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Event: 1878

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

Decreased, Eye size

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
Decreased, Eye size

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

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
Organ term

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
microphthalmia eye decreased

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
TPOi eye size KeyEvent Lucia Vergauwen (send email) Under development: Not open for comment. Do not cite
Inhibition of Fyna leading to increased mortality KeyEvent Vid Modic (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
zebrafish Danio rerio 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
Larval development

Sex Applicability

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

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

Animals show a wide variation in relative eye size (compared to body size) both within and between species. Eye size is directly related to visual ability. Eye size, and in particular the eye to body ratio, gives a lot of information about the quality of vision of the individual but also about its lifestyle. For example, eye size provides information on nocturnal and diurnal lifestyles in mammals (Kirk, 2006). Previous studies of eye design suggest a common organizing principle about how the activity pattern is reflected in the size and shape of the eyes (Hall, 2008).

Large eyes generally have greater visual sensitivity as they have relatively large corneas and lenses, e.g. in primates (e.g (Kirk, 2006; Ross and Kirk, 2007), birds (e.g (Brooke et al., 1999; Hall, 2008), lizards (Hall, 2008), fish (e.g. (Bejarano-Escobar et al., 2010; Karvonen and Seppälä, 2008) and other species. Increasing the size of the whole eye can increase resolution or sensitivity without having to decrease the other. For example, a larger eye with a longer focal length may be more sensitive without loss of acuity, or it may be more acute without loss of sensitivity. However, a constraint for large eyes is that they must always fit inside an animal's head and are associated with increased development and maintenance costs (Caves et al., 2017).

Microphthalmia is a congenital ocular deformation characterized by abnormally small eyes, with or without structural abnormalities (Le et al., 2012). Microphthalmia can occur as a consequence of a number of potential mechanisms, including but not limited to general developmental delay, increased cell death, reduced cell proliferation, and reduced cell differentiation within the developing eye (Stenkamp et al., 2002).

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). ?
  • Use of plasticine spherical ball (Brooke et al., 1999)
  • Ocular biometry (Kang and Wildsoet, 2016)
  • Relative eye size: larger corneal diameters relative to the axial length or larger eye diameter relative to body length (Baumann et al., 2016; Hall, 2008) determined by morphological analysis with electromicroscopy or analysis of digital images
  • Morphological live imaging + Aqueous outflow tract visualization (Chawla et al., 2018)

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

Taxonomic applicability: Applicable to a large range of species. For instance, eye length is positively correlated with visual acuity across mammals  (Heesy and Hall 2010; Veilleux and Kirk 2014), birds (Hall and Heesy 2011), and  fishes (Baumann et al., 2016; Caves et al., 2017; Corral-López et al., 2017).

Sex applicability: Difference in male/female probably due to general differences in body size, highlighted by some studies (Corral-López et al., 2017; Svanbäck and Johansson, 2019).

Evidence for Perturbation by Stressor


  • Zebrafish embryos treated with RA inhibitor diethylaminobenzaldehyde (DEAB) just before bud formation [i.e.,  ̴9 h postfertilization (hpf)] develop microphthalmia (Le et al., 2012).


  • Inhibiting RA synthesis with citral in zebrafish embryos at 6- to 7-somite stage [i.e.,  ̴11hpf], results in absence of the ventral retina (Marsh-Armstrong et al., 1994).
  • Zebrafish embryos treated with RA inhibitor citral just before bud formation [i.e.,  ̴9 h postfertilization (hpf)] develop microphthalmia (Le et al., 2012).


  • (Kashyap et al., 2008) data suggest that the microphthalmic phenotype seen in zebrafish embryos after ethanol exposure during retinal neurogenesis, is due to a cumulative impact of three mechanisms: general delay, increased cell death within the lens, and decreased retinal cell differentiation.


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

Baumann, L., Ros, A., Rehberger, K., Neuhauss, S.C.F., Segner, H., 2016. Thyroid disruption in zebrafish (Danio rerio) larvae: Different molecular response patterns lead to impaired eye development and visual functions. Aquat. Toxicol. 172, 44–55.

Bejarano-Escobar, R., Blasco, M., DeGrip, W.J., Oyola-Velasco, J.A., Martín-Partido, G., Francisco-Morcillo, J., 2010. Eye development and retinal differentiation in an altricial fish species, the senegalese sole (Solea senegalensis, Kaup 1858). J. Exp. Zool. Part B Mol. Dev. Evol. 314 B, 580–605.

Brooke, M.D.L., Hanley, S., Laughlin, S.B., 1999. The scaling of eye size with body mass in birds. Proc. R. Soc. B Biol. Sci. 266, 405–412.

Caves, E.M., Sutton, T.T., Johnsen, S., 2017. Visual acuity in ray-finned fishes correlates with eye size and habitat. J. Exp. Biol. 220, 1586–1596.

Chawla, B., Swain, W., Williams, A.L., Bohnsack, B.L., 2018. Retinoic acid maintains function of neural crest–derived ocular and craniofacial structures in adult zebrafish. Investig. Ophthalmol. Vis. Sci. 59, 1924–1935.

Corral-López, A., Garate-Olaizola, M., Buechel, S.D., Kolm, N., Kotrschal, A., 2017. On the role of body size, brain size, and eye size in visual acuity. Behav. Ecol. Sociobiol. 71.

Hall, M.I., 2008. Comparative analysis of the size and shape of the lizard eye. Zoology 111, 62–75.

Kang, P., Wildsoet, C.F., 2016. Acute and short-term changes in visual function with multifocal soft contact lens wear in young adults. Contact Lens Anterior Eye 39, 133–140.

Karvonen, A., Seppälä, O., 2008. Eye fluke infection and lens size reduction in fish: A quantitative analysis. Dis. Aquat. Organ. 80, 21–26.

Kashyap, B., Frederickson, L.C., & Stenkamp, D.L., 2008. Mechanisms for persistent microphthalmia following ethanol exposure during retinal neurogenesis in zebrafish embryos. Vis. Neurosci. 24(3), 409–421.

Kirk, E.C., 2006. Effects of activity pattern on eye size and orbital aperture size in primates. J. Hum. Evol. 51, 159–170.

Le, H.G., Dowling, J.E., & Cameron, D.J., 2012. Early retinoic acid deprivation in developing zebrafish results in microphthalmia. Vis. Neurosci. 29(4–5), 219–228.

Marsh-Armstrong, N., Mccaffery, P., Gilbert, W., Dowling, J.E., & Dräger, U.C., 1994. Retinoic acid is necessary for development of the ventral retina in zebrafish. Proc. Natl. Acad. Sci. U S A. 91(15), 7286–7290.

Ross, C.F., Kirk, E.C., 2007. Evolution of eye size and shape in primates. J. Hum. Evol. 52, 294–313.

Stenkamp, D.L., Frey, R.A., Mallory, D.E., & Shupe, E.E., 2002. Embryonic Retinal Gene Expression in Sonic-You Mutant Zebrafish. Dev. Dyn., 225, 344–350.

Svanbäck, R., Johansson, F., 2019. Predation selects for smaller eye size in a vertebrate: Effects of environmental conditions and sex. Proc. R. Soc. B Biol. Sci. 286.

Wold, M., Beckmann, M., Poitra, S., Espinoza, A., Longie, R., Mersereau, E., Darland, D.C., Darland, T., 2017. The longitudinal effects of early developmental cadmium exposure on conditioned place preference and cardiovascular physiology in zebrafish. Aquat. Toxicol. 191, 73–84.