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Relationship: 2144


The title of the KER should clearly define the two KEs being considered and the sequential relationship between them (i.e., which is upstream and which is downstream). Consequently all KER titles take the form “upstream KE leads to downstream KE”.  More help

Inhibition, Aromatase leads to Reduction, E2 Synthesis by the undifferentiated gonad

Upstream event
Upstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help
Downstream event
Downstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

This table is automatically generated upon addition of a KER to an AOP. All of the AOPs that are linked to this KER will automatically be listed in this subsection. Clicking on the name of the AOP in the table will bring you to the individual page for that AOP. More help
AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Aromatase inhibition leads to male-biased sex ratio via impacts on gonad differentiation adjacent High Kelvin Santana Rodriguez (send email) Under Development: Contributions and Comments Welcome

Taxonomic Applicability

Select one or more structured terms that help to define the biological applicability domain of the KER. In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. Authors can indicate the relevant taxa for this KER in this subsection. The process is similar to what is described for KEs (see pages 30-31 and 37-38 of User Handbook) More help
Term Scientific Term Evidence Link
Oreochromis niloticus Oreochromis niloticus Low NCBI
zebrafish Danio rerio Moderate NCBI

Sex Applicability

Authors can indicate the relevant sex for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of the User Handbook). More help
Sex Evidence
Unspecific Moderate

Life Stage Applicability

Authors can indicate the relevant life stage for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of User Handbook). More help
Term Evidence
before or during gonadal sex differentiation High

Key Event Relationship Description

Provide a brief, descriptive summation of the KER. While the title itself is fairly descriptive, this section can provide details that aren’t inherent in the description of the KEs themselves (see page 39 of the User Handbook). This description section can be viewed as providing the increased specificity in the nature of upstream perturbation (KEupstream) that leads to a particular downstream perturbation (KEdownstream), while allowing the KE descriptions to remain generalised so they can be linked to different AOPs. The description is also intended to provide a concise overview for readers who may want a brief summation, without needing to read through the detailed support for the relationship (covered below). Careful attention should be taken to avoid reference to other KEs that are not part of this KER, other KERs or other AOPs. This will ensure that the KER is modular and can be used by other AOPs. More help

Aromatase (cyp19; EC is a cytochrome P450 enzyme that is rate limiting for estrogen synthesis (Simpson et al. 1994; Miller 1988; Payne and Hale, 2004).  The expression and activity of aromatase in the bipotential gonad, and subsequent autocrine and/or paracrine signaling mediated by the action of estrogens on the estrogen receptor (or lack thereof), are thought to be key regulators of sex determination and gonadal differentation in vertebrates (Angelopoulou et al. 2012; Nakamura 2010). 

Evidence Supporting this KER

Assembly and description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for overall assessment of the AOP (see pages 49-56 of the User Handbook). To do this, biological plausibility, empirical support, and the current quantitative understanding of the KER are evaluated with regard to the predictive relationships/associations between defined pairs of KEs as a basis for considering WoE (page 55 of User Handbook). In addition, uncertainties and inconsistencies are considered. More help

   See below

Biological Plausibility
Define, in free text, the biological rationale for a connection between KEupstream and KEdownstream. What are the structural or functional relationships between the KEs? For example, there is a functional relationship between an enzyme’s activity and the product of a reaction it catalyses. Supporting references should be included. However, it is recognised that there may be cases where the biological relationship between two KEs is very well established, to the extent that it is widely accepted and consistently supported by so much literature that it is unnecessary and impractical to cite the relevant primary literature. Citation of review articles or other secondary sources, like text books, may be reasonable in such cases. The primary intent is to provide scientifically credible support for the structural and/or functional relationship between the pair of KEs if one is known. The description of biological plausibility can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured (see page 40 of the User Handbook for further information).   More help

Literature search to date has provided relatively little direct evidence of 17β-estradiol (E2) production in the bipotential gonad or decreases in E2 production upon aromatase inhibition in the same. However, given the well-established role of aromatase as a rate limiting enzyme for E2 production (Simpson et al. 1994; Payne and Hale, 2004) and the close association between aromatase expression and activity and gonadal sex determination/differentiation (Angelopoulou et al. 2012; Nakamura 2010), it is highly plausible that local estrogen production in the bipotential gonad plays a significant role in gonadal differentiation.  However, particularly for species with genetic sex determination, it is one just one of multiple determinants that ultimately influence differentiation of the gonad (Angelopoulou et al. 2012, Sarre et al. 2004).

Uncertainties and Inconsistencies
In addition to outlining the evidence supporting a particular linkage, it is also important to identify inconsistencies or uncertainties in the relationship. Additionally, while there are expected patterns of concordance that support a causal linkage between the KEs in the pair, it is also helpful to identify experimental details that may explain apparent deviations from the expected patterns of concordance. Identification of uncertainties and inconsistencies contribute to evaluation of the overall WoE supporting the AOPs that contain a given KER and to the identification of research gaps that warrant investigation (seep pages 41-42 of the User Handbook).Given that AOPs are intended to support regulatory applications, AOP developers should focus on those inconsistencies or gaps that would have a direct bearing or impact on the confidence in the KER and its use as a basis for inference or extrapolation in a regulatory setting. Uncertainties that may be of academic interest but would have little impact on regulatory application don’t need to be described. In general, this section details evidence that may raise questions regarding the overall validity and predictive utility of the KER (including consideration of both biological plausibility and empirical support). It also contributes along with several other elements to the overall evaluation of the WoE for the KER (see Section 4 of the User Handbook).  More help

Even in some of the most studied classes with regard to the role of aromatase in gonad differentiation (e.g., fish, reptiles) there is considerable variability in the determinants that influence aromatase expression and work with estrogen signaling to influence differentiation. These are not easily predicted from phylogenetic relationships (Angelopoulou et al. 2012, Sarre et al. 2004). Thus susceptibility and relative sensitivities may vary considerably between species.

Response-response Relationship
This subsection should be used to define sources of data that define the response-response relationships between the KEs. In particular, information regarding the general form of the relationship (e.g., linear, exponential, sigmoidal, threshold, etc.) should be captured if possible. If there are specific mathematical functions or computational models relevant to the KER in question that have been defined, those should also be cited and/or described where possible, along with information concerning the approximate range of certainty with which the state of the KEdownstream can be predicted based on the measured state of the KEupstream (i.e., can it be predicted within a factor of two, or within three orders of magnitude?). For example, a regression equation may reasonably describe the response-response relationship between the two KERs, but that relationship may have only been validated/tested in a single species under steady state exposure conditions. Those types of details would be useful to capture.  More help

To date, none of the studies reviewed have offered insight into the quantitative relationship between aromatase inhibition and estradiol synthesis by the undifferentiated, bipotential, gonad.

This sub-section should be used to provide information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). This can be useful information both in terms of modelling the KER, as well as for analyzing the critical or dominant paths through an AOP network (e.g., identification of an AO that could kill an organism in a matter of hours will generally be of higher priority than other potential AOs that take weeks or months to develop). Identification of time-scale can also aid the assessment of temporal concordance. For example, for a KER that operates on a time-scale of days, measurement of both KEs after just hours of exposure in a short-term experiment could lead to incorrect conclusions regarding dose-response or temporal concordance if the time-scale of the upstream to downstream transition was not considered. More help
  • Based on studies in adults, post-differentiation, effects on estradiol production can be detected within a few hours of administering an aromatase inhibitor in vivo (Schroeder et al. 2017; Skolness et al. 2011).
  • Based on in vitro studies, significant reductions in aromatase activity and/or E2 synthesis can be detected in 90 min or less (Villeneuve et al. 2006).
Known modulating factors
This sub-section presents information regarding modulating factors/variables known to alter the shape of the response-response function that describes the quantitative relationship between the two KEs (for example, an iodine deficient diet causes a significant increase in the slope of the relationship; a particular genotype doubles the sensitivity of KEdownstream to changes in KEupstream). Information on these known modulating factors should be listed in this subsection, along with relevant information regarding the manner in which the modulating factor can be expected to alter the relationship (if known). Note, this section should focus on those modulating factors for which solid evidence supported by relevant data and literature is available. It should NOT list all possible/plausible modulating factors. In this regard, it is useful to bear in mind that many risk assessments conducted through conventional apical guideline testing-based approaches generally consider few if any modulating factors. More help

Aromatase expression during gonadal differentiation is subject to both environmental and genetic controls to various degrees depending on species (Angelopoulou et al. 2012, Sarre et al. 2004). However, generalizable relationships that account for effects of specific parameters in the response-response relationships underlying this KER are currently unknown.

Known Feedforward/Feedback loops influencing this KER
This subsection should define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits? In some cases where feedback processes are measurable and causally linked to the outcome, they should be represented as KEs. However, in most cases these features are expected to predominantly influence the shape of the response-response, time-course, behaviours between selected KEs. For example, if a feedback loop acts as compensatory mechanism that aims to restore homeostasis following initial perturbation of a KE, the feedback loop will directly shape the response-response relationship between the KERs. Given interest in formally identifying these positive or negative feedback, it is recommended that a graphical annotation (page 44) indicating a positive or negative feedback loop is involved in a particular upstream to downstream KE transition (KER) be added to the graphical representation, and that details be provided in this subsection of the KER description (see pages 44-45 of the User Handbook).  More help

Aromatase expression and E2 synthesis in adults is known to be under feedback regulation via the brain-pituitary-gonadal axis (Villeneuve et al. 2009; Ankley et al. 2009; Villeneuve et al. 2013; Yu et al. 2020; Norris 1997; Miller 1988; Callard et al. 2001).

It is unclear whether these same feedback mechanisms are active during gonadal differentiation.

Domain of Applicability

As for the KEs, there is also a free-text section of the KER description that the developer can use to explain his/her rationale for the structured terms selected with regard to taxonomic, life stage, or sex applicability, or provide a more generalizable or nuanced description of the applicability domain than may be feasible using standardized terms. More help

Life Stage

The life stage applicable to this key event relationship is developing embryos and juveniles prior to or during the gonadal differentiation stage. This key event relationship is not applicable to sexually differentiated adults. 


Because this KER is relevant prior to or during sexual differentiation, the relationship is relevant to animals with an undetermined (non-specific) sex.

Taxonomic Applicability 

Phylogenetic analysis among mammalian, amphibian, reptile, bird, and fish has shown that aromatase is well conserved among all vertebrates (Wilson JY et al., 2005). Additionally, CYP19 was detected in the amphioxus suggesting that it has possible origin in primitive chordates. However this relationship probably only applicable to vertebrates. 

As noted above, even in some of the most studied classes with regard to the role of aromatase in gonad differentiation (e.g., fish, reptiles) there is considerable variability in the determinants that influence aromatase expression and work with estrogen signaling to influence differentiation. These are not easily predicted from phylogenetic relationships (Angelopoulou et al. 2012, Sarre et al. 2004). Thus susceptibility and relative sensitivities may vary considerably between species.


List of the literature that was cited for this KER description using the appropriate format. Ideally, the list of references should conform, to the extent possible, with the OECD Style Guide (OECD, 2015). More help

Angelopoulou R, Lavranos G, Manolakou P. Sex determination strategies in 2012: towards a common regulatory model? Reprod Biol Endocrinol. 2012 Feb 22;10:13. doi: 10.1186/1477-7827-10-13. PMID: 22357269; PMCID: PMC3311596.

Ankley GT, Bencic DC, Cavallin JE, Jensen KM, Kahl MD, Makynen EA, Martinovic D, Mueller ND, Wehmas LC, Villeneuve DL. Dynamic nature of alterations in the endocrine system of fathead minnows exposed to the fungicide prochloraz. Toxicol Sci. 2009 Dec;112(2):344-53. doi: 10.1093/toxsci/kfp227. Epub 2009 Sep 18. PMID: 19767443.

Callard GV, Tchoudakova AV, Kishida M, Wood E. Differential tissue distribution, developmental programming, estrogen regulation and promoter characteristics of cyp19 genes in teleost fish. J Steroid Biochem Mol Biol. 2001 Dec;79(1-5):305-14. doi: 10.1016/s0960-0760(01)00147-9. PMID: 11850237.

Miller WL. Molecular biology of steroid hormone synthesis. Endocr Rev. 1988 Aug;9(3):295-318. doi: 10.1210/edrv-9-3-295. PMID: 3061784.

Nakamura M. The mechanism of sex determination in vertebrates-are sex steroids the key-factor? J Exp Zool A Ecol Genet Physiol. 2010 Aug 1;313(7):381-98. doi: 10.1002/jez.616. PMID: 20623803.

Norris, D. O. Vertebrate Endocrinology, 3rd ed.; Academic Press: San Diego, CA, 1997.

Payne AH, Hales DB. Overview of steroidogenic enzymes in the pathway from cholesterol to active steroid hormones. Endocr Rev. 2004 Dec;25(6):947-70. doi: 10.1210/er.2003-0030. PMID: 15583024.

Sarre SD, Georges A, Quinn A. The ends of a continuum: genetic and temperature-dependent sex determination in reptiles. Bioessays. 2004 Jun;26(6):639-45. doi: 10.1002/bies.20050. PMID: 15170861.

Schroeder AL, Ankley GT, Habib T, Garcia-Reyero N, Escalon BL, Jensen KM, Kahl MD, Durhan EJ, Makynen EA, Cavallin JE, Martinovic-Weigelt D, Perkins EJ, Villeneuve DL. Rapid effects of the aromatase inhibitor fadrozole on steroid production and gene expression in the ovary of female fathead minnows (Pimephales promelas). Gen Comp Endocrinol. 2017 Oct 1;252:79-87. doi: 10.1016/j.ygcen.2017.07.022. Epub 2017 Jul 21. PMID: 28736226; PMCID: PMC6010346.

Skolness SY, Durhan EJ, Garcia-Reyero N, Jensen KM, Kahl MD, Makynen EA, Martinovic-Weigelt D, Perkins E, Villeneuve DL, Ankley GT. Effects of a short-term exposure to the fungicide prochloraz on endocrine function and gene expression in female fathead minnows (Pimephales promelas). Aquat Toxicol. 2011 Jun;103(3-4):170-8. doi: 10.1016/j.aquatox.2011.02.016. Epub 2011 Mar 2. PMID: 21470553.

Simpson ER, Mahendroo MS, Means GD, Kilgore MW, Hinshelwood MM, Graham-Lorence S, Amarneh B, Ito Y, Fisher CR, Michael MD, et al. Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocr Rev. 1994 Jun;15(3):342-55. doi: 10.1210/edrv-15-3-342. PMID: 8076586.

Villeneuve DL, Breen M, Bencic DC, Cavallin JE, Jensen KM, Makynen EA, Thomas LM, Wehmas LC, Conolly RB, Ankley GT. Developing predictive approaches to characterize adaptive responses of the reproductive endocrine axis to aromatase inhibition: I. Data generation in a small fish model. Toxicol Sci. 2013 Jun;133(2):225-33. doi: 10.1093/toxsci/kft068. Epub 2013 Mar 14. PMID: 23492810.

Villeneuve DL, Mueller ND, Martinović D, Makynen EA, Kahl MD, Jensen KM, Durhan EJ, Cavallin JE, Bencic D, Ankley GT. Direct effects, compensation, and recovery in female fathead minnows exposed to a model aromatase inhibitor. Environ Health Perspect. 2009 Apr;117(4):624-31. doi: 10.1289/ehp.11891. Epub 2008 Dec 12. PMID: 19440503; PMCID: PMC2679608.

Villeneuve DL, Knoebl I, Kahl MD, Jensen KM, Hammermeister DE, Greene KJ, Blake LS, Ankley GT. Relationship between brain and ovary aromatase activity and isoform-specific aromatase mRNA expression in the fathead minnow (Pimephales promelas). Aquat Toxicol. 2006 Mar 10;76(3-4):353-68. doi: 10.1016/j.aquatox.2005.10.016. Epub 2005 Dec 2. PMID: 16330110.

Wilson JY, McArthur AG, Stegeman JJ. Characterization of a cetacean aromatase (CYP19) and the phylogeny and functional conservation of vertebrate aromatase. Gen Comp Endocrinol. 2005 Jan 1;140(1):74-83. doi: 10.1016/j.ygcen.2004.10.004. PMID: 15596073.

Yin Y, Tang H, Liu Y, Chen Y, Li G, Liu X, Lin H. Targeted Disruption of Aromatase Reveals Dual Functions of cyp19a1a During Sex Differentiation in Zebrafish. Endocrinology. 2017 Sep 1;158(9):3030-3041. doi: 10.1210/en.2016-1865. PMID: 28575219.

Yu Q, Peng C, Ye Z, Tang Z, Li S, Xiao L, Liu S, Yang Y, Zhao M, Zhang Y, Lin H. An estradiol-17β/miRNA-26a/cyp19a1a regulatory feedback loop in the protogynous hermaphroditic fish, Epinephelus coioides. Mol Cell Endocrinol. 2020 Mar 15;504:110689. doi: 10.1016/j.mce.2019.110689. Epub 2019 Dec 28. PMID: 31891771.