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Event: 1506

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

Testicular atrophy

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
Testicular atrophy

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
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
Testicular atrophy Testis 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
Histone deacetylase inhibition leading to testicular atrophy AdverseOutcome Shihori Tanabe (send email) Open for citation & comment WPHA/WNT Endorsed


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
Rattus norvegicus Rattus norvegicus Moderate NCBI
Mus musculus Mus musculus Moderate 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
Adult, reproductively mature Moderate

Sex Applicability

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

It is hypothesized that the testicular effects of 1,6-dimethoxyhexane (DMH) are caused by its metabolism to methoxyacetic acid (MAA) [Wade et al., 2006; Poon et al., 2004]. MAA produces testicular and thymic atrophy such as the decrease in size [Miller et al., 1982; Moss et al., 1985]. The spermatogenic stages in which the toxicity of MAA is induced are on the patchytene spermatocytes immediately before and during meiotic division, which are Stages XII-XIV of the cycle in the rat and the early pachytene spermatocytes at stages I-IV of the cycle. Dead germ cells can be seen as soon as 12 hours after the treatment of MAA [Casarett & Doull’s, 7th edition].

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). ?
  • Testicular atrophy can be assessed by testicular volume measurement using an orchidometer, rulers, calipers, and ultrasonography or by testis weighing and histopathologic examination.
  • The testis weight is measured to detect testicular atrophy [Foster et al., 1983].
  • The urinary zinc excretion and testicular zinc content are examined since zinc concentration has been shown to play an important role in the production of testicular injury  [Foster et al., 1983].
  • The testicular tissue structure is observed whether there are normal germinal epithelial cells and Leydig cells [Mercantepe et al., 2018]. The testis is fixed for observations by light microscopy or transmission electron microscopy [McDowell and Trump, 1976; Mercantepe et al., 2018].
  • Changes in sperm are measured by computer-assisted sperm analysis [Foote et al., 1995].
  • For the assessment of sperm morphology, eosin-stained sperm collected from the cauda epididymis is observed. At least 200 sperm on each slide were examined for the proportion of sperm with abnormal head (overhooked, blunt hook, banana-shaped, amorphous, or extremely oversized) or tail (twisted, bent, corkscrew, double/multiple) [Wade et al., 2006].
  • For the measurement of the total number of condensed spermatids per testis, a weighed portion of the parenchyma from the left testis was homogenized [Wade et al., 2006]. Sperm or homogenization-resistant spermatid nuclei densities were calculated from the average number of nuclei and were expressed as total or as per gram of epididymis or testis weight [Wade et al., 2006].
  • For the determination of total LDH and LDH-X in the supernatant of the homogenized testis fragment, enzyme activity was measured by monitoring the extinction of NAD absorbance [Wade et al., 2006].

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
  • The decrease in testis weight associated with testicular cell damage was induced by ethylene glycol monomethyl ether (EGME) or MAA treatment in rats (Rattus norvegicus) [Foster et al., 1983].
  • The number of spermatocytes, principally pachytene cells, is decreased by EGME treatment in CD-1 mice (Mus musculus) and CD rats (Rattus norvegicus) [Anderson et al., 1987].
  • The testicular lesions induced by 2-methoxyethanol (or EGME) were observed in rats (Rattus norvegicus) and guinea pigs (Cavia porcellus), which are different in onset, characteristics, and severity [Ku et al., 1984].
  • Spermatogenesis was disrupted by EGME treatment in rabbits (Oryctolagus cuniculus) [Foote et al., 1995].
  • Testicular toxicity such as spermatocyte death in seminiferous tubule stages I-IV and stages XII-XIV was induced by dimethoxyhexane (DMH) treatment in Sprague-Dawley rats (Rattus norvegicus) [Wade et al., 2006].

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

The testicular atrophy assessment is important for assessing the side effects of the medicines such as anti-cancer drugs, as well as the hazard and risk of chemicals. The testicular atrophy including a decrease in testis weight and sperm count, fertility, decrease in morphology and function of the sperm, can become one of the main endpoints as the adverse effects of the therapeutics. The unexpected effects of the therapeutics may be predicted with this Adverse Outcome (AO). In terms of chemical risk assessment, the AO may be related to the health effects caused by the usage of pesticides or biocides.


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

Anderson, D. et al. (1987), "Effect of ethylene glycol monomethyl ether on spermatogenesis, dominant lethality, and F1 abnormalities in the rat and the mouse after treatment of F0 males", Teratog Carcinog Mutagen 7:141-158

Casarett & Doull’s Toxicology, the Basic Science of Poisons, 7th Edition, Edited by Curtis D. Klaassen, Chapter 20 Toxic responses of the reproductive system

Foote, R.H. et al. (1995), "Ethylene glycol monomethyl ether effects on health and reproduction in male rabbits", Reprod Toxicol 9:527-539

Foster, P.M. et al. (1983), "Testicular toxicity of ethylene glycol monomethyl and monoethyl ethers in the rats", Toxicol Appl Pharmacol 69:385-399

Ku, W.W. et al. (1994), "Comparison of the testicular effects of 2-methoxyethanol (ME) in rats and guinea pigs", Exp Mol Pathol 61:119-133

McDowell, E.M. and Trump, B.F. (1976), "Histologic fixatives suitable for diagnostic light and electron microscopy", Arch Pathol Lab Med 100:405-414

Mercantepe, T. et al. (2018), "Protective effects of amifostine, curcumin and caffeic acid phenethyl ester against cisplatin-induced testis tissue damage in rats", Exp Ther Med 15:3404-3412

Miller, R. et al. (1982), "Toxicity of methoxyacetic acid in rats", Fundam Appl Toxicol 2:158-160

Moss, E.J. et al. (1985), "The role of metabolism in 2-methoxyethanol-induced testicular toxicity", Toxicol Appl Pharmacol 79:480-489

Poon, R. et al. (2004), "Short-term oral toxicity of pentyl ether, 1,4-diethoxybutane, and 1,6-dimethoxyhexane in male rats", Toxicol Sci 77:142-150

Wade, M.G. et al. (2006), "Testicular toxicity of candidate fuel additive 1,6-dimethoxyhexane: comparison with several similar aliphatic ethers", Toxicol Sci 89:304-313