**2. Vertebrate model animals**

### **2.1. Studies on rodents**

Owing to close taxonomic proximity, rodents, including rat, mice and hamster, resemble most with of humans among all other commonly used vertebrate models, and many symptoms of human conditions can be replicated in mice and rats. For that reason, rodents occupy the most preferred model animal in biomedical research, and keeping pace with this global trend, BPA researchers also rely on rodents to unravel the BPA effects on mammals.

#### *2.1.1. Effects on reproductive system*

from 1.0 to 10 mg/L (Environment Canada 2008), and BPA is declared as 'moderately toxic' and 'toxic' to aquatic biota by the European Commission and the United States Environmental Protection Agency (US EPA), respectively [4], Commission of the European Communities 1996]. Moreover, environmentally relevant concentrations (12 mg/L or lower) of BPA were also found to be harmful as far as wildlife is concerned [5]. BPA exerts its effect through direct binding to estrogen receptor (ER) in a wide range of species that includes invertebrates, fish, amphibians, reptiles, birds and mammals [6]. BPA binds both ERα and ERβ receptors, with

The toxicokinetics of BPA exposure reveal that after oral administration in human, BPA is metabolized rapidly in the intestine and liver. BPA is not completely metabolized via Phase I reactions, but it is rapidly conjugated with glucuronic acid (Phase II metabolism) to pro‐ duce non‐active BPA‐glucuronide in the gut wall and liver. Little amount of BPA also reacts with sulphate to form BPA‐sulphate compound. The formation of BPA conjugates with other chemical moieties is a detoxification process [8, 9]. The BPA conjugates formed in the liver reach the kidney through blood circulation and then excreted in the urine with terminal half‐ lives of less than 6 hours [10, 11]. According to a declaration made in 2010 by U.S. Food and Drug Administration, exposure to BPA is alarming because of possible health hazards it exerts on brain, behaviour and prostate gland of foetuses, infants and children. The European Food Safety Authority (EFSA) reviewed new scientific information on BPA in the years 2008, 2009, 2010, 2011 and 2015, concluding on each occasion the known level of exposure to BPA to be hazardous. In February 2016, France announced that it intends to propose BPA as a

Owing to difficulty in doing research on human subjects, researchers prefer to use model organisms to test the toxic effect of xenobiotic agents in living system. This approach is also popular in the research on BPA as the agent is ubiquitously present in our 'plastic wrapped world' and no perfect control subject could be obtained in natural environment. Several model organisms from different taxa are in use for studying the effects of BPA on their life history, morphological traits, reproductive functioning, neural functioning and behaviour. The outcome of these studies helps to anticipate the probable adversity that BPA inflicts in human body. Keeping all these factors in mind, a critical review on latest research works is presented here to understand the deleterious effects of BPA exposure on different vertebrate and invertebrate model organisms that could facilitate the understanding of human health

Owing to close taxonomic proximity, rodents, including rat, mice and hamster, resemble most with of humans among all other commonly used vertebrate models, and many symptoms of human conditions can be replicated in mice and rats. For that reason, rodents occupy the most preferred model animal in biomedical research, and keeping pace with this global trend, BPA

REACH Regulation candidate substance of very high concern (SVHC).

hazards due to exposure to this xenoestrogen and endocrine disruptor BPA.

researchers also rely on rodents to unravel the BPA effects on mammals.

**2. Vertebrate model animals**

**2.1. Studies on rodents**

approximately 10‐fold higher affinity to ERβ [7].

4 Bisphenol A Exposure and Health Risks

Almost all xenobiotic agents have been tested for their toxicity in rodents to anticipate the probable effects on human body owing to taxonomic closeness of rodents and human as pri‐ mate. There is extensive evidence that BPA imperils development of reproductive system in male rats and mice, although there appear to be species, strain and dose differences in the sensitivity of specific outcomes to BPA [3]. There are numerous studies of the effects of low doses of BPA on the development of the female and male reproductive organs in rats and mice. Findings include chromosomal abnormalities in oocytes in females [12, 13] and long‐ term effects on accessory reproductive organs that are not observed until mid‐life, such as uterine fibroids and para‐ovarian cysts [14]. In Newbold's study [14], outbred female CD‐1 mice were treated on days 1–5 with subcutaneous injections of BPA (10, 100 or 1000 μg/kg/ day). At 18 months of age, ovaries and reproductive tract tissues exhibited significant increase in cystic ovaries and cystic endometrial hyperplasia in the BPA‐treated group. Progressive proliferative lesion of the oviduct and cystic mesonephric (Wolffian) duct remnants was also seen in BPA‐treated groups [15].

The effect of BPA on male reproductive organs and function includes decrease in testoster‐ one secretion [16] and sperm production [17, 18]. Impacts on other reproductive structures include reduction in the size of the epididymis at a dose of 2 ng/g and enlargement of the size of prostate ducts in the male foetuses when pregnant females were exposed to a dose of 10 μg/ kg BPA/day [19, 20]. These findings are consistent with effects of low doses of positive con‐ trol chemicals, such as diethylstilbestrol (DES) and ethinyl estradiol. Moreover, the testicular function impairment includes germ cell sloughing, disruption of the blood‐testis‐barrier and germ cell apoptosis [21, 22].

Impairment in testicular function is also evident in other studies [23, 24, 25]. The effects of BPA resemble more or less the estrogenic effects on the testes [18, 26, 27] with reduction in daily sperm production [28], deformed acrosomal vesicles, acrosomal caps, acrosomes and nuclei of the spermatids. Tohei et al. [29] reported that plasma concentration of testosterone was decreased, and LH was increased in rats after administration of BPA. Testicular content of inhibin was decreased. The testicular response to human chorionic gonadotropin (hCG) for progesterone and testosterone release was also decreased in BPA‐treated rats. These results suggest that BPA directly inhibits testicular functions by disrupting the pathway of negative feedback regulation.

Studies have revealed that BPA exposure also affects the female systems, and it is found to be associated with a number of anomalies like polycystic ovarian syndrome [30], endome‐ triosis [31] and anovulation. Studies have also been conducted to evaluate effects of BPA on development of mammary gland. *In utero* exposure to 25 and 250 μg BPA/kg body weight showed changes in the mammary glands of CD1 mice, including a significant increase in the percentage of gland ducts, terminal ducts, terminal end buds and alveolar buds at 6 months of age [32]. Perinatal exposure to 25 and 250 ng BPA/kg body weight showed increased area of terminal end buds relative to the gland ductal area [33]. Studies in both rats and mice have shown that BPA induces change in mammary gland morphology that may predispose ani‐ mals to develop cancer [34, 35]. **Table 1** shows the summary of results from experiments on reproductive system of laboratory rodents.


**Table 1.** Summary table of the various effects of BPA exposure on reproductive system of laboratory rodents.

#### *2.1.2. Effects on nervous system*

BPA has both indirect and direct effects on the nervous system. Since gonadal hormones in conjunction with other neurotrophins regulate cell death, neuronal migration, neurogenesis and neurotransmitter plasticity [36], BPA, in disrupting sex hormone functions, can affect brain development. Estrogen plays a major role in development and differentiation of certain parts of male and female brains. Male and female brains are exposed to different amounts of estrogen during development, and this appears to shape some regions of the brain differently. One of these regions is the hypothalamus, which controls a variety of basic functions includ‐ ing hunger, mood and sex drive. Due to its estrogenic and antiandrogenic activities, BPA can interfere with the dimorphic development of the neuronal networks of male and female brain regulating [37] the activation of hypothalamic estrogen or androgen receptors, testosterone‐ activating enzymes and hippocampal aromatase expression [38].

As BPA disrupts thyroid function, it can also affect the development of the nervous system because thyroid hormones regulate prenatal and neonatal development of the brain [39]. Juvenile hypothyroidism due to BPA exposure leads to diminutive dentritic growth in hippo‐ campal neurons of rat brain, resulting in cognitive defects including impaired memory, defec‐ tive perception and attention problems [40]. In a prenatal study [41] of brain development in mice treated with BPA in a dose 20 μg/kg, body revealed decrease in growth in the ventricular zone of the BPA‐treated offspring, whereas in the cortical plate, growth was increased. In addition, the expression of thyroid Receptor gene TRα (and other genes) was significantly upregulated in the cortical area of the BPA‐treated group. BPA induces cortical plate growth via upregulation of the thyroid pathway. In doing so, BPA might have disrupted normal neo‐ cortical development by accelerating neuronal differentiation and migration. BPA exposure may also interfere with the development and expression of normal sex differences in cogni‐ tive function, via inhibition of estrogen‐dependent hippocampal synapse formation in female rat [42] and testosterone‐induced hippocampal synapse formation in male mice [43].

In addition, BPA may directly cause neurodegeneration. BPA enhances hydroxyl radical for‐ mation in the rat brain [44], and it is induced by 1‐methyl‐4‐phenylpyridinium ion (MPP+) [45]. This leads to neurodegeneration of the *substantia nigra* and produces acute Parkinsons like symptoms. In this study, 10 μM BPA was infused into the rat striatum to generate OH radical, and *in vivo* micro‐dialysis technique was used for evaluating toxic effects on nervous tissues. In another study [46], BPA was shown to increase intracellular reactive oxygen species at a concentration of 1, 10, 25 and 50 μmol/L and induce apoptosis at a concentration of 100 μmol/L in mesencephalic neuronal cell culture. Besides, BPA has a significant impact on the dopaminergic system and hippocampal‐associated cognitive functions. **Table 2** represents the various observations on the nervous system of laboratory rodents exposed to BPA.

#### *2.1.3. Effects on chromosomes*

*2.1.2. Effects on nervous system*

**Affected area Model; time and route of exposure**

6 Bisphenol A Exposure and Health Risks

Ovary Mice; developmental, pellet

Mammary gland Mice, Rats; developmental,

Testes Mice, rats ; developmental,

Prostate gland CF‐1 mice, CD‐1 mice;

Epididymis CF‐1 mice; developmental, oral

Ovaries and reproductive tract

tissues

implantation

injection

CD‐1 mice; developmental,

injection, minipump

adult, oral, injection

developmental, oral

Blood Rats, adult, oral ↓ Plasma testosterone and

BPA has both indirect and direct effects on the nervous system. Since gonadal hormones in conjunction with other neurotrophins regulate cell death, neuronal migration, neurogenesis and neurotransmitter plasticity [36], BPA, in disrupting sex hormone functions, can affect brain development. Estrogen plays a major role in development and differentiation of certain parts of male and female brains. Male and female brains are exposed to different amounts of estrogen during development, and this appears to shape some regions of the brain differently. One of these regions is the hypothalamus, which controls a variety of basic functions includ‐ ing hunger, mood and sex drive. Due to its estrogenic and antiandrogenic activities, BPA can interfere with the dimorphic development of the neuronal networks of male and female brain regulating [37] the activation of hypothalamic estrogen or androgen receptors, testosterone‐

**Table 1.** Summary table of the various effects of BPA exposure on reproductive system of laboratory rodents.

**Effect Citation**

Susiarjo et al. [12]

Newbold et al. [14]

Tohei et al. [29]

al. [20]

↓ size vom Saal et al. [19]

Markey et al. [32]; Munoz‐ de‐Toro et al. [33]; Soto et al. [13]; Durando et al. [34]; Murray et al. [35]

Akingbemi et al. [16]; Aikawa et al. [17]; Toyama et al. [18]; Al‐Hiyasat et al. [23]; Chitra et al. [24]; Sakaue et al. [26]

Thayer et al. [27]; Timms et

Disruption of early oogenesis

hyperplasia

motility

↑ LH

volume

Seminiferous tubules C57BL/6 mice; adult, oral Disrupted Takao et al. [25]

Increase in cystic ovaries and cystic endometrial

Enhanced growth and differentiation

Decreased testosterone secretion and sperm production ; deformed sperm with reduced

↑ weight, ↑ prostate duct

As BPA disrupts thyroid function, it can also affect the development of the nervous system because thyroid hormones regulate prenatal and neonatal development of the brain [39]. Juvenile hypothyroidism due to BPA exposure leads to diminutive dentritic growth in hippo‐ campal neurons of rat brain, resulting in cognitive defects including impaired memory, defec‐ tive perception and attention problems [40]. In a prenatal study [41] of brain development in mice treated with BPA in a dose 20 μg/kg, body revealed decrease in growth in the ventricular

activating enzymes and hippocampal aromatase expression [38].

Recently, researches have unravelled the fact that maternal exposure to a very low dose (20 ng/g body weight) of BPA disrupts alignment of chromosomes during meiosis in the embry‐ onic oocyte during formation of the primary follicles. This abnormality was also observed in mice that were housed in polycarbonate cages and that were provided water in polycarbonate bottles that had been damaged by exposure to a harsh detergent during washing [47]. This find‐ ing suggests that exposure to BPA during the time that meiosis resumes in the mid‐cycle surge by luteinizing hormone (LH) can result in an increase in foetal aneuploidy and subsequent spontaneous abortion in humans [47]. The effect of BPA on aneuploidy has also been exam‐ ined in cell culture [48–51]. In the study by Tsutsui et al. [48, 49], treatment of Syrian hamster


**Table 2.** Summary table of the various effects of BPA exposure on nervous system of laboratory rodents.

embryo cells with BPA (100 μM) for 48 hours resulted in statistically significant increases in the percentage of aneuploid metaphases with chromosome losses. Reports are also available that revealed delay in the meiotic cell cycle, possibly by a mechanism that degrades cen‐ trosomal proteins and thus perturbs the spindle microtubule organization and chromosome segregation in mouse oocyte during meiosis. When cultured cells were exposed to BPA dur‐ ing the transition from meiosis‐I to meiosis‐II, a delay in meiosis‐I had been observed. This transition phase usually lasts for 8–10 hours in mice, but for BPA‐exposed culture, 53% of cells remained in meiosis‐I. Insignificant counts of cells were found in anaphase [52].

#### *2.1.4. Effects on behaviour*

With inevitable effects of BPA on nervous system, behavioural patterns of rodents are reported to be affected by BPA exposure. An increase in defensive aggression was reported in the off‐ spring of male Sprague‐Dawley rat whose mother was offered oral BPA dose (40 μg/kg/day) throughout gestation [53]. In addition, increased aggressiveness (using a composite score of aggression) in male CD‐1 mouse offspring was evident as a result of oral administration of low dose of BPA (2 and 20 ng/g of body weight) to pregnant females on gestation days 11–17 [54, 55].

A series of studies demonstrated that prenatal and neonatal exposure to BPA upregulates activities of the dopamine system and induced hyperactivity among the experimental rat [56]. Support to this primary report came from the study [57] that revealed prenatal and neonatal exposure of mice to BPA caused upregulation of dopamine D1 receptors, produced hyper‐ locomotion and increased rewarding responses induced by methamphetamine. Narita et al. [58] demonstrated that exposure of mice to BPA during either organogenesis or lactation, but not implantation and parturition, significantly enhanced the morphine‐induced hyperactivity and rewarding effects. In a rat model, Ishido et al. [59] demonstrated that neonatal exposure to BPA (87 nmol/10 μl/rat) caused significant hyperactivity at 4–5 weeks of age, and signifi‐ cantly decreased gene expression of dopamine transporter at 8 weeks.

Negishi et al. [60] demonstrated that BPA impaired both passive and active avoidance learn‐ ing among offspring of Fisher 344 rats that were fed a low dose of BPA (0.1 mg/kg/day orally) during pregnancy and lactation. There are also evidences of depressed maternal behaviour in female exposed [61, 62]. There are also reports by Dessi‐Fulgheri et al. [63] about decrease in play behaviour of juvenile Sprague‐Dawley rats due to exposure of BPA. Authors observed a masculinization of female behaviour in two behavioural categories, that is, play with females and sociosexual exploration, an effect probably mediated by the estrogenic activity of BPA in the central nervous system.

Foetal/neonatal exposure to low doses of BPA causes sex differences in brain structure, chem‐ istry and behaviour. BPA interferes with the normal processes of sexual differentiation, with brain changes in both male and female rat and mice [61, 64]. Evidence of anatomical altera‐ tions in brain sexual differentiation was evident in male and female offspring born to mother exposed to 25 or 250 ng BPA/kg body weight per day [65]. In Fujimoto's experiment, prenatal exposure to BPA affected male rats and abolished sex differences in rearing behaviour in the open‐field test and struggling behaviour in the forced swimming test. **Table 3** shows the sum‐ mary of the experimental results on the behavioural aspects of laboratory rodents.


**Table 3.** Summary table of the various effects of BPA exposure on behaviour of laboratory rodents.

#### *2.1.5. Other miscellaneous effects*

embryo cells with BPA (100 μM) for 48 hours resulted in statistically significant increases in the percentage of aneuploid metaphases with chromosome losses. Reports are also available that revealed delay in the meiotic cell cycle, possibly by a mechanism that degrades cen‐ trosomal proteins and thus perturbs the spindle microtubule organization and chromosome segregation in mouse oocyte during meiosis. When cultured cells were exposed to BPA dur‐ ing the transition from meiosis‐I to meiosis‐II, a delay in meiosis‐I had been observed. This transition phase usually lasts for 8–10 hours in mice, but for BPA‐exposed culture, 53% of cells

With inevitable effects of BPA on nervous system, behavioural patterns of rodents are reported to be affected by BPA exposure. An increase in defensive aggression was reported in the off‐ spring of male Sprague‐Dawley rat whose mother was offered oral BPA dose (40 μg/kg/day) throughout gestation [53]. In addition, increased aggressiveness (using a composite score of aggression) in male CD‐1 mouse offspring was evident as a result of oral administration of low dose of BPA (2 and 20 ng/g of body weight) to pregnant females on gestation days 11–17 [54, 55].

A series of studies demonstrated that prenatal and neonatal exposure to BPA upregulates activities of the dopamine system and induced hyperactivity among the experimental rat [56]. Support to this primary report came from the study [57] that revealed prenatal and neonatal exposure of mice to BPA caused upregulation of dopamine D1 receptors, produced hyper‐ locomotion and increased rewarding responses induced by methamphetamine. Narita et al. [58] demonstrated that exposure of mice to BPA during either organogenesis or lactation, but not implantation and parturition, significantly enhanced the morphine‐induced hyperactivity and rewarding effects. In a rat model, Ishido et al. [59] demonstrated that neonatal exposure to BPA (87 nmol/10 μl/rat) caused significant hyperactivity at 4–5 weeks of age, and signifi‐

Negishi et al. [60] demonstrated that BPA impaired both passive and active avoidance learn‐ ing among offspring of Fisher 344 rats that were fed a low dose of BPA (0.1 mg/kg/day orally) during pregnancy and lactation. There are also evidences of depressed maternal behaviour in female exposed [61, 62]. There are also reports by Dessi‐Fulgheri et al. [63] about decrease in play behaviour of juvenile Sprague‐Dawley rats due to exposure of BPA. Authors observed a masculinization of female behaviour in two behavioural categories, that is, play with females and sociosexual exploration, an effect probably mediated by the estrogenic activity of BPA in

Foetal/neonatal exposure to low doses of BPA causes sex differences in brain structure, chem‐ istry and behaviour. BPA interferes with the normal processes of sexual differentiation, with brain changes in both male and female rat and mice [61, 64]. Evidence of anatomical altera‐ tions in brain sexual differentiation was evident in male and female offspring born to mother exposed to 25 or 250 ng BPA/kg body weight per day [65]. In Fujimoto's experiment, prenatal exposure to BPA affected male rats and abolished sex differences in rearing behaviour in the open‐field test and struggling behaviour in the forced swimming test. **Table 3** shows the sum‐

mary of the experimental results on the behavioural aspects of laboratory rodents.

remained in meiosis‐I. Insignificant counts of cells were found in anaphase [52].

cantly decreased gene expression of dopamine transporter at 8 weeks.

*2.1.4. Effects on behaviour*

8 Bisphenol A Exposure and Health Risks

the central nervous system.

There are evidences on effects of BPA on subsequent activity of enzymes in tissues and thus metabolic processes [66–69]. Study showed very low dose (10 μg/kg) of BPA stimulates insulin production and secretion, which is then followed by insulin resistance at a dose of 100 μg/kg in mice [70]. In the study by Sakurai et al. [71], a high dose of BPA has been revealed to stimulate an increase in the glucose transporter and glucose uptake into adipocytes in cell culture. Study showed that perinatal exposure to a low dose of BPA increased adipogenesis in female rats at weaning [72].

BPA appears to possess complex immuno‐modulating effects. It may stimulate or suppress the immune system. It may also alter immune response pathways. There is extensive evidence that BPA modulates both T helper 1 and T helper 2 cytokine production and alters antibody production [73–75]. Yamashita et al. [76] used immune cells from BALB/c mice and demon‐ strated that BPA induces innate immune response by increasing cytokine synthesis, including tumour necrosis factor (TNF) and IL‐1 in macrophages, and stimulates both T and B cells in adaptive response pathway. Using IL‐2 and IFN‐γ as markers for Th1 response and IL‐4 for Th2 response, the authors found that BPA stimulated Th1 cells to produce IFN‐γ and Th2 cells to express IL‐4. The authors inferred that BPA does not selectively activate the Th1 or Th2 path. BPA also enhances Th1 or Th2 response *in vivo*, depending on the doses [74, 77]. In addition, prenatal exposure to BPA was shown to augment both Th1 and Th2 responses in adulthood [74]. BPA has been reported to modulate immune function at doses between 2.5 and 30 μg/kg/day [70, 73].

#### **2.2. Studies on zebrafish**

Zebrafish (*Danio rerio*) as vertebrate model system is popular for studying developmental events. The reasons for choosing zebrafish in developmental biology research include its easy maintenance and rearing, prolific fecundity, transparent embryo, absence of placenta that eases the study of morphological characters and even teratogenic effects on anatomy due to experimental exposure to xenotoxicants. Researchers have taken this opportunity to facilitate their understanding in the effects of BPA on vertebrate model. Summary of the results of experiments on zebrafish model is given in **Table 4**.

#### *2.2.1. Effects on development and reproduction*

Laboratory studies showed that BPA causes developmental and reproductive effects in zebrafish. There are evidences of delayed hatching, altered axial curvature and tail malfor‐ mation in zebrafish embryos following exposure of fertilized eggs to BPA [78]. In a study by William et al. [79], BPA altered early dorso‐ventral patterning, segmentation and brain development in zebrafish embryos at a concentration of 50 μM within 24 hours of exposure.


**Table 4.** Summary table of the various effects of BPA exposure on zebrafish (*Danio rerio*).

Perturbations in expression of cytochrome P450 aromatase activity have also been observed in zebrafish. Estrogen synthesized in the brain by the action of P450 aromatase is known to have organizing effects on the developing central nervous system. In fish, estrogen increases the predominant brain isoform (P450aromB), implying that xenoestrogens like BPA could act as neurodevelopmental toxicants by altering the expression of P450aromB [80].

Lora et al. [81] found several alterations in the zebrafish testes including a pronounced degen‐ eration of all cellular components, an increase in the percentage of the Sertoli cells and a marked decrease in the percentage of germ cells due to exposure of BPA. Histological studies also showed severe deterioration of ovarian tissue such as disintegration of vesicular struc‐ tures of mature oocytes, irregularities at cytoplasm, reduction in the number of primary and developing oocytes, deformation at the ooplasm and structure of the mature oocytes and irregularities at nucleolus. The number of the atretic oocytes increased due to BPA exposure. Structurally distorted and less developed oocytes were also observed [82]. A study by Laing et al. [83] documented significant increase in egg production, together with a reduced rate of fertilization in zebrafish exposed to BPA, associated with considerable alterations in the tran‐ scription of genes involved in reproductive function and epigenetic processes in both liver (vtg1, esr2b, hdac3, mbd2, mecp2 and dnmt1) and gonad tissue (esr2a, cyp19a1a and amh). Their study demonstrated how BPA disrupts reproductive processes in zebrafish. BPA can also disrupt zebrafish oocyte maturation by a novel nongenomic estrogenic mechanism [84]. BPA exerts this nongenomic estrogenic action on zebrafish oocytes directly through bind‐ ing to the membrane estrogen receptor Gper and activating a Gper‐dependent Egfr/Mapk3/1 pathway. BPA activates this pathway by increasing phosphorylation of Mapk3/1and cAMP concentrations in zebrafish oocytes. Activation of this pathway prevents the resumption of meiotic maturation in fish oocytes [83]. Study showed that BPA downregulated oocyte matu‐ ration‐promoting signals through changes in the chromatin structure mediated by histone modifications in zebrafish [85].

#### *2.2.2. Effects on nervous system and behaviour*

maintenance and rearing, prolific fecundity, transparent embryo, absence of placenta that eases the study of morphological characters and even teratogenic effects on anatomy due to experimental exposure to xenotoxicants. Researchers have taken this opportunity to facilitate their understanding in the effects of BPA on vertebrate model. Summary of the results of

Laboratory studies showed that BPA causes developmental and reproductive effects in zebrafish. There are evidences of delayed hatching, altered axial curvature and tail malfor‐ mation in zebrafish embryos following exposure of fertilized eggs to BPA [78]. In a study by William et al. [79], BPA altered early dorso‐ventral patterning, segmentation and brain development in zebrafish embryos at a concentration of 50 μM within 24 hours of exposure.

**Effect Citation**

Altered William et

Hua and Lin [78]

al. [79]

Laing et al. [83]

Lora et al. [81]

Yon and Akbulut [82]

[83]

al. [84]

Kinch et al. [89]

Saili et al. [90]

Santangeli et al. [85]

Delayed hatching, altered axial curvature, tail malformation

Reduced rate of fertilization, increased egg production

number of sustentacular cells, decreased percentage of

increased number of atretic follicles, distorted and less developed oocytes

Increased neurogenesis and

Increased activity, learning

Adult, Altered Laing et al.

hyperactivity

modification

deficit

germ cells

**Endpoint Life stage and route** 

**of exposure**

a plate

Fertilized eggs, directly in a plate

Embryo, directly in

Breeding adult, in aquarium

Testes Adult, in aquarium Degenerated, increased

Ovary Adult, in aquarium Deteriorated ovarian tissues,

Oocyte maturation Adult, in aquarium Disrupted Fitzgerald et

experiments on zebrafish model is given in **Table 4**.

*2.2.1. Effects on development and reproduction*

10 Bisphenol A Exposure and Health Risks

Hatching, axial curvature, tail morphology

Early dorso‐ventral patterning, segmentation and brain development

Fertilization and egg production

Transcription of genes involved in reproductive

Larval hyperactivity, Adult learning behaviour

Hypothalamus Embryo, directly in

**Table 4.** Summary table of the various effects of BPA exposure on zebrafish (*Danio rerio*).

culture plate

Embryo, directly in culture plate

Oocyte maturation Adult, in aquarium Disrupted by chromatin

function

**Effects on nervous system and behaviour**

**Effects on chromosomes**

**Effects on development and reproduction**

> Zebrafish has been used extensively to elucidate basic mechanisms underlying behavioural toxicology [86]. Zebrafish was also employed as a model for identifying sex‐specific effects on social interactions induced by developmental BPA exposure [87, 88]. A study by Kinch et al. [89] revealed that treatment of embryonic zebrafish with very low‐dose BPA (0.0068 μM, 1000‐fold lower than the accepted human daily exposure) resulted in 180% increase in neu‐ rogenesis within the hypothalamus. Fish embryos exposed to BPA exhibit hyperactivity with ontogenetic growth possibly due to the accelerated neural growth. The authors also found that these effects are probably not due to an effect on estrogen receptors (or estrogen‐like receptors) but may be due to its deleterious effects on the synthesis of key enzyme in steroid hormone synthesis, Aromatase B. This study also demonstrated that developmental BPA exposure led to larval hyperactivity or learning deficits in adult zebrafish [90]. There are evidences for tem‐ perature‐specific impairment of swimming performance, disturbances in muscle activity and gene expression in zebrafish due to exposure of BPA [91]. This result suggests that BPA toxic‐ ity is compounded with the effects of climate change.

#### *2.2.3. Other miscellaneous effects*

BPA can alter sex ratio of zebrafish by inducing feminization of the fry [92]. Zebrafish embryos exposed to BPA also showed signs of feminized brains [86]. Kinch et al. [93] investigated mor‐ phological changes to developing zebrafish caused by exposure to BPA including changes in body length, pericardia (heart) and the head. Na et al. [94] observed a significant damage in the liver of zebrafish after 96 hours of exposure to BPA. This result further confirmed that liver was the target organ of BPA.
