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Event: 1101

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Altered, Amphibian metamorphosis

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Altered, Amphibian metamorphosis

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
metamorphosis delayed
metamorphosis increased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
TPO inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
NIS inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
IYD inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
DIO1 inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
DIO2 inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
DIO3 inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
Pendrin inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
DUOX inhib alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
Hepatic nuclear receptor activation alters metamorphosis AdverseOutcome Jonathan Haselman (send email) Under Development: Contributions and Comments Welcome
TH displacement from serum TTR leading to altered amphibian metamorphosis AdverseOutcome Jonathan Haselman (send email) Under development: Not open for comment. Do not cite
TH displacement from serum TBG leading to altered amphibian metamorphosis AdverseOutcome Jonathan Haselman (send email) Under development: Not open for comment. Do not cite


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. 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
African clawed frog Xenopus laevis High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
Development High

Sex Applicability

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

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

Vertebrate metamorphosis is a biological transformation process that transitions an organism from one life stage to another; it is defined by growth of new tissues, programmed death of other tissues and physiological transformation of yet other tissues (Laudet, 2011; Brown and Cai, 2007). In the case of most amphibians, metamorphosis mediates the transition from aquatic to terrestrial life, while in bony and jawless fish, metamorphosis mediates transitions between life stages that offer various advantages for survival and reproduction. In vertebrates, metamorphosis is orchestrated by the hypothalamus-pituitary-thyroid (HPT) axis involving complex timing of gene expression/repression within various tissues, whereas in some cases across taxonomic classes, metamorphosis has been shown to be controlled very differently by the HPT axis.

Thyroid hormone-mediated amphibian metamorphosis can be characterized by three phases during larval development: (1) pre-metamorphosis, (2) pro-metamorphosis and (3) metamorphic climax. All three of these phases coincide with activity states of the HPT axis. Pre-metamorphosis is characterized by a fully aquatic organism with low-level function of the thyroid gland and very low circulating levels of thyroid hormone. Pro-metamorphosis is characterized by the onset of full thyroid axis function and the initiation of rising levels of thyroid hormone in the plasma, with consequential changes in anatomy and physiology defining the transition from aquatic to terrestrial life. Metamorphic climax occurs when circulating thyroid hormone levels peak, which subsequently decrease to levels maintained homeostatically as adults. This climax period also represents the time at which all anatomical and physiological changes induced by thyroid hormone have either been initiated or are already completed. Detailed descriptions of these processes are reviewed by Brown and Cai (2007).

Altered metamorphosis occurs when these thyroid hormone-mediated processes are perturbed, primarily during pro-metamorphosis and metamorphic climax. These perturbations can lead to either, delayed/arrested development, accelerated development or asynchronous development depending on the xenobiotic mode of action or MIE. Genetic defects or xenobiotic exposure that reduce thyroid hormone synthesis can delay metamorphosis, and in extreme cases, can completely arrest development. The most profound impacts on TH-mediated metamorphosis have be demonstrated through inhibition of key proteins in the TH synthesis pathway including the sodium-iodide symporter (Tietge et al., 2005, 2010; Hornung et al., 2010) and thyroperoxidase (Degitz et al., 2005; Tietge et al., 2010, 2013; Hornung et al., 2010, 2015). Alternatively, agonism of the thyroid axis through inhibition of negative feedback at the level of the hypothalamus-pituitary, or premature activation of thyroid receptor-mediated transcription can accelerate metamorphosis (Degitz et al., 2005), which can lead to asynchronous development due to errors in gene expression timing across the various metamorphic tissues. Asynchronous development can also occur due to inhibition of deiodinase (DIO) enzymes in peripheral tissues. DIO enzymes are responsible for activation and catabolism of TH; when dio gene expression profiles are altered, or the enzymes themselves undergo chemical inhibition, the imbalance of prohormone (T4), active hormone (T3) and inactive hormone (rT3, T2) can cause aberrant tissue development.

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Rates of metamorphosis in model amphibian species, Xenopus laevis, are measured multiple ways, both of which rely on a developmental staging atlas developed by Nieuwkoop and Faber (NF)(1994). The method utilized within the 21 d Amphibian Metamorphosis Assay regulatory test guideline (OECD, 2009; US EPA 2009) relate the distribution of developmental stage of control larvae to the distributions of developmental stages of treated/exposed larvae. These data are typically analyzed for differences from control using non-parametric statistical approaches such as the Kruskal-Wallis test followed by Dunn's test for pairwise comparisons. The method utilized within the Larval Amphibian Growth and Development Assay regulatory test guideline (OECD, 2015; US EPA 2015) relate the number of days to reach metamorphic climax (NF stage 62) in control larvae to the number of days to NF stage 62 in treated/exposed larvae. These data are typically analyzed for differences from control using a Cox mixed-effects proportional hazard model.

Asynchronous development is identified as disruption of the relative timing of morphogenic milestones and/or somatic development within a single larvae undergoing metamorphosis. The inability to identify an organism's developmental stage based on accepted criteria, such as outlined in Nieuwkoop and Faber (1994) for Xenopus sp. or Gosner (1960) for anurans, constitutes evidence of asynchronous development and would be counted as an incidence. Evaluations of severity are possible but the accuracy and resolution of the results would depend on the experience of the observer. One possible statistical approach for analyzing these data collected from a regulatory test guideline (OECD, 2009, 2015) would be a Rao-Scott-Cochran-Armitage by slices test (Green et al., 2014), as is often used for analysis of histopathology incidence and severity data.  

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help


Xenopus laevis

Evidence for Perturbation by Stressor


Methimazole exposure during prometamorphosis in Xenopus laevis causes delayed/arrested metamorphosis.

Stressor:48 Propylthiouracil

Propylthiouracil exposure during prometamorphosis in Xenopus laevis causes delayed/arrested metamorphosis.


Mercaptobenzothiazole exposure during prometamorphosis in Xenopus laevis causes delayed/arrested metamorphosis.


Perchlorate exposure during prometamorphosis in Xenopus laevis causes delayed/arrested metamorphosis.

Regulatory Significance of the Adverse Outcome

An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP. For KEs that are designated as an AO, one additional field of information (regulatory significance of the AO) should be completed, to the extent feasible. If the KE is being described is not an AO, simply indicate “not an AO” in this section.A key criterion for defining an AO is its relevance for regulatory decision-making (i.e., it corresponds to an accepted protection goal or common apical endpoint in an established regulatory guideline study). For example, in humans this may constitute increased risk of disease-related pathology in a particular organ or organ system in an individual or in either the entire or a specified subset of the population. In wildlife, this will most often be an outcome of demographic significance that has meaning in terms of estimates of population sustainability. Given this consideration, in addition to describing the biological state associated with the AO, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to describe regulatory examples using this AO. More help

Altered metamorphosis is a critical apical endpoint evaluated as part of regulatory test guideline studies (OECD, 2009, 2015; US EPA 2009, 2015). Measurable effects on metamorphic rates can be an indication of endocrine disruption, and more specifically thyroid disruption, due to the requirement of thyroid hormone for amphibians to undergo metamorphosis. Although this outcome is evaluated at the level of the individual organism, delayed or arrested metamorphosis can have implications toward population-level effects; however, significant effects on metamorphic rates are typically considered in a weight-of-evidence evaluation to determine a chemical's potential to cause thyroid disruption. 


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

Brown, D.D. and Cai, L., 2007. Amphibian metamorphosis. Developmental biology, 306(1), pp.20-33.

Degitz, S.J., Holcombe, G.W., Flynn, K.M., Kosian, P.A., Korte, J.J. and Tietge, J.E., 2005. Progress towards development of an amphibian-based thyroid screening assay using Xenopus laevis. Organismal and thyroidal responses to the model compounds 6-propylthiouracil, methimazole, and thyroxine. Toxicological sciences, 87(2), pp.353-364.

Gosner, K.L., 1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica16(3), pp.183-190.

Green, J.W., Springer, T.A., Saulnier, A.N. and Swintek, J., 2014. Statistical analysis of histopathological endpoints. Environmental toxicology and chemistry33(5), pp.1108-1116.

Hornung, M.W., Degitz, S.J., Korte, L.M., Olson, J.M., Kosian, P.A., Linnum, A.L. and Tietge, J.E., 2010. Inhibition of thyroid hormone release from cultured amphibian thyroid glands by methimazole, 6-propylthiouracil, and perchlorate. Toxicological Sciences, 118(1), pp.42-51.

Laudet, V., 2011. The origins and evolution of vertebrate metamorphosis. Current Biology, 21(18), pp.R726-R737.

Nieuwkoop, P.D. and Faber, J., 1994. Normal Table of Xenopus laevis (Daudin) Garland Publishing. New York252.

OECD. (2009). Test No. 231: Amphibian Metamorphosis Assay, OECD Guidelines for the Testing of Chemicals, Section 2. OECD Publishing, Paris.

OECD. (2015). Test No. 241: The Larval Amphibian Growth and Development Assay (LAGDA), OECD Guidelines for the Testing of Chemicals, Section 2. OECD Publishing, Paris.

Tietge, J.E., Butterworth, B.C., Haselman, J.T., Holcombe, G.W., Hornung, M.W., Korte, J.J., Kosian, P.A., Wolfe, M. and Degitz, S.J., 2010. Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis. Aquatic Toxicology, 98(1), pp.44-50.

Tietge, J.E., Holcombe, G.W., Flynn, K.M., Kosian, P.A., Korte, J.J., Anderson, L.E., Wolf, D.C. and Degitz, S.J., 2005. Metamorphic inhibition of Xenopus laevis by sodium perchlorate: effects on development and thyroid histology. Environmental Toxicology and Chemistry, 24(4), pp.926-933.

Tietge, J.E., Degitz, S.J., Haselman, J.T., Butterworth, B.C., Korte, J.J., Kosian, P.A., Lindberg-Livingston, A.J., Burgess, E.M., Blackshear, P.E. and Hornung, M.W., 2013. Inhibition of the thyroid hormone pathway in Xenopus laevis by 2-mercaptobenzothiazole. Aquatic toxicology, 126, pp.128-136.

U.S. EPA. (2009). OCSPP 890.1100: Amphibian Metamorphosis Assay (AMA), Endocrine Disruptor Screening Program Test Guidelines, 890 Series. Available at:, ID: EPA-HQ-OPPT-2009-0576-0002. Accessed March 20, 2020.

U.S. EPA. (2015). OCSPP 890.2300: Larval Amphibian Growth and Development Assay (LAGDA), Endocrine Disruptor Screening Program Test Guidelines, 890 Series. Available at:, ID: EPA-HQ-OPPT-2014-0766-0020. Accessed March 20, 2020.