Endocrine Disruptor Impact on Zebrafish Larvae: Posterior Lateral Line System as a New Target

Abstract

Endocrine-disrupting chemicals (EDCs), including polychlorinated biphenyls (PCBs), bisphenol A (BPA), pharmaceutical drugs, and pesticides, affect a variety of hormone-regulated physiological pathways in humans and wildlife. The occurrence of these EDCs in the aquatic environment is linked with vertebrates’ health alteration. EDCs exhibit lipophilic characteristics and bind to hydrophobic areas of steroid receptors, such as the estrogen receptor, which are involved in vertebrate developmental regulation. Mainly, EDCs modify the transcription of several genes involved in individual homeostasis. Zebrafish conserve many developmental pathways found in humans, which makes it an appreciated model system for EDCs research studies, especially on early organ development. In the current chapter, we emphasize on latest published papers of EDCs effects on lateral line regeneration in zebrafish larvae. Similarly, we describe other special impacts of EDCs exposure. In conclusion, we make the case that the zebrafish lateral line exposed to EDCs can provide important insights into human health.

1. Introduction

During the last two decades, special attention from scientists and the public has been increasingly given to the harmful effects that may result from exposure of humans or wildlife to chemicals, having the property of interfering with the endocrine system. According to McLachlan John [1], one of the very first references to the problematic posed by what are called “endocrine disruptors” is that of Roy Hertz [2] who considered “that the fact should be taken into account that the use of hormones in animal feed risked exposing certain individuals to these substances, when they should never have been in contact with such molecules in their lifetime, that we were in the process of creating a steroid cycle in our environment, and that we had to seriously consider the implications this could have for our development, our growth and perhaps for our reproductive function.”

The presence of endocrine disruptors (EDCs) in the environment has raised many questions within the scientific community because of the risks they represent for humans and ecosystems [3]. Indeed, EDCs can cause “adverse effects on the health of an organism or its progeny in relation to changes in endocrine function” [4]. EDCs can act on all stages of endocrine regulation, from the hormones synthesis to their action in target tissues [3]. They include a large category of chemical substances such as natural and synthetic hormones or synthetic molecules such as pesticides, polychlorinated biphenyls (PCBs), or even alkylphenols from the industrial activities increasing as well as the pharmaceuticals used in everyday life.

Aquatic environments are often the ultimate outlet for many anthropogenic chemicals [5]. Several studies, carried out in the laboratory or in the natural environment, have revealed harmful effects of these substances on fauna and in particular on the physiology of fish reproduction [6]. Numerous studies have established a direct link between the presence of EDCs in the environment and alterations in reproductive function in certain aquatic organisms [6]. In fish, disturbances of hormone-regulated proteins, as well as histological alterations in the male gonads, have been demonstrated and linked to exposure to estrogen mimetics [7]. Indeed, certain EDCs are compounds that act as agonists of estrogen receptors (ERs) such as 17α-ethinylestradiol (EE2). These estrogen-mimetic compounds also include certain pesticides, alkylphenols, or bisphenols. These EDCs can thus mimic the action of endogenous natural estradiol and are able to induce the expression of estrogen-regulated genes [8].

While most of the research has focused on the EDCs effects on peripheral organs and more specifically on the gonad [9], this chapter aims to focus on the latest published research on the impact of EDCs on the peripheral nervous system of zebrafish larvae, more precisely, the posterior lateral line PLLs regeneration after exposure to environmental pollutants.

2. Posterior lateral line (PLL)

2.1 Definition and organization

The mechanosensory lateral line system is found in more than 25,000 fishes species—the Chondrichthyes (sharks, skates, rays, and chimeras), the “Agnatha” (the jawless fishes, the hagfishes, and lampreys), and the Osteichthyes (the bony fishes). The lateral line systems of the Osteichthyes and Chondrichthyes are more evolved, with superficial neuromasts on the skin and canal neuromasts in a series of lateral line canals on the head and trunk. The lateral line system of the lampreys is composed exclusively of superficial neuromasts located in grooves on the skin of the head and trunk. The lateral line system is a mechanosensory organ that detects the water vibration up to about 300 Hz [10], and relative pressure to the body. The information collected will be used for the detection of prey, schooling behavior, and avoiding predators [11]. Thus, the lateral line is composed of mechanoreceptors organs called neuromasts, which are distributed over the fish body. Neuromasts comprise a core of sensory hair cells surrounded by nonsensory support cells (Figure 1). Each neuromast is innervated by a branch of the lateral line nerve (axonal nerve). Generally, the lateral line is subdivided in two—the anterior lateral line (ALL) which comprises the neuromasts present on the head, jaw, and opercle, and its sensory neurons form the ALL ganglion, rostral to the ear, and the posterior lateral line (PLL) which contains the neuromasts on the trunk and tail and its sensory neurons form the PLL ganglion, caudal to the ear. Both the ALL and the PLL comprise several branches.

Figure 1.

Zebrafish posterior lateral line organization (PLL).

Each lateral line neuromast contains a central sensory hair cell progenitor embedded within a rosette formed by apical attachment and constriction of surrounding epithelial support cells. Lateral line hair cells are surrounded by a group of support cells. Many of these cells are located basally to the hair cells and project interdigitating processes between them, acting to isolate hair cells from one another. In addition to serving as the source for new hair cells in the event of damage, they also provide structural and trophic support. Lateral line hair cells also share structural, functional, and molecular similarities with the hair cells in the vertebrate inner ear [12]. They are innervated by both afferent and efferent nerve fibers on their basal surfaces, which emanate from the lateral line ganglion and branch off at each neuromast [13]. The lateral line system has become increasingly popular as a model for studying hair cell biology relating to human hearing and balance disorders (Figure 1).

2.2 PLL development

In zebrafish, the posterior lateral line system genesis (PLL) is started at the end of 24 hpf by the migration of cells cluster (100 cells) called the primordium PLL (PLLp) under the skin near the ear to the end of the tail [14]. During this step, the PLLp periodically deposits neuromasts (L1, L2 … L7) along the body and will finish its migration, by the establishment of 2–3 terminal neuromasts (TN) at the level of the tail. In addition, each neuromast is formed by the sensory hair cells in the center. The pLLP is prearranged along its migratory axis—the posterior third cells (head zone) are highly proliferative and mesenchymal type, while those of the anterior two-thirds (leakage zone) are placed in epithelial rosettes [15]. These rosette cells called protoneuromasts will give rise to hair cells or support cells after they are deposited (Figure 2).

Figure 2.

Zebrafish neuromast anatomy and posterior lateral line development.

The sensory hair cells are formed from the central progenitor hair cells which after division produce pairs of differentiated hair cells sensitive to water movement. Support cells surround the central sensory hair cells and provide structural support [16]. Also, these internal support cells are themselves surrounded by mantle cells or external support cells. These cells form internal support cells during their proliferation (Figure 1) [17].

2.2.1 Genetic control

The PLLp migration is genetically regulated by several genes including those of the chemokine Cxcl12a and its receptors Cxcr4b and Cxcr7b. Cxcl12a is expressed in cells along the horizontal myoseptum, while PLLp expresses both Cxcr4b and Cxcr7b. The PLLp migration is inhibited following disruption of expression of Cxcl12a and these receptors [18]. Dambly-Chaudière and Ghysen [19] have shown that from an affinity point of view, Cxcr7b has more affinity for Cxcl12a (expressed in the trailing zone) than for Cxcr4b (expressed in the head zone). Cxcr7b to serve as a molecular sink due to this difference in binding affinity, preventing Cxcr4b receptors expressed near the leak area from binding to the chemokine [20]. Thus, through the primordium an expression gradient of Cxcl12a is generated, the binding of which can involve the polymerization of actin in the direction of migration [21].

The PLLp migration is dependent on the canonical signaling interaction between Wnt and FGF. In the head zone, the Wnt signaling is important while in the tail zone it is the FGF signaling which is essential. Wnt signaling results in Cxcr4b expression and proliferation mediation in the head area, allowing the primordium to maintain its size throughout its migration [22]. These proliferating cells migrate from the head area and grow throughout the pPLL, thus depositing several protoneuromasts. The number of neuromasts generated is reduced and the speed of neuromast placement decreases following disruption of proliferation [23, 24]. Wnt signaling also controls the expression of Fgf3 and Fgf10a ligands [25]. FGF signaling drives the morphogenesis of epithelial rosettes which will give rise to neuromasts at the level of the leakage zone [26]. Studies have shown that inhibition of FGF signaling inhibits the formation of rosettes, and consequently the formation of neuromasts [27]. Recently, Yanicostas et al. [28] reported that primordial migration is inhibited following the inactivation of kal1a, which is a homologous zebrafish gene encoding the extracellular matrix protein Anosmin-1a and known to be an activator of FGF signaling [29]. Likewise, the expression of kal1a is similar to that of cxcr4b, important in the head region and less essential in the tail region, but independent of CXCR4b/SDF1a signaling.

2.2.2 Estrogen receptors implication

In zebrafish, the most studied nuclear receptors are estrogen receptors (ERs). Three genes encoding these receptors have been identified—one encoding an ERα ortholog in mammals and two orthologs encoding ERβ (called ERβ1 and ERβ2) [30]. These receptors are found scattered throughout several regions of the body such as the gonads, liver, and nervous system [30]. Several studies have shown that all estrogen receptor isoforms exhibit high expression levels specifically in lateral line neuromasts [31]. Research work on the importance of ERs in the establishment of PLL has shown that a disturbance in the development of neuromasts by the absence of hair cells, occurs following the temporary suppression of the expression of ERβ2 by a morpholino, which could be linked to aberrant activation of the Notch signaling pathway in embryos [32]. The temporary suppression by a morpholino of the expression of ERβ2 led to the disturbing development of these neuromasts (absence of hair cells), which could be related to aberrant activation of the Notch signaling pathway in embryos treated with morpholino [32]. Likewise, developmental defects and early embryo mortality occur following the suppression of ERα expression by application of morpholino from the translation of maternal transcripts [33]. Recently, a mutation in the gene encoding ERβ2 made it possible to identify a new mutant zebrafish line [34]. Deformed sexual intercourse (dominance of the adult male population), testes of altered morphologies, an imbalance in hormone levels, and an altered immune system, are the results of this mutation [34].

3. Axonal nerve PLL

3.1 Axonal nerve regeneration

Several tissues in fish exhibit a remarkable capacity for regeneration after injury, including the retina, cardiac tissue, and neurons. The lateral line which is a sensory system located on the surface and used to detect the movement of water shows a robust regenerative capacity. In addition, O’Brien et al. [35] approved that all cell types in this system can be genetically, physically, or chemically modified. Neuromasts deposited on the body surface of zebrafish larvae are innervated by sensory axons (PLL nerve) [11]. The superficial development of the nerve allows localized lesion, thus, the dynamics of complete regeneration of axons has been studied in zebrafish larvae 24 hours after axotomy [36]. Several works have studied the involvement of different cell types in the dynamics of degeneration and regeneration of the lateral line nerve (PLL). The inhibition of Schwann cell expression after chemical depletion inhibits the binding of neuregulin to the Erb receptor, which causes the exhaustion of nerves in peripheral Schwann cells [37]. In addition, the use of a transgenic mutant fish “named leo1” (no development of neural crest derivatives according to Nguyen et al. [38] leads to incoherent axonal regeneration. Also, inhibition of macrophages (by morpholino: molecule used to modify gene expression) causes very slow regeneration of the PLL nerve (more than 6 hours). In doubly mutant individuals characterized by the absence of Schwann cells and macrophages, axotomy of the nerve is followed by the death of these individuals [36].

The PLL nerve undergoes Wallerian degeneration (WD) after axotomy (Figure 3) with a succession of three phases; a delay phase (phase 1), a fragmentation phase (phase 2) followed by a final clearance phase (phase 3). The two phases of fragmentation and clearance begin approximately 3–5 hours post-axotomy respectively. In zebrafish, Wallerian degeneration (WD) of posterior lateral line axons occurs much faster than that studied in mammals and Drosophila [36]. Wallerian degeneration (WD) occurs in an orderly and stereotypical fashion involving described genetic control in the central nervous system (CNS) and peripheral (PNS) after trauma, stroke, or infection [39]. After axon cleavage, acute axonal degeneration (AAD) can occur at both adjacent ends of the lesion [40]. After ADA, it has been reported that the detached fragment remains intact during the delay phase or “lag phase.” Following this latency phase, the axons will rapidly degenerate followed by a cut in the endoplasmic reticulum, degradation of the neurofilament, and swelling of the mitochondria of the axonal fragments. During the final phase, these fragments are removed using phagocytic cells. At the level of the PLL system, Schwann cells and macrophages play an important role in the process of Wallerian degeneration (WD). Thus, Schwann cells decrease myelin lipid synthesis in the first 12 hours after axotomy [41] and inhibit myelin protein production for 48 hours [42].

Figure 3.

Wallerian degeneration (WD) steps.

4. Sensory hair cells

The hair cells of neuromasts are mechanosensory cells that are able to detect the water movements and transform the energy generated into electrical signals transmitted automatically to the brain. Usually, the hair cell is highly polarized, both apicobasal and in the plane of the epithelium. In its structure, it is characterized by a crescent-shaped stereociliary bundle and a large single kinocilium, on the apical side of the cell facing the otic lumen (Figure 4). Due to the morphology and function of hair cells in the lateral line system, these cells are very similar to those in the inner ear of mammals [12]. Numerous researchers have demonstrated that lateral line hair cells are sensitive to exposure to aminoglycosides [43, 44], in vivo imaging of fish lateral line hair cells zebra was first used by [45] who observed the death and regeneration of hair cells induced by neomycin. Harris et al. [17] then developed additional assays to quantify the death and regeneration of hair cells in the lateral line, establishing a basis for genetic and chemical studies aimed at detecting modulators of hair cell sensitivity to ototoxic exposures and to perform further testing. Other research works [46] have been studied the lateral line response of zebrafish following exposure to ototoxic compounds, such as aminoglycosides and cisplatin. The zebrafish lateral line system is, therefore, a rapid and efficient model for evaluating the effects of a large number of pharmaceuticals on mechanical-sensory hair cells [32].

Figure 4.

Hair cell of neuromast showing its functional asymmetry and its afferent/efferent innervation.

4.1 Sensory hair cell regeneration

4.1.1 Cell differentiation and proliferation

Neuromast hair cells are functional in zebrafish 3 days after fertilization [19] and contain 8–20 hair cells 5 days after fertilization [17]. They are surrounded by nonsensory support cells, with basal nuclei and apical projections that intersect between them [47]. Rubel et al. [48] report that hair cells regenerate after damage via trans-differentiation or proliferation of carrier cells. Studies performed using tritium and bromodeoxyuridine (BrdU) labeling techniques have shown that lateral line hair cells can undergo continuous proliferation [49]. After acoustic trauma, fish can regenerate hair cells within 1–2 weeks [50]. However, the regenerative potential of neuromast hair cells can be considered a dose-dependent response depending on the level of damage [51]. Several studies have shown that hair cells can regenerate from mitotic divisions and the proliferation of supporting cells. At the zebrafish lateral line, hair cells normally undergo programmed cell death during development but are restored from support cells to the periphery after the S phase is produced [45]. The proliferating supporting cells can either remain on the periphery or migrate inward and their number increases after drug-induced hair cell death. Further studies on the zebrafish lateral line have shown that the newly formed hair cells are the result of the proliferation of supporting (carrier) cells and that there are two sets of these cells within the neuromasts [52]; one group of cells is centrally located and considered the progenitor of hair cells, the other is peripheral whose function is unknown. This suggests that there may be functional specializations between populations of neuromast support cells. Hair cells can also regenerate from the trans-differentiation of carrier cells. By applying high levels of damage to neuromasts, hair cell replacement allows surrounding support cells to divide [53].

4.1.2 Regeneration mechanisms

The determination of genes and molecular mechanisms controlling the regeneration of hair cells via differentiation and proliferation of support cells has received great interest. Recently, DNA microarrays and next-generation sequencing (high throughput sequencing) have been used to identify which genes are activated after the destruction of hair cells. Thus, following the exposure of zebrafish to noise, a DNA chip was produced in order to follow the change in gene transcripts [54]. A modification of the genes encoding growth hormone and genes for myosin (light and heavy chains) and the major histocompatibility complex have been observed. Liang et al. [55] have shown that the “stat3/socs3” pathway can modulate the production of lateral line hair cells during development and of the adult inner ear during regeneration. In mouse models, Stat3 effectors may be involved in hair cell survival [56], but no role of Stat3 in hair cell regeneration has yet been reported in mammals.

Chemical screening techniques (chemical screening) have also been used to identify compounds that increase or inhibit hair cell regeneration in the lateral line of zebrafish. Chemical screening has been used to identify synthetic glucocorticoid activators that promote hair cell regeneration by increasing mitotic activity [57]. This study also identified inhibitors that reduced hair cell regeneration or prevented cell proliferation. Research on zebrafish has shown that the “Wnt” signaling pathway is involved in the regeneration of hair cells. At the neuromast level, inhibition of “Wnt/β-catenin” signaling reduces proliferation and differentiation of hair cells while activation of “Wnt” increases the number of hair cells and promotes reintegration of support cells in the cycle of hair cells and their proliferation [58]. In addition, activation of Wnt/β-catenin causes increased regeneration of hair cells [59]. The size of the neuromast is also regulated by a negative feedback loop that integrates the “Wnt” signaling activity [60] and promotes the proliferation of surrounding cells.

Jiang et al. [61] showed that analysis of RNA transcripts expressed in the zebrafish lateral line following neomycin-induced damage showed that Wnt/β-catenin signaling is weakly regulated at onset, but becomes highly regulated later, suggesting that “Wnt” is necessary for hair cell proliferation, but not immediately after hair cell damage. The research focused on “Wnt” signaling in zebrafish indicates that the “Sox2” transcription factor is involved in the proliferation and trans-differentiation of hair cells [59]. Thus, it has been shown that the newly formed hair cells originate from the proliferation of “Sox2 positive” cells. The “Sox2” factor is highly expressed in most progenitor cells of proliferating neuromasts [53] and is also required for trans-differentiation of carrier cells [62]. Another regulator of “Wnt/β-catenin” signaling is “ErbB/Neuregulin,” which may act to regulate zebrafish lateral line interneuromast cells proliferation and neuromast development [63]. Other work suggests that the “Notch” pathway modulates regeneration because inhibition of “Notch” signaling can cause regeneration of hair cells [64]. Jiang et al. [61] studying lateral line hair cell regeneration in zebrafish found that “Notch” signaling is inhibited immediately after hair cell damage. In addition, constitutive expression of “Notch” may prevent the proliferation of hair cells from carrier progenitor cells, while the elimination of “Notch” activity produces an increased number of cellular progenitors and hair cells [16]. Wada et al. [60] reported that the proliferation pathways of hair cells, Wnt, Notch, and Erb are key components common in zebrafish and mouse models involved in the regulation of hair cell regeneration. In addition, the cell cycle inhibitor p27kip is known to regulate the regeneration of mammalian hair cells [65].

4.2 Regeneration after contaminant exposure

The lateral line system has been used to study the various compounds’ toxicity, including aminoglycosides [44], trace metals [51], cisplatin [66], and endocrine disruptors [67, 68, 69] during the zebrafish early life stages. The hair cells death by necrosis and apoptosis, the new hair cells differentiation and proliferation following exposure to toxic substances have been described by several studies [44, 51, 66, 67, 68, 69].

4.2.1 Metal

Hernandez et al. [51] have shown that copper is toxic to the zebrafish lateral line following exposure to concentrations varying between 1 and 50 M resulting in the death of hair cells of neuromasts. During the first 5 minutes of copper sulfate “CuSO4” exposure (5 M), they appear signs of damage [70]. Thus, morphological changes resulting from the onset of apoptosis and necrosis have been recorded [51]. Exposure of zebrafish larvae to “CuSO4” for 2 hours, followed by assessment of hair cell regeneration over the next 5 days, resulted in robust regeneration in neuromasts of the anterior lateral line, whereas posterior lateral line neuromasts showed little regeneration, suggesting a differential regeneration in the lateral line within the same animal [51]. In other work, they have shown that copper attenuates hair cell regeneration in part by reducing cell proliferation [71]. Also, neuromasts did not regenerate upon continuous exposure to copper [71]. These results suggest that copper is toxic to both hair cells and support cells, and its presence in waterways can negatively affect fitness. Other work has shown that the exposure of zebrafish larvae to cadmium causes an alteration of the lateral line, including a regeneration deficit associated with a change in behavior such as rheotaxis or escape reactions [72]. Exposure to 5 mg/L cadmium for 2 days severely damaged the nervous system of sea bass (Dicentrarchus labrax) [73].

4.2.2 Aminoglycosides

It was during the twenty-first century that the zebrafish lateral line was considered a model system for determining the toxicity of therapeutic drugs via hair cells. Williams and Holder [45] agreed that neomycin is toxic to lateral line hair cells. In addition, Harris et al. [17] showed that the response to neomycin is dose-dependent, identical in each neuromast, and that the sensitivity of hair cells depends on the age of the fish. Thus, in fish aged 4 days post fertilization (dpf), hair cells are less sensitive to neomycin than those of 5 dpf or more [74].

Van Trump et al. [75] also showed that hair cells in the lateral line are sensitive to damage caused by aminoglycosides. Research on the toxicity mechanisms of its molecules has shown that the swelling of mitochondria, loss of mitochondrial membrane potential, and the need for Bcl2 proteins associated with mitochondria suggest that aminoglycosides activate mitochondrial-dependent cell death pathways [76]. Also, the duration of hair cell death differs considerably for different aminoglycoside molecules, showing that toxicity pathways are initiated via the activation of different intracellular signaling cascades [46]. Actually, treatment with aminoglycosides lateral line of fish species has become a standard tool in behavioral studies designed to study lateral line function [77].

4.2.3 Cisplatin

The cisplatin toxicity study, which is a platinum-based chemotherapeutic agent used in the treatment of various tumors, has shown that there is a proportional relationship between the dose and the time of hair cell loss [66]. Thus, hair cells damage is related to intracellular accumulation of cisplatin as well as little work that reports cascading activation of cell death in lateral line hair cells treated with cisplatin. Molecules with antioxidant properties, such as N-acetyl L-cysteine and D-methionine, exhibit protective capacity for hair cells in zebrafish exposed to cisplatin, suggesting that cisplatin can induce oxidative stress pathways. These oxidative stress pathways have been implicated in the ototoxicity of cisplatin and aminoglycosides, suggesting some conservation of cell death mechanisms between different classes of ototoxic drugs [78]. Nuclear condensation and mitochondrial swelling are the consequences of apoptotic cell death [66].

4.2.4 Endocrine disruptors

Endocrine disruptors chemicals (EDCs) are natural or synthetic compounds found in the environment that disrupt the levels and distribution of endogenous hormones in exposed organisms [79], which can alter development and/or reproduction in humans and wildlife. Among the endocrine-disrupting compounds, we can cite natural hormones (17 beta-estradiol, E2) and synthetic [17α-ethinylestradiol (EE2)], pesticides [for example, dichlorodiphenyltrichloroethane (DDT)], polychlorinated biphenyls (PCB), bisphenol A (BPA), phthalates, flavonoids and polycyclic musks [80]. Zebrafish are used as a model organism to study endocrine disruptors and assess environmental risks [81]. The main mechanism of action triggered by these molecules is an agonist or antagonist interaction with ER. The concentration of vitellogenin (VTG), an egg yolk precursor protein, which is produced and secreted by the liver, absorbed by the ovary, and changed by developing eggs to form egg yolk, is the most common biomarker most widely used for EDC activity [81].

Bisphenol-A (BPA), polychlorinated biphenyls (PCBs), 17α-ethinylestradiol (EE2), and pesticides are widespread aquatic pollutants and can deeply affect the lateral line of zebrafish via disruption of endocrine system signaling. Bisphenol-A (BPA) is found in polycarbonate plastics and epoxy resins, as well as in the coatings of some cans. Researchers examined its impact on lateral line regeneration and found that it is toxic to hair cells in zebrafish larvae and that exposure delays regeneration [67]. BPA also attenuates hair cell regeneration after aminoglycoside damage, suggesting that BPA is toxic to supporting cells [71]. Hayashi et al. [67] found in another study that PCB-95 had no effect on lateral line development or hair cell survival. Rather, BPA had a significant effect on the survival and regeneration of hair cells. BPA-induced hair cell loss is both dose-dependent and temporal. Experimental laboratory studies suggest that BPA kills hair cells via activation of oxidative stress pathways, similar to previous reports of BPA toxicity in other tissues. In addition, Hayashi et al. [67] observed that hair cells killed with neomycin did not regenerate normally when BPA was present, suggesting that BPA in aquatic environments could interfere with innate regenerative responses in fish.

In other studies, Nasri et al. [68] examined the effect of the pesticide A6 derived from naturally-occurring α-terthienyl, and structurally related to the endocrine-disrupting pesticides anilinopyrimidines, on living zebrafish larvae. Results show that A6 decreases larval survival and affects central neurons at micromolar concentrations. In the lateral line system, researchers found that A6 alters the axonal and sensory cell regeneration at nanomolar concentrations. In addition, A6 has accumulated in lateral line neurons and hair cells. In addition, the examination of 17α-ethinylestradiol (EE2) effects at pico- to nano-molar concentrations on early nervous system development of zebrafish larvae showed that EE2 disrupted axonal nerve regeneration and hair cell regeneration. Upregulation of gene expression of estrogen and progesterone receptors has been recorded. In contrast, downregulation of the tyrosine hydroxylase, involved in the synthesis of neurotransmitters, occurred concomitant with diminution of proliferating cells. Collectively, EE2 modifies nervous system development, both centrally and peripherally, with negative effects on regeneration and swimming behavior [69].

5. Conclusions

The lateral line is a sensory system utilized at a variety of aquatic vertebrates especially in zebrafish, to detect changes in surrounding water flow. This sense, which utilizes mechanotransduction, mediates a wide variety of behaviors from predator detection to schooling. Its position on the body surface allows experimental laboratory studies. The regeneration of functional mechanosensory cells after damage or following pollutants exposure offers the potential to uncover processes involved in the maintenance, proliferation, and differentiation of sensory precursors. The latest research has approved that estrogen receptors are involved in the control of lateral line development and regeneration. These results support the use of this sensory system as a target for research on environmental estrogenic endocrine disruptors.

Conflict of interest

The authors declare no conflict of interest.