Estrogens affect brain development of vertebrates, not only by impacting activity and morphology of existing circuits, but also by modulating embryonic and adult neurogenesis. The issue is complex as estrogens can not only originate from peripheral tissues, but also be locally produced within the brain itself due to local aromatization of androgens. In this respect, teleost fishes are quite unique because aromatase is expressed exclusively in radial glial cells, which represent pluripotent cells in the brain of all vertebrates. Expression of aromatase in the brain of fish is also strongly stimulated by estrogens and some androgens. This creates a very intriguing positive auto-regulatory loop leading to dramatic aromatase expression in sexually mature fish with elevated levels of circulating steroids. Looking at the effects of estrogens or anti-estrogens in the brain of adult zebrafish showed that estrogens inhibit rather than stimulate cell proliferation and newborn cell migration. The functional meaning of these observations is still unclear, but these data suggest that the brain of fish is experiencing constant remodeling under the influence of circulating steroids and brain-derived neurosteroids, possibly permitting a diversification of sexual strategies, notably hermaphroditism. Recent data in frogs indicate that aromatase expression is limited to neurons and do not concern radial glial cells. Thus, until now, there is no other example of vertebrates in which radial progenitors express aromatase. This raises the question of when and why these new features were gained and what are their adaptive benefits. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Estrogens are known as steroid hormones affecting the brain in many different ways and a wealth of data now document effects on neurogenesis. Estrogens are provided by the periphery but can also be locally produced within the brain itself due to local aromatization of circulating androgens. Adult neurogenesis is described in all vertebrate species examined so far, but comparative investigations have brought to light differences between vertebrate groups. In teleost fishes, the neurogenic activity is spectacular and adult stem cells maintain their mitogenic activity in many proliferative areas within the brain. Fish are also quite unique because brain aromatase expression is limited to radial glia cells, the progenitor cells of adult fish brain. The zebrafish has emerged as an interesting vertebrate model to elucidate the cellular and molecular mechanisms of adult neurogenesis, and notably its modulation by steroids. The main objective of this review is to summarize data related to the functional link between estrogens production in the brain and neurogenesis in fish. First, we will demonstrate that the brain of zebrafish is an endogenous source of steroids and is directly targeted by local and/or peripheral steroids. Then, we will present data demonstrating the progenitor nature of radial glial cells in the brain of adult fish. Next, we will emphasize the role of estrogens in constitutive neurogenesis and its potential contribution to the regenerative neurogenesis.Finally, the negative impacts on neurogenesis of synthetic hormones used in contraceptive pills production and released in the aquatic environment will be discussed.

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The lagoon of El Bibane (Tunisia) is a Mediterranean coastal area that is affected by an increasing human impact. Thus, an estimation of its biodiversity and an ecological quality assessment are very urgent. This study investigates the meiofaunal and nematode communities of the El Bibane lagoon both from a taxonomic and a functional point of view. Nematodes were the dominant group as is common in brackish water systems. The nematode community was made up of 62 genera in 22 families. Xyalidae, Chromadoridae and Cyatholaimidae were the richest and most abundant families. The taxonomic composition and biodiversity of nematodes were comparable to those found in other European and Mediterranean transitional areas. Both meiofauna and nematodes showed a clear subdivision between marginal and central areas. The central stations, represented by fine sediments and higher amounts of organic carbon, seemed to be characterized by low meiofaunal densities, nematode diversity and a strong dominance of 1B and 2B trophic guilds. These trends could be related not only to the grain size and organic load, but also to a possible influence of tidal currents that can transport pollutants in the central area of the lagoon coming from the adjacent coastal areas. Shannon diversity and life strategies are not always in agreement in defining the ecological quality, but an overall worse ecological quality was detected especially at S6. The results of this study are an important starting point for the future monitoring of the potential and actual human impact on the El Bibane area over time.

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