Aop: 43


Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [AO]”. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Disruption of VEGFR Signaling Leading to Developmental Defects

Short name
A short name should also be provided that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
Developmental Vascular Toxicity

Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.'  Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you.  Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help


List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help

USEPA - National Center for Computational Toxicology, Research Triangle Park NC, USA

              LEAD: Tom Knudsen -
                    Nancy Baker (Leidos) -
                    Richard Spencer (GDIT) -

Finnish Centre for Alternative Methods, University of Tampere, Tampere Finland

                    Tuula Heinonen - 

The Dow Chemical Company, Midland MI, USA

                    Rob Ellis-Hutchings - 

University of Aberdeen, Aberdeen, Scotland UK

                    Neil Vargesson –                     

National Toxicology Program/NICEATM-ICCVAM, Research Triangle Park NC, USA

                    Nicole Kleinstreuer –

Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate.  More help
Tom Knudsen   (email point of contact)


List user names of all  authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact.  More help
  • Tom Knudsen
  • Nancy Baker


The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help
Author status OECD status OECD project SAAOP status
Open for citation & comment EAGMST Under Review 1.6 Included in OECD Work Plan
This AOP was last modified on January 25, 2022 12:16
The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page Revision Date/Time
Inhibition, VegfR2 January 17, 2022 10:05
Reduction, Angiogenesis January 17, 2022 10:09
Impairment, Endothelial network January 18, 2022 13:21
Insufficiency, Vascular January 18, 2022 12:48
Increased, Developmental Defects January 18, 2022 13:12
Inhibition, VegfR2 leads to Reduction, Angiogenesis January 18, 2022 12:51
Reduction, Angiogenesis leads to Impairment, Endothelial network January 18, 2022 12:54
Impairment, Endothelial network leads to Insufficiency, Vascular January 18, 2022 13:05
Insufficiency, Vascular leads to Increased, Developmental Defects January 18, 2022 13:07
Vatalanib November 29, 2016 18:42
Sunitinib malate Sunitinib (INN) November 29, 2016 18:42


In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

BACKGROUND: The cardiovascular system is the first functional organ system to develop in the vertebrate embryo, reflecting its critical role during normal development and pregnancy. Elucidating an AOP for embryonic vascular disruption must consider the stepwise events underlying blood vessel patterning. Vascular development commences in the early embryo with in situ formation of nascent vessels from angioblasts, leading to a primary capillary plexus (vasculogenesis). After the onset of blood circulation, the primary vascular pattern is further expanded as new vessels sprout from pre-existing vessels (angiogenesis). Both processes, vasculogenesis and angiogenesis, are regulated by genetic signals and environmental factors dependent on anatomical region, physiological state, and developmental stage of the embryo. The developing vascular network is further shaped into a hierarchical system of arteries and veins, through progressive effects on blood vessel arborization, branching, and pruning (angioadaptation). These latter influences include hemodynamic forces, regional changes in blood flow, local metabolic demands and growth factor signals. Disruptions in embryonic vascular patterning-adaptation may result in adverse pregnancy outcomes, including birth defects, angiodysplasias and cardiovascular disease, intrauterine growth restriction or prenatal death. Some chemicals may act as potential vascular disrupting compounds (pVDCs) altering the expression, activity or function of molecular signals regulating blood vessel development and remodeling. Critical pathways involve receptor tyrosine kinases (e.g., growth factor-signaling), G-protein coupled receptors (e.g., chemokine signaling), and GPI-anchored receptors (e.g. uPAR system).

DESCRIPTION: This AOP focuses on the regulation and disruption of vasculogenesis-angiogenesis during embryonic development via disruption of the VEGF-signaling pathway. VEGFA binding to its cognate receptor (VEGFR2) triggers angiogenic sprouting, growth and fusion during early development, and in flow-sensing adaptation of vascular development during later development. VEGFR2 inhibition, the postulated molecular initiating event (MIE) for this AOP, may be invoked by effects on VEGFA production, mobility, or receptor binding, and by effects on VEGFR2 cellular expression, molecular function or post-receptor signal transduction pathways. Downstream key events (KE) include altered cell fate and behavior of 'endothelial tip cells' (exploratory behavior, cell migration) and endothelial 'stalk cells' (cell proliferation, apoptosis). KE relationships (KERs) leading to vascular insufficiency then involve local interactions with other cell types (stromal cells, macrophages), the extracellular matrix (ECM) and micro-physiology (hemodynamics, metabolism). Adverse outcomes (AO) would ultimately vary by anatomical region, organ system, gestational stage and state of the embryo, fetus or placenta when an MIE is invoked.

RELEVANCE and APPLICATION: Angiogenesis and vascular disruption is a broad concept. The intended use of this AOP in a regulatory context is the predictive toxicology of developmental hazards, especially for integrating data from high-throughput screening (HTS) assays into cell agent-based models for predicting dysmorphogenesis. As part of an integrated assessment of toxicity, this AOP can identify useful information for assessing adverse outcomes relevant to risk assessment and efficient use of resources for validation through predictive models linking developmental toxicity to vascular disruption. AOP-based computer models that simulate vascular development can usher-in new virtual screening techniques to predict what might happen to a developing embryo when exposed to chemicals across different dose-time-stage scenarios, including the range of effects and how cellular injury propagates across development.

Background (optional)

This optional subsection should be used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development. The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Functionalization of the ToxCast pVDC predictive signature

The ToxCast putative Vascular Disrupting Chemicals (pVDC) signature will be described here and parts will be incorporated into the relevant Key Events sections.

The sectors of the ToxPI are color-represented by features from ToxCast HTS assays indicated by the target of the assays, the characteristics as follows.

Vascular cell adhesion molecule 1 (VCAM1): the pVDC signature aggregates assays from the BioMAP Systems Predictive Toxicology panel [Houck et al., 2009, Kunkel et al., 2004] focusing here on chemical disruption of endothelial VCAM1 expression following stimulation by cytokines-growth factors. This assay endpoint is an in vitro surrogate for inflammatory cell recruitment per endothelial dysfunction and has been probed across five different cell systems: 4H (HUVECs stimulated with IL-4 + histamine); 3C (HUVECs stimulated with IL-1β + TNFα + IFNϒ); CASM3C (primary human coronary artery smooth muscle cells stimulated with IL-1β + TNFα + IFNϒ); LPS (HUVECs co-cultured with monocytes and stimulated with bacterial endotoxin); and hDFCGF (human dermal fibroblasts stimulated with IL-1β + TNFα + IFNϒ and EGF + bFGF + PDGF-BB)[Knudsen and Kleinstreuer, 2011, Kleinstreuer et al., 2014].

Angiogenic cytokines and chemokines: the pVDC signature aggregates features for LPS-induced TNFα protein expression (see BioMAP descriptor above), nuclear factor-kappa B (NFkB) mediated reporter gene activation (Attagene; cis- configuration), and caspase 8 enzymatic activity (NovaScreen; inhibition or activation). TNFα is a proinflammatory cytokine that can promote angiogenesis indirectly through NFkB-mediated expression of angiogenic growth factors, or inhibit angiogenesis by direct effects on endothelial proliferation and survival. The pVDC signature also aggregates features for signaling activity of the pro-angiogenic cytokines interleukin-1 alpha (IL1a, a macrophage-derived activator of TNFα) and interleukin 6 (IL6). These cytokines act through the G-protein coupled receptors (GPCRs) IL1R and IL6R, respectively. CXCL8 (chemokine (C-X-C motif) ligand 8), formerly known as interleukin 8 (IL8), is angiogenic through its cognate GPCRs (CXCR1, CXCR2). In contrast to CXCL8, the chemokines CXCL9 (alias MIG, monokine induced by IFNϒ) and CXCL10 (alias IP10, interferon-inducible cytokine IP-10) are considered anti-angiogenic through their cognate receptor, CXCR3.

Angiogenic growth factors: FGFs and VEGFs exert their effects on endothelial cell proliferation, migration, and differentiation via specific binding to receptor tyrosine kinases VEGFR and FGFR. The pVDC signature has features for liganding VEGFR1, VEGFR2, and VEGFR3 based on receptor kinase activity (RTK, inhibition or activation) from the NovaScreen biochemical profile [Sipes et al. 2013] and for down-regulation of VEGFR2 expression in the 4H BioMAP system (HUVECs stimulated with IL-4 + histamine, B). VEGFR1 is a non-signaling VEGF-A decoy receptor that can be cleaved from the cell surface; VEGFR2 is the most important VEGF-A receptor and a master switch for developmental angiogenesis; and VEGFR3 is a VEGF-C receptor up-regulated by Notch signals. The pVDC signature includes features for the basic helix-loop-helix transcription factors Aryl Hydrocarbon Receptor (AhR) and Hypoxia Inducible Factor-1 alpha (HIF1a) that are upstream regulators of VEGF gene expression during ischemia or hypoxia. HIF1a and AhR are measured in reporter assays (Attagene). In addition to HIF1a and AhR, the pVDC signature has features for the estrogen receptor alpha (ERa), also a trans-activator of VEGF expression. This included human ERa binding activity (NovaScreen), ERa reporter trans-activation (Attagene) and ERE (estrogen responsive element) reporter cis-activation (Attagene).

Angiogenic sprouting: the ephrins (EFNA1 and EFNB2 in particular) couple VEGF signaling to angiogenic sprouting during early development of the embryonic vasculature (vasculogenesis, angiogenesis). The ToxCast pVDC signature included features for EPH-receptor tyrosine kinase biochemical activity (increased or decreased) for receptors EPHA1, EPHA2, EPHB1 and EPHB2 via their cognate cell membrane-anchored ligands (EFNAs). In contrast to the ephrin system, a number of chemicals had activity on diverse assays for urokinase-type plasminogen activator (uPA). That system, consisting of uPA (4 features) and its GPI-anchored receptor, uPAR (8 features) - both assayed in the BioMAP System [Kleinstreuer et al. 2014], functions in VEGFR2-induced changes to focal adhesion and extracellular matrix (ECM) degradation at the leading edge of endothelial cells during angiogenic sprouting. Binding of uPA to uPAR results in serine-protease conversion of plasminogen to plasmin that initiates a proteolytic cascade leading to degradation of the basement membrane and angiogenic sprouting. The uPA proteolytic cascade is suppressed by the serine protease inhibitor, endothelial plasminogen activator inhibitor type 1 (PAI1). The PAI1/uPA/uPAR assays report chemical effects on the system (up or down) across diverse cellular platforms: 4H, 3C, CASM3C, and hDFCGF noted above; BE3C (human bronchial epithelial cells stimulated with IL-1β + TNFα + IFNϒ); and KF3T (human keratinocytes + fibroblasts stimulated with IL-1β + TNFα + IFNϒ + TGF-β). The pVDC signature has features for thrombomodulin (THBD) and the thromboxane A2 (TBXA2) receptor that participate in the regulation of endothelial migration during angiogenic sprouting. THBD is a type I transmembrane glycoprotein that mediates regulator of uPA/uPAR and TBXA2 is an angiogenic eicosanoid generated by endothelial cyclooxygenase-2 (COX-2) following VEGF- or bFGF stimulation. THBD protein expression was monitored in the 3C and CASM3C BioMAP systems (up, down) and TBXA2 was assayed for ligand binding in the NovaScreen platform.

Endothelial cell migration and proliferation: the pVDC signature includes assays for human primary vascular cultures (endothelial and vascular smooth muscle cells). Assays for nuclear localization of beta-catenin (CTNB) are based on the principle that nuclear translocation activates pathways important for endothelial cell migration, proliferation and survival during capillary network formation in HUVEC cells [Muller et al. 2002; Masckauchan et al. 2005].

Vascular stabilization: The signature has features for transforming growth factor-beta 1 (TGF-b), which regulates vascular morphogenesis and integrity, and for Tie2 - a receptor tyrosine kinase activated by the angiopoietins (ANG1, ANG2) that function stabilize nascent vasculature. The pVDC signature has features for the anti-angiogenic phosphatases PTEN (phosphatase and tensin homolog), PTPN11 (tyrosine-protein phosphatase non-receptor type 11) and PTPN12, and endothelial-specific receptor tyrosine protein phosphatase beta (PTPRB). Matrix metalloproteinases (MMPs) 1/2/9 aggregate features on biochemical activity and cellular function of zinc-dependent endopeptidases MMP1, MMP2 and MMP9 that facilitate angiogenesis through ECM degradation by activated endothelial cells.

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help


Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help
Key Events (KE)
This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page.  More help
Adverse Outcomes (AO)
An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP.  More help
Sequence Type Event ID Title Short name
1 MIE 305 Inhibition, VegfR2 Inhibition, VegfR2
2 KE 28 Reduction, Angiogenesis Reduction, Angiogenesis
3 KE 110 Impairment, Endothelial network Impairment, Endothelial network
4 KE 298 Insufficiency, Vascular Insufficiency, Vascular
5 AO 1001 Increased, Developmental Defects Increased, Developmental Defects

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button.  Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the user will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.).  Click ‘Create [adjacent/non-adjacent] relationship.’  The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and WoE summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help


The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help
Life stage Evidence
Conception to < Fetal High
Pregnancy High

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
human Homo sapiens Moderate NCBI
mouse Mus musculus High NCBI
rats Rattus norvegicus Low NCBI
zebrafish Danio rerio High NCBI

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help

The cardiovascular system is the first organ system to function in the vertebrate embryo, reflecting its critical role during organogenesis [Chan et al. 2002; Jin et al. 2005; Walls et al. 2008]. Blood vessel development commences in the early (sexually undifferentiated) embryo with de novo assembly of angioblasts into a primary capillary plexus (vasculogenesis). With the onset of blood circulation, the primary vascular pattern is further expanded as new vessels sprout from pre-existing vessels (angiogenesis). Both processes, vasculogenesis and angiogenesis, are developmentally regulated by genetic signals and environmental factors dependent on anatomical region, physiological state, and gestational age of the embryo-fetus [Shalaby et al. 1995; Patan, 2000; Jin et al. 2005; Knudsen and Kleinstreuer, 2011; Eberlein et al. 2021]. Disruption of embryonic vascular development is a potential framework for adverse outcome pathways (AOPs) in developmental toxicity [Knudsen and Kleinstreuer, 2011; Kleinstreuer et al. 2013; Saili et al. 2019; Zurlinden et al. 2020]. Developmental angiogenesis is supported by evidence in genetic models of abnormal vascularization leading to severe developmental phenotypes [Fong et al. 1995; Shalaby et al. 1995; Carmeliet et al. 1996; Maltepe et al. 1997; Abbott and Buckalew, 2000; Chan et al. 2002; Coultas et al. 2005; van den Akker et al. 2007; Eberlein et al. 2021]. This may include cell signals and responses driving formation of the primitive capillary network in the early embryo and extraembryonic membranes (vasculogenesis), the subsequent expansion and patterning of the embryonic and placental vasculature (angiogenesis), and its further stabilization, specialization, and remodeling during growth, organogenesis and differentiation. Additional evidence comes from dysmorphogenesis induced with known anti-angiogenic compounds across multiple vertebrate species (e.g., zebrafish, frog, chick, mouse, rat) [Therapontos et al. 2009; Jang et al. 2009; Rutland et al. 2009; Tal et al. 2014; Vargesson, 2015; Beedie et al. 2016; Ellis-Hutchings et al. 2017; Kotini et al. 2020] and human studies of malformations correlated with genetic and/or environmental factors that target vascular development [Husain et al. 2008; Gold et al. 2011; Vargesson and Hootnick, 2017]. An analysis of pharma compounds to which women of reproductive age were exposed identified vascular disruption as one of six potential mechanisms of teratogenesis [van Gelder et al. 2010]. This AOP is focused on disruption of ‘developmental angiogenesis’ from the perspective of dysmorphogenesis leading to severe developmental defects. Although uterine-decidual vascularization is critically important for healthy pregnancy outcomes, the emphasis here is the direct role on anatomical development of the embryo proper.  

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence.The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications.  More help

The Vascular Endothelial Growth Factor (VEGF) pathway is a critical regulatory system for assembly of embryonic blood vessels [Fong et al. 1995; Shalaby et al. 1995; Carmeliet et al. 1996; Ferrara, et al. 1996; Argraves et al. 2002; Hogan et al. 2004; Roberts et al. 2004; Chung and Ferrara, 2011; Shibuya, 2013; Chapell et al. 2016; Jin et al. 2017; Queisser et al. 2021]. The VEGF growth factors belong to the platelet-derived growth factor supergene family. VEGF-A, the major regulator for angiogenesis, binds receptor tyrosine kinases VEGFR-1 (Flt-1) and VEGFR2 (KDR/Flk-1) to regulate endothelial cell proliferation, survival, migration, tubular morphogenesis, and sprouting [Hogan et al. 2004; Douglas et al. 2009; Shibuya, 2013]. This pro-angiogenic effect is locally fashioned as VEGF gradients where the soluble VEGFR1 (sFlt-1) is released from the cell surface as an endogenous VEGF inhibitor that sets up VEGF-A corridors in the developing embryo [Roberts et al. 2004; Chappell et al. 2016]. Genetic studies have shown that perturbing the VEGF signaling system can invoke varying degrees of adverse consequences, ranging from congenital angiodysplasia to fetal malformations and embryolethality [Fong et al. 1995; Ferrara et al. 1996; Eshkar-Oren et al. 2015; Jin et al. 2017]. Filopodial sprouting of the endothelial cell tip (EC-tip) is the critical VEGFR2 responsive cell type specifically in this AOP [Belair et al. 2016a and 2016b]; however, other relevant cell types include: angioblasts (AB) as direct precursors to primary endothelial cells; endothelial ‘stalk’ cells (EC-stalk) proliferate in the wake of an angiogenic sprout; macrophage/microglial cells (MCs) release cytokines, chemokines, and growth factors; and stromal cells (SCs) are recruited to the nascent vascular wall for vessel stabilization. As such, the VEGF gradient/response system influences a multicellular dimension determined by cellular patterns of VEGF expression and processing (eg, MCs, SCs) and biochemical corridors set up by the extracellular matrix and the VEGFR1 decoy receptor (eg, EC-stalk). Evidence supporting an AOP for chemical disruption is available for thalidomide, estrogens, endothelins, dioxin, retinoids, cigarette smoke, and metals among other compounds [Kleinstreuer et al. 2011; Knudsen and Kleinstreuer, 2011; Shirinifard et al. 2013; Tal et al. 2014 and 2017; McCollum et al. 2017; Toimela et al. 2017; Mahony et al. 2018; Saili et al. 2019; Zurlinden et al. 2020]. Although not all compounds with developmental toxicity show an in vitro vascular bioactivity signature, many 'putative vascular disruptor compounds' (pVDCs) invoke adverse developmental consequences [Kleinstreuer et al. 2011 and 2013]. The molecular and cellular biology of human vascular development, stabilization and remodeling is amenable to in vitro assays with human cells [Bishop et al. 1999; Sarkanen et al. 2010; Kleinstreuer et al. 2014; Belair et al. 2016a and 2016b; Nguyen et al. 2017; Toimela et al. 2017; Pauty et al. 2018; van Duinen et al. 2019a and 2019b; Zurlinden et al. 2020], pluripotent stem cells induced to endothelial differentiation [Belair et al. 2015; Sinha and Santoro, 2018; Li et al. 2018; Galaris et al. 2021], and endothelial-specific reporter zebrafish [Tran et al. 2007; Shirinifard et al. 2013; Tal et al. 2014 and 2017; Beedie et al. 2017; McCollum et al. 2017]. An integrated portfolio of assays is thus available to cover many aspects of the angiogenic cycle and its ramifications during neurovascular development [Bautch and James, 2009; Eichman and Thomas, 2013; Saili et al. 2017; Uwamori et al. 2017; van Duinen et al. 2019; and Zurlinden et al. 2020]. Evidence is also available to support the essentiality of this AOP outside the embryo proper, such as uterine angiogenesis [Douglas et al. 2009; Araujo et al. 2021], placentation [Abbott and Bucklew, 2000; Chen and Zheng, 2014], and human pregnancies complicated by preeclampsia and small-for-gestational age infants [Andraweera et al. 2012].

Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together.  Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook.  More help

Weight of evidence for the MIE and AO are strong; the intermediate KEs have in some cases strong evidence but in other cases weaker evidence, due to the lack of quantitative information. the KERs are biologically plausible. Several manuscripts have been published recently that bolster weight of evidence [Belair et al. 2016; Nguyen et al. 2017; Tal et al. 2017; McCollum et al. 2017; Ellis-Hutchings et al. 2017; Saili et al. 2019; Zurlinden et al. 2020].

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help



Considerations for Potential Applications of the AOP (optional)

At their discretion, the developer may include in this section discussion of the potential applications of an AOP to support regulatory decision-making. This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale.To edit the “Considerations for Potential Applications of the AOP” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Considerations for Potential Applications of the AOP” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page or 'Update and continue' to continue editing AOP text sections.  The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help


List the bibliographic references to original papers, books or other documents used to support the AOP. More help


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Andraweera, P. H., G. A. Dekker, S. D. Thompson and C. T. Roberts (2012). Single-nucleotide polymorphisms in the KDR gene in pregnancies complicated by gestational hypertensive disorders and small-for-gestational-age infants. Reproductive sciences (Thousand Oaks, Calif.) 19(5): 547-5545. PMID: 22344734.

Araujo Júnior, E., A. C. Zamarian, A. C. Caetano, et al. (2021). Physiopathology of late-onset fetal growth restriction. Minerva obstetrics and gynecology 73(4): 392-4084. PMID: 33876907.

Argraves, W. S., A. C. Larue, P. A. Fleming and C. J. Drake (2002). VEGF signaling is required for the assembly but not the maintenance of embryonic blood vessels. Developmental dynamics : an official publication of the American Association of Anatomists 225(3): 298-3043. PMID: 12412012.

Bautch, V. L. and J. M. James (2009). Neurovascular development: The beginning of a beautiful friendship. Cell adhesion & migration 3(2): 199-2042. PMID: 19363295.

Beedie, S. L., A. J. Diamond, L. R. Fraga, et al. (2017). Vertebrate embryos as tools for anti-angiogenic drug screening and function. Reproductive toxicology (Elmsford, N.Y.) 70: 49-59. PMID: 27888069.

Beedie, S. L., C. Mahony, H. M. Walker, et al. (2016). Shared mechanism of teratogenicity of anti-angiogenic drugs identified in the chicken embryo model. Scientific reports 6: 30038-30038. PMID: 27443489.

Belair, D. G., M. J. Miller, S. Wang, et al. (2016). Differential regulation of angiogenesis using degradable VEGF-binding microspheres. Biomaterials 93: 27-37. PMID: 27061268.

Belair, D. G., M. P. Schwartz, T. Knudsen and W. L. Murphy (2016). Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays. Acta biomaterialia 39: 44554-44554. PMID: 27181878.

Belair, D. G., J. A. Whisler, J. Valdez, et al. (2015). Human vascular tissue models formed from human induced pluripotent stem cell derived endothelial cells. Stem cell reviews and reports 11(3): 511-5253. PMID: 25190668.

Bishop, E. T., G. T. Bell, S. Bloor, et al. (1999). An in vitro model of angiogenesis: basic features. Angiogenesis 3(4): 335-3444. PMID: 14517413.

Carmeliet, P., V. Ferreira, G. Breier, et al. (1996). Abnormal blood vessel development and lethality in embryos lacking a single VEGF allele. Nature 380(6573): 435-4396573. PMID: 8602241.

Chan, J., P. E. Bayliss, J. M. Wood and T. M. Roberts (2002). Dissection of angiogenic signaling in zebrafish using a chemical genetic approach. Cancer cell 1(3): 257-2673. PMID: 12086862.

Chappell, J. C., J. G. Cluceru, J. E. Nesmith, et al. (2016). Flt-1 (VEGFR-1) coordinates discrete stages of blood vessel formation. Cardiovascular research 111(1): 84-931. PMID: 27142980.

Chen, D. B. and J. Zheng (2014). Regulation of placental angiogenesis. Microcirculation (New York, N.Y. : 1994) 21(1): 15-251. PMID: 23981199.

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Douglas, N. C., H. Tang, R. Gomez, et al. (2009). Vascular endothelial growth factor receptor 2 (VEGFR-2) functions to promote uterine decidual angiogenesis during early pregnancy in the mouse. Endocrinology 150(8): 3845-38548. PMID: 19406950.

Eberlein, J., L. Herdt, J. Malchow, et al. (2021). Molecular and Cellular Mechanisms of Vascular Development in Zebrafish. Life (Basel, Switzerland) 11(10)10. PMID: 34685459.

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Ellis-Hutchings, R. G., R. S. Settivari, A. T. McCoy, et al. (2017). Embryonic vascular disruption adverse outcomes: Linking high throughput signaling signatures with functional consequences. Reproductive toxicology (Elmsford, N.Y.) 70: 82-96. PMID: 28527947.

Eshkar-Oren, I., S. Krief, N. Ferrara, et al. (2015). Vascular patterning regulates interdigital cell death by a ROS-mediated mechanism. Development (Cambridge, England) 142(4): 672-6804. PMID: 25617432.

Ferrara, N., K. Carver-Moore, H. Chen, et al. (1996). Heterozygous embryonic lethality induced by targeted inactivation of the VEGF gene. Nature 380(6573): 439-4426573. PMID: 8602242.

Fong, G. H., J. Rossant, M. Gertsenstein and M. L. Breitman (1995). Role of the Flt-1 receptor tyrosine kinase in regulating the assembly of vascular endothelium. Nature 376(6535): 66-706535. PMID: 7596436.

Galaris, G., J. H. Thalgott, E. Teston and F. P. G. Lebrin (2021). In vitro Three-Dimensional Sprouting Assay of Angiogenesis using Mouse Embryonic Stem Cells for Vascular Disease Modeling and Drug Testing. Journal of visualized experiments : JoVE(171)171. PMID: 34057453.

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Jin, D., D. Zhu, Y. Fang, et al. (2017). Vegfa signaling regulates diverse artery/vein formation in vertebrate vasculatures. Journal of genetics and genomics = Yi chuan xue bao 44(10): 483-49210. PMID: 29037991.

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Kleinstreuer, N. C., J. Yang, E. L. Berg, et al. (2014). Phenotypic screening of the ToxCast chemical library to classify toxic and therapeutic mechanisms. Nature biotechnology 32(6): 583-5916. PMID: 24837663.

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