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Prenatal regression of the trophotaenial placenta in a viviparous fish, Xenotoca eiseni.

ABSTRACT: The trophotaenial placenta is a branching, ribbon-like structure that extends from the perianal region of the embryo in viviparous teleost fishes belonging to the family Goodeidae. It is a hindgut-derived pseudoplacenta, which contributes to absorbing maternal nutrients during the prenatal stage. The trophotaeniae are known to reduce at birth; however, no previous study has evaluated the removal mechanisms. We report here the analysis of the trophotaeniae using the goodeid fish species Xenotoca eiseni. The X. eiseni trophotaenia consists of an epidermal cell layer, mesenchyme, vasculature, and circulating erythrocytes. The trophotaeniae had preliminary regressed when the embryo was born. Immunohistochemistry indicated that caspase3-activated cells with fragmented nuclei are present in the regressed processes of the fry immediately after birth, but not in the vasculature and blood cells. This finding suggests that the trophotaenia is rapidly resorbed by apoptosis in the last phase of the pregnancy and that its circulatory pathway is maintained. Such prenatal regression of pseudoplacentae has not been reported in other viviparous vertebrates. On the other hand, similar apoptotic remodeling in the gut has been reported in amphibians, which is regulated by thyroid hormone. Thus, apoptotic regression of the trophotaeniae might occur in a manner similar to amphibian metamorphosis.

SUBMITTER: Iida A

PROVIDER: S-EPMC4297964 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Disoriented animals and humans use both the environmental geometry and visual landmarks to guide their spatial behavior. Although there is a broad consensus on the use of environmental geometry across various species of vertebrates, the nature of disoriented landmark-use has been greatly debated in the field. In particular, the discrepancy in performance under spontaneous choice conditions (sometimes called "working memory" task) and training over time ("reference memory" task) has raised questions about the task-dependent dissociability of mechanisms underlying the use of landmarks. Until now, this issue has not been directly addressed, due to the inclusion of environmental geometry in most disoriented navigation paradigms. In the present study, therefore, we placed our focus on landmark-based navigation in fish (Xenotoca eiseni), an animal model that has provided fruitful research in spatial reorientation. We began with a test of spontaneous navigation by geometry and landmarks (Experiment 1), showing a preference for the correct corner, even in the absence of reinforced training. We then proceeded to test landmarks without the influence of informative geometry through the use of square environments (Experiment 2-4), varying the numerosity of present landmarks, the distance of landmarks from the target corner, and the type of task (i.e., spontaneous cued memory or reference memory). We found marked differences in landmark-use in the absence of environmental geometry. In the spontaneous memory task, visual landmarks acquired perceptive salience (and attracted the fish) but without serving as a spatial cue to location when they were distal from the target. Across learning in the reference memory task, the fish overcame these effects and gradually improved in their performance, although they were still biased to learn visual landmarks near the target (i.e., as beacons). We discuss these results in relation to the existing literature on dissociable mechanisms of spatial learning.

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Prenatal regression of the trophotaenial placenta in a viviparous fish, Xenotoca eiseni.

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