Ocular lesions in leptin receptor-deficient medaka (Oryzias latipes)

Polyethylene microbeads are more critically toxic to the eyes and reproduction than the kidneys or growth in medaka, Oryzias latipes

Many studies using experimental and wild animals have reported negative effects of microplastic beads (MPs) ingestion. However, data regarding the lowest observed adverse effect levels (LOAELs) of MPs remain limited. Our aim was to evaluate the adverse effect levels of polyethylene MPs (10-63 μm diameter) with respect to growth, reproduction, and the eyes and kidneys of medaka (Oryzias latipes) under breeding conditions to contribute to future research involving LOAEL determinations. Fish were exposed to 0.009 mg-MPs (approximately 1000 particles)/L to 0.32 mg-MPs (approximately 40,000 particles)/L for 12 weeks. The eyes and kidneys were evaluated by histopathologic analysis. Although histologic analyses indicated an absence of MPs in the tissues, the eyes and kidneys as well as reproduction were adversely affected by increasing MP concentrations. The number of spawned eggs decreased, and changes were noted in the eyes of fish exposed to ≥0.032 mg-MPs/L under breeding conditions. The eyes exhibited thinning of the optic nerve fiber layer and dilatation of retinal capillaries compared with medaka not treated with MPs. Changes in the kidneys were observed in fish exposed to ≥0.065 mg-MPs/L. The mesangial matrix in the glomerulus of the kidneys was expanded compared with non-treated medaka, suggesting a deterioration in renal function. Analyses of an oxidative stress marker in the tissues indicated that lesion progression was associated with increased oxidative stress. Furthermore, a comparison of adverse effect levels suggested that MPs were more toxic to the eyes and reproduction than the kidneys or growth. Our data should prove useful for determining the LOAELs of polyethylene beads on vertebrates and enhance understanding of the mechanism underlying the biological toxicity of polyethylene MPs.

Intact in vivo visualization of telencephalic microvasculature in medaka using optical coherence tomography

To date, various human disease models in small fish-such as medaka (Oryzias lapties)-have been developed for medical and pharmacological studies. Although genetic and environmental homogeneities exist, disease progressions can show large individual differences in animal models. In this study, we established an intact in vivo angiographic approach and explored vascular networks in the telencephalon of wild-type adult medaka using the spectral-domain optical coherence tomography. Our approach, which required neither surgical operations nor labeling agents, allowed to visualize blood vessels in medaka telencephala as small as about 8 µm, that is, almost the size of the blood cells of medaka. Besides, we could show the three-dimensional microvascular distribution in the medaka telencephalon. Therefore, the intact in vivo imaging via optical coherence tomography can be used to perform follow-up studies on cerebrovascular alterations in metabolic syndrome and their associations with neurodegenerative disease models in medaka.

Renal lesions in leptin receptor-deficient medaka (Oryzias latipes)

The aim of this study was to elucidate the renal lesions of leptin receptor-deficient medaka showing hyperglycemia and hypoinsulinemia and to evaluate the usefulness of the medaka as a model of diabetic nephropathy. Leptin receptor-deficient medaka at 20 and 30 weeks of age showed hyperglycemia and hypoinsulinemia; they also showed a higher level of plasma creatinine than the control medaka. Histopathologically, dilation of glomerular capillary lumina and of afferent/efferent arterioles was observed in leptin receptor-deficient medaka at 20 weeks of age, and then glomerular enlargement with cell proliferation and matrix expansion, formation of fibrin cap-like lesions, glomerular atrophy with Bowman’s capsule dilation, and renal tubule dilation were observed at 30 weeks of age. These histopathological characteristics of leptin receptor-deficient medaka were similar to the characteristics of kidney lesions of human and rodent models of type II diabetes mellitus, making leptin receptor-deficient medaka a useful model of diabetic nephropathy.