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Altered, retinal layer structure leads to Altered, Visual function
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Thyroperoxidase inhibition leading to altered visual function via altered retinal layer structure||adjacent||High||Low||Lucia Vergauwen (send email)||Open for citation & comment||EAGMST Under Review|
Life Stage Applicability
Key Event Relationship Description
The structure of the vertebrate retina is well conserved and consists of the following layers: The retinal pigment epithelium (RPE), the photoreceptor layer (PRL), the outer plexiform layer (OPL), the inner nuclear layer (INL), the inner plexiform layer (IPL) and the ganglion cell layer (GCL). Each of these layers has a specific function for the physiology of the visual system. The RPE serves to protect and maintain the photoreceptors and absorbs excess light. The photoreceptors in the PRL consist of a light-receiving outer segment (OS) and the inner segment (IS), which contains the cell bodies. They send their signals to the bipolar cells in the INL, which transmit the signal to the ganglion cells. These form the optic nerve and are responsible for transmitting signals to the optic nerves. In both plexiform layers, the retinal neurons form their synaptic connections (Bibliowicz et al. 2011).
To study the eye, the zebrafish (Danio rerio) is at the forefront of many studies as a model organism. In zebrafish, eye development begins around 12 hpf (Houbrechts et al., 2016b) and by 72 hpf the layers of the retina are well developed (Malicki et al., 2016). Functional vision is established by 4-5 dpf (Brockerhoff, 2006; Chhetri et al., 2014).
Evidence Collection Strategy
Evidence Supporting this KER
It should be emphasised that all layers of the retina are interdependent. The RPE plays an essential role in the retinoid cycle for the photoreceptors (PRL), which perceive the light stimulus and transmit it via the bipolar cells to the ganglia (IPL), which form the optic nerve and transmit the signal to the optic nerve (Connaughton 2005). If these key sites of the phototransduction pathway are disrupted by, for example, endocrine disruptors, it stands to reason that there would be a significant impact on the optical sense and it is plausible that disorders of the eye structure can lead to visual disorders.
Uncertainties and Inconsistencies
Often, high variances occur in the results of behavioural studies that may be due to a variety of factors including genetic differences, variation in feeding status, etc. It is also difficult to compare data from different laboratories in such experiments. Similarly, extrapolating data from fish to mammalian data is particularly difficult for behavioural studies.
Known modulating factors
Temporal evidence is supported by the studies of Houbrechts et al. (2016) and Van Camp et al. (2018) in genetic knockdown and knockout zebrafish respectively. Houbrechts et al. (2016) used a DIO 1 and 2 knockdown, which causes transient hypothyroidism. At 3 dpf they showed altered retinal layer structure and at 4 dpf they showed an altered response to light. By 7 dpf both the retinal layer structure and the response to light had returned to normal. Van Camp et al. (2018) used a DIO2 knockout model causing permanent hypothyroidism. They did shown both altered numbers of rods and cones in the retina and an altered response to light at 7 dpf.
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Taxonomic applicability: The visual system of the zebrafish follows the typical organisation of vertebrates and is often used as a model to study human eye diseases. Although there are some differences in eye structure between zebrafish and humans, it is plausible to assume that a functioning eye structure is important for visual function across all vertebrates and invertebrates that have eyes.
Life stage applicability: The first visual responses based on retinal functionality appear around 70 hpf in zebrafish (Schmitt and Dowling 1999). It is plausible to assume that alterations of the eye structure would result in altered visual function across all life stages, but such alterations are most likely to occur during the development of the normal eye structure, which occurs in the embryo-eleutheroembryo phase.
Sex applicability: Zebrafish are undifferentiated gonochorists since both sexes initially develop an immature ovary (Maack and Segner, 2003). Immature ovary development progresses until approximately the onset of the third week. Later, in female fish immature ovaries continue to develop further, while male fish undergo transformation of ovaries into testes. Final transformation into testes varies among male individuals, however finishes usually around 6 weeks post fertilization. Effects on visual function resulting from altered eye structure during early development are therefore expected to be independent of sex.
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