial ROS formation, mitochondrial protein oxidation, and mitochondrial and nuclear DNA damage, without interfering with mitochondrial electron transport or cellular bioenergetics. The ability of paroxetine to improve hyperglycemic endothelial cell injury was unique among serotonin reuptake blockers and can be attributed to its antioxidant effect, which primarily resides within its sesamol moiety. Paroxetine maintained the ability of vascular rings to respond to the endothelium-dependent relaxant acetylcholine, both during in vitro hyperglycemia and ex vivo, in a rat model of streptozotocin-induced diabetes. Thus, the current work identifies a novel pharmacological action of paroxetine as a protector of endothelial cells against hyperglycemic injury and raises the potential of repurposing of this drug for the experimental therapy of diabetic cardiovascular complications.

Lens-free computational imaging of capillary morphogenesis within three-dimensional substrates

Endothelial cells cultured in three-dimensional (3-D) extracellular matrices spontaneously form microvessels in response to soluble and matrix-bound factors. Such cultures are common for the study of angiogenesis and may find widespread use in drug discovery. Vascular networks are imaged over weeks to measure the distribution of vessel morphogenic parameters. Measurements require micron-scale spatial resolution, which for light microscopy comes at the cost of limited field-of-view (FOV) and shallow depth-of-focus (DOF). Small FOVs and DOFs necessitate lateral and axial mechanical scanning, thus limiting imaging throughput. We present a lens-free holographic on-chip microscopy technique to rapidly image microvessels within a Petri dish over a large volume without any mechanical scanning. This on-chip method uses partially coherent illumination and a CMOS sensor to record in-line holographic images of the sample. For digital reconstruction of the measured holograms, we implement a multiheight phase recovery method to obtain phase images of capillary morphogenesis over a large FOV (24 mm2) with ≈ 1.5 μm spatial resolution. On average, measured capillary length in our method was within approximately 2% of lengths measured using a 10 × microscope objective. These results suggest lens-free on-chip imaging is a useful toolset for high-throughput monitoring and quantitative analysis of microvascular 3-D networks.

Phenotype-based high-content chemical library screening identifies statins as inhibitors of in vivo lymphangiogenesis

Lymphangiogenesis plays an important role in promoting cancer metastasis to sentinel lymph nodes and beyond and also promotes organ transplant rejection. We used human lymphatic endothelial cells to establish a reliable three-dimensional lymphangiogenic sprouting assay with automated image acquisition and analysis for inhibitor screening. This high-content phenotype-based assay quantifies sprouts by automated fluorescence microscopy and newly developed analysis software. We identified signaling pathways involved in lymphangiogenic sprouting by screening the Library of Pharmacologically Active Compounds (LOPAC)(1280) collection of pharmacologically relevant compounds. Hit characterization revealed that mitogen-activated protein kinase kinase (MEK) 1/2 inhibitors substantially block lymphangiogenesis in vitro and in vivo. Importantly, the drug class of statins, for the first time, emerged as potent inhibitors of lymphangiogenic sprouting in vitro and of corneal and cutaneous lymphangiogenesis in vivo. This effect was mediated by inhibition of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and subsequently the isoprenylation of Rac1. Supplementation with the enzymatic products of HMG-CoA reductase functionally rescued lymphangiogenic sprouting and the recruitment of Rac1 to the plasma membrane.

Osteogenic stimulatory conditions enhance growth and maturation of endothelial cell microvascular networks in culture with mesenchymal stem cells

To optimize culture conditions for in vitro prevascularization of tissue-engineered bone constructs, the development of organotypic blood vessels under osteogenic stimulatory conditions (OM) was investigated. Coculture of endothelial cells and mesenchymal stem cells was used to assess proangiogenic effects of mesenchymal stem cells on endothelial cells. Four different culture conditions were evaluated for their effect on development of microvascular endothelial cell networks. Mineralization, deposition of extracellular matrix, and perivascular gene expression were studied in OM. After 3 days, endothelial cells established elongated capillary-like networks, and upregulated expression of vascular markers was seen. After 15 days, all parameters evaluated were significantly increased for cultures in OM. Mature networks developed in OM presented lumens enveloped by basement membrane-like collagen IV, with obvious mineralization and upregulated perivascular gene expression from mesenchymal stem cells. Our results suggest osteogenic stimulatory conditions to be appropriate for in vitro development of vascularized bone implants for tissue engineering.

Characterization of a novel angiogenic model based on stable, fluorescently labelled endothelial cell lines amenable to scale-up for high content screening

BACKGROUND

Blood vessel formation is important for many physiological and pathological processes and is therefore a critical target for drug development. Inhibiting angiogenesis to starve a tumour or promoting 'normalization' of tumour vasculature in order to facilitate delivery of anticancer drugs are both areas of active research. Recapitulation of vessel formation by human cells in vitro allows the investigation of cell-cell and cell-matrix interactions in a controlled environment and is therefore a crucial step in developing HCS (high content screening) and HTS (high throughput screening) assays to search for modulators of blood vessel formation. HUVECs (human umbilical-vein endothelial cells) exemplify primary cells used in angiogenesis assays. However, primary cells have significant limitations that include phenotypic decay and/or senescence by six to eight passages in culture, making stable integration of fluorescent markers and large-scale expansion for HTS problematic. To overcome these limitations for HTS, we developed a novel angiogenic model system that employs stable fluorescent endothelial cell lines based on immortalized HMECs (human microvascular endothelial cell). We then evaluated HMEC cultures, both alone and co-cultured with an EMC (epicardial mesothelial cell) line that contributes vascular smooth muscle cells, to determine the suitability for HTS or HCS.

RESULTS

The endothelial and epicardial lines were engineered to express a panel of nuclear- and cytoplasm-localized fluorescent proteins to be mixed and matched to suit particular experimental goals. HMECs retained their angiogenic potential and stably expressed fluorescent proteins for at least 13 passages after transduction. Within 8 h after plating on Matrigel, the cells migrated and coalesced into networks of vessel-like structures. If co-cultured with EMCs, the branches formed cylindrical-shaped structures of HMECs surrounded by EMC derivatives reminiscent of vessels. Network formation measurements revealed responsiveness to media composition and control compounds.

CONCLUSIONS

HMEC-based lines retain most of the angiogenic features of primary endothelial cells and yet possess long-term stability and ease of culture, making them intriguing candidates for large-scale primary HCS and HTS (of ~10000-1000000 molecules). Furthermore, inclusion of EMCs demonstrates the feasibility of using epicardial-derived cells, which normally contribute to smooth muscle, to model large vessel formation. In summary, the immortalized fluorescent HMEC and EMC lines and straightforward culture conditions will enable assay development for HCS of angiogenesis.

Endothelial Cell Vascular Smooth Muscle Cell Co-Culture Assay For High Throughput Screening Assays For Discovery of Anti-Angiogenesis Agents and Other Therapeutic Molecules

Drug development for many diseases would be aided greatly by accurate in vitro model systems that replicate key elements of in vivo physiology. The recent development of co-culture systems of endothelial cells and vascular smooth muscle cells can be extended to high throughput systems for the identification of compounds for angiogenesis, vascular repair and hypertension. In this review, the various co-culture systems are reviewed and biological interactions between endothelial cells and vascular smooth muscle cells are discussed. Key considerations in the design of high throughput systems are presented and selected examples are discussed.

Efficient in vivo vascularization of tissue-engineering scaffolds

The success of tissue engineering depends on the rapid and efficient formation of a functional blood vasculature. Adult blood vessels comprise endothelial cells and perivascular mural cells that assemble into patent tubules ensheathed by a basement membrane during angiogenesis. Using individual vessel components, we characterized intra-scaffold microvessel self-assembly efficiency in a physiological in vivo tissue engineering implant context. Primary human microvascular endothelial and vascular smooth muscle cells were seeded at different ratios in poly-L-lactic acid (PLLA) scaffolds enriched with basement membrane proteins (Matrigel) and implanted subcutaneously into immunocompromised mice. Temporal intra-scaffold microvessel formation, anastomosis and perfusion were monitored by immunohistochemical, flow cytometric and in vivo multiphoton fluorescence microscopy analysis. Vascularization in the tissue-engineering context was strongly enhanced in implants seeded with a complete complement of blood vessel components: human microvascular endothelial and vascular smooth muscle cells in vivo assembled a patent microvasculature within Matrigel-enriched PLLA scaffolds that anastomosed with the host circulation during the first week of implantation. Multiphoton fluorescence angiographic analysis of the intra-scaffold microcirculation showed a uniform, branched microvascular network. 3D image reconstruction analysis of human pulmonary artery smooth muscle cell (hPASMC) distribution within vascularized implants was non-random and displayed a preferential perivascular localization. Hence, efficient microvessel self-assembly, anastomosis and establishment of a functional microvasculture in the native hypoxic in vivo tissue engineering context is promoted by providing a complete set of vascular components.