Morphological and immunohistochemical comparison of the pituitary gland between a tropical Paracheirodon axelrodi and a subtropical Aphyocharax anisitsi characids (Characiformes: Characidae)

Laura Rincón Camacho¹, Andrea G. Pozzi¹, Eliane G. de Freitas², Akio Shimizu³ and Matías Pandolfi¹

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Abstract​

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Introduction​

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The understanding of the reproductive function, especially focused on the brain-pituitary-gonadal axis, is essential to acquire knowledge about the reproductive biology of different species and fundamental for the support of commercial aquaculture. In captivity, reproduction can be modulated by manipulating environmental parameters, such as water temperature, conductivity and pH, among others (Fiszbein et al., 2010; Mylonas et al., 2010). However, the biological aspects of several teleost species are not well-known, and thus it results very difficult to simulate the environmental conditions required for their reproduction in captivity.

The global trade in ornamental fish is dominated (90%) by freshwater fishes, most of which are sourced from breeding facilities located in developing countries, typically in Asia or South America. However, some fish are still obtained from natural (wild) sources and the exact percentage of wild-caught fish is difficult to quantify given a lack of reliable data (Evers et al., 2019). Generally, these species, collected from their natural environment, are difficult to grow, so it is important to know the morphological and physiological key components of neuroendocrine systems to better regulate the reproduction of these species.

In some commercial species, for example, reproduction can be enhanced by several strategies which can modify the synthesis and release of pituitary hormones (Mylonas et al., 2010; Zohar et al., 2010). In this sense, identifying and characterizing cell types that produce pituitary hormones will enhance our knowledge on fish reproductive physiology and will constitute a solid basis for future reproductive induction with hormonal treatments. Authors said that heterologous antisera can be used in these kind of studies, since their specificity and absence of cross reactivity can be demonstrated (Laiz-Carrión et al., 2003; Pandolfi et al., 2005; Honji et al., 2015).

The pituitary gland is an important endocrine organ that presents two distinctive regions: the neurohypophysis, consisting of neurosecretory terminals principally from the hypothalamus, and the adenohypophysis, constituted mainly by secretory cells. In teleost fish, these endocrine cells are arranged in groups that produce different hormones and are located at specific regions of the adenohypophysis (Van Oordt, Peute, 1983; Laiz-Carrión et al., 2003; Pandolfi et al., 2005).

Cardinal tetra Paracheirodon axelrodi (Schultz, 1956) is a characid species endemic from South America, which inhabits the Amazon, Orinoco and Rio Negro basins (tropical habitat). It inhabits middle water layers with temperatures between 23°C and 27°C, pH ranging from 5.5 to 7, hardness oscillating between 5 dH and 12 dH (Anjos, Anjos, 2006) and it is an omnivore species (Walker, 2004; Marshall et al., 2008). Female spawning occurs in shaded territories and each spawning event can range from 154 to 562 oocytes. It is worth mentioning that this species has an asynchronous oogenesis, since more than two oocyte stages can be present simultaneously (Anjos, Anjos, 2006; Brito, Bazzoli, 2009).

Bloodfin tetra Aphyocharax anisitsi Eigenmann, Kennedy, 1903 is a characid species that inhabits the basin of La Plata’s river in South America (subtropical habitat). In this habitat, the water temperature range is between 18°C and 28°C, pH ranges from 5.4 to 7.9 and hardness of 30 dH (Burgess, 2004; Casciotta et al., 2005). The reproductive biology and the development of sexual dimorphic structures in this species have been already described (Gonçalves et al., 2005). Other works on its biodiversity and distribution in Argentina report that this is a vulnerable species in the Mesopotamia (López et al., 2005), but it is not a threatened species in La Plata basin (Zayas, Cordiviola, 2007), suggesting that their conservation status may differ among different habitats.

Both species, cardinal and bloodfin tetra, exhibit high market value and an increasing global importance for ornamental aquaculture (Evers et al., 2019) as they correspond to the fish order of greatest export interest in countries such as Brazil, Colombia, Venezuela and Argentina (Mancera-Rodríguez, Álvarez-León, 2008; Panné-Huidobro, Ferino, 2017). Some studies have focused on understanding the basic biology of both species, P. axelrodi (Burton et al., 1998; Walker, 2004; Anjos, Anjos, 2006; Oliveira et al., 2008; Brito, Bazzoli, 2009; Tovar-Bohorquez et al., 2009; Gómez-Ramírez et al., 2011; Obando-Bulla et al., 2013) and A. anisitsi (Gonçalves et al., 2005; López et al., 2005; Zayas, Cordiviola, 2007). However, some structures related to reproduction, such as the pituitary gland, have never been explored.

Taking into account the importance of deepening the knowledge of structures related to reproduction, the brain atlas (Obando-Bulla et al., 2013; Rincón et al., 2016^a, 2017) and also a histological and anatomical comparison of the pineal complex in P. axelrodi and A. anisitsi (Rincón et al., 2016b) have been recently published. In this context, considering that we intend to continue assessing the brain-pituitary-gonadal axis, the present study assesses a morphological, anatomical and immunohistochemical characterization of the pituitary gland of both species for the first time.

Material and methods

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Animals. Adults of Paracheirodon axelrodi and Aphyocharax anisitsi were obtained from commercial aquaria. In both cases, animals were housed in community aquaria mimicking their natural conditions: 25°C for both species, pH 6.0 to 7.0 for A. anisitsi and pH 5.0 to 6.0 for P. axelrodi. Fish were fed twice a day with commercial fish pellets (Tetra ®) and acclimatized for at least one month before being used. All procedures described in the following sections were conducted in accordance with international standards, Guide for Care and Use of Laboratory Animals (NRC, 2011) on animal welfare as well as being compliant with local regulations (CICUAL, Comision Institucional para el Cuidado y Uso de Animales de Laboratorio).

Eight adult specimens, 5 females and 3 males of P. axelrodi, of standard length (L_S) of 2.86 ± 0.10 cm, total length (L_T) of 3.36 ± 0.38 cm and weight of 0.42 ± 0.17 g were used. Two females and two males were used for classical histological procedures and three females and one male for immunohistochemical assays. Four females and four adult males of A. anisitsi of L_S 3.08 ± 0.04 cm, L_T of 3.71 ± 0.06 cm and weight of 0.57 ± 0.03 g were processed. Two females and two males were used for histological studies and two females and two males for immunohistochemical procedures. Voucher specimens are deposited at the Ichthyological collection of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN-Ict 12710), Buenos Aires, Argentina.

Histological analysis of the pituitary gland. Animals were anesthetized with benzocaine (0.1 g/l) and killed by decapitation. To preserve both pituitary´s histology and its anatomical relationship with the brain, whole heads were dissected and fixed in Bouin’s solution for 24h at 4°C in the dark. Afterwards, heads were dehydrated through an ascending series of ethanol, clarified with xylene and embedded in Paraplast ® (Sigma). Embedded heads were sagitally and transversally sectioned at 7 µm and mounted on gelatine-coated glass slides. Then, these sections were deparaffinized in xylene, rehydrated through a descending ethanol series and stained with haematoxylin eosin (H-E), Masson trichrome (MT) and periodic acid-Schiff (PAS). Finally, slides were mounted in DPX (Sigma), examined using a Nikon Microphot FX microscope and digitally photographed (Coolpix 4500, Nikon; Japan).

Immunohistochemical analysis of the pituitary gland. For this study, the specimens were processed as described for the histological analysis. Briefly, heads (with pituitaries) were sagittal sectioned at 7 µm and mounted on different slides. They were deparaffinized in xylene, and rehydrated through a graded ethanol series to phosphate-buffered saline (PBS, pH 7.4). Sections were treated with 3% H₂O₂ for ten minutes to saturate endogenous peroxidase activity, after which they were incubated with 5% non-fat dry milk to block unspecific binding sites. Then, slides were incubated overnight at 4°C, in a moist chamber, with different specific primary antisera (antiserum and dilutions used are detailed on Tab. 1). After washing with PBS, sections were incubated for 45 min with a biotinylated anti-rabbit IgG diluted 1:600 (Dako), at room temperature in a moist chamber. Amplification of the signal was achieved by incubating the sections with peroxidase-conjugated streptavidin (STRP-HRP) (Dako) diluted 1:600, at room temperature in a moist chamber, and visualized with 0.1% 3.3’-diaminobenzidine with 0.03% H₂O₂. Sections were slightly counterstained with haematoxylin, mounted in DPX and examined.

TABLE 1 | Primary antisera dilutions used in the immunohistochemical analysis. All the antisera used in this study were kindly provided by 1 Dr. A. Shimizu. National Research Institute of Fisheries Science, Fisheries Research Agency, Kanazawa, Yokohama, Japan. 2NIDDK. The National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Marylan. ACTH: adrenocorticotropic hormone; FSH: follicle-stimulating hormone; GH: growth hormone; LH: luteinizing hormone; NS, not supplied; PRL: prolactin; SL: somatolactin.

For β-FSH and β-LH antigen immunoreactivity was recovered. Before the block of endogenous peroxidase activity, samples were heated for 10 min at 90°C with citrate buffer (10mM, pH=6) for epitope unmasking, cooled at room temperature (RT) and washed with PBS. Finally, the slides continued with the immunohistochemical method described above. Negative controls (data not shown) were achieved by omission of the primary antisera and preabsorption tests performed on other species showed antisera specificity (see Pandolfi et al., 2006; Honji et al., 2013). Furthermore, the specificity of the antisera could also be inferred from the localization of the immunostained cells, which agree with previously reports in other neotropical fish species (Borella et al., 2009; Honji et al., 2013; Nóbrega et al., 2017).

Results​

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Anatomical description of the pituitary gland. The pituitary glands of both P. axelrodi and A. anisitsi appears attached to the ventral region of the hypothalamus (H) by a thick infundibular stalk and consists of two components: the neurohypophysis (NH) and the adenohypophysis (ADH). In both species, the NH consists of a well-developed infundibular stem, and the ADH is globular with its three areas located in an anterior-posterior position. According to these characteristics, the pituitary gland of both species is of the leptobasic type (Fig. 1C-E). The ADH is subdivided in three different areas: a rostral part of the gland, rostral pars distalis (RPD), a central part, the proximal pars distalis (PPD), and a posterior part, pars intermedia (PI). Additionally, each of these areas are invaded by the NH, in a greater proportion in the PPD and PI than RPD, and present different cell types with distinct staining characteristics (Figs. 1, 2). Most of the individuals used for this study were mature males and females. Ovaries had vitellogenic oocytes in greater proportion than earlier oogenetic stages, while testis presented mature spermatozoa in their lumen. No differences were observed in the distribution of pituitary cells with respect to the different phases of the reproductive cycle of individuals (data not shown).

FIGURE 1| Microphotographs of sagittal sections of the pituitary gland of Paracheirodon axelrodi. A. Stained with Haematoxylin-Eosin (H-E), C. Masson trichrome (MT), and E. Periodic acid-Schiff (PAS). Microphotographs of sagittal sections of the pituitary gland of Aphyocharax anisitsi. B. Stained with H-E, D. MT, and F. PAS. RPD: rostral pars distalis; PPD: proximal pars distalis; PI: pars intermedia; NH: neurohypophysis.

FIGURE 2| Details of various components of the pituitary gland of Paracheirodon axelrodi and Aphyocharax anisitsi. Microphotographs of sagittal sections of the A. RPD, B. PPD in P. axelrodi and C. PPD and D. PI in A. anisitsi stained with Haematoxylin-Eosin (H-E). Microphotographs of sagittal sections of the E. PPD and F. PI of P. axelrodi and G. PPD and H. PI in A. anisitsi stained with periodic acid-Schiff (PAS). The delimited gray area corresponds to the neurohypophysis. Black arrowhead: blood vessel; RPD: rostral pars distalis; PPD: proximal pars disalis; PI: pars intermedia. Bars = 10µm.

Histochemistry and immunohistochemistry. Rostral pars distalis (RPD). In both species, PRL and ACTH cells are spherical, acidophilic and chromophobic (with H-E and MT) (Figs. 1A-D, 2B, C), and PAS-negative (Figs. 1E, F; 2E, G). Immunoreactive (ir) PRL and ACTH cells are observed in the dorsal and ventral portions of A. anisitsi (Fig. 3 A-D) and P. axelrodi (Fig. 4A-D). In the most dorsal portion, ir-ACTH cells are close to the neurohypophysis, while in the more ventral region they are isolated or grouped where PRL cells are settled. Particularly, in P. axelrodi, ir-βFSH and ir-βLH cells are observed in the dorsal part of the RPD. These cells are spindle-shaped, intermingled with PRL cells and they are in close relationship with the interdigitations of the neurohypophysis (Figs. 4A, B; 6A-D). In the negative control, no ir-PRL or ir-ACTH cells were detected (data not show).

FIGURE 3| Microphotographs of sagittal sections in the pituitary gland of Aphyocharax anisitsi showing the location of PRL- A. and ACTH- C. ir from the RPD and MSH- C. and SL- F. ir from the PI. Gray box shows the area of detail microphotographs. Details of PRL- B. and ACTH- D. ir from the RPD and MSH- E. and SL- G. ir from the PI. ACTH: adrenocorticotropic hormone; Black star: neurohypophysis; MSH: melanocyte-stimulating hormone; PI: pars intermedia; PPD: proximal pars distalis; PRL: prolactin; RPD: rostral pars distalis; SL: somatolactin.

FIGURE 4| Microphotographs of sagittal sections in the pituitary gland of Paracheirodon axelrodi showing the location of PRL- A. and ACTH- C. ir from the RPD and MSH- C. and SL- F. ir from the PI. Gray box shows the area of detail microphotographs. Details of PRL- B. and ACTH- D. ir from the RPD and MSH- E. and SL- G. ir from the PI. ACTH: adrenocorticotropic hormone; Black star: neurohypophysis; MSH: melanocyte-stimulating hormone; PI: pars intermedia; PPD: proximal pars distalis; PRL: prolactin; RPD: rostral pars distalis; SL: somatolactin.

Proximal pars distalis (PPD). In both species, one group of cells are ovoid with large central nucleus, larger, acidophilic (H-E), blue with MT (Figs. 1A-D; 2B, C) and PAS-negative (Figs. 2E, G). These cells correspond with the location of ir-GH cells (Figs. 5E, F; 6E, F). Another group of cells are basophilic (H-E), violet with MT (Figs. 1A-D; 2B, C) and PAS-positive (Figs. 2E, G). These cells overlap with ir-βFSH and ir-βLH cells (Figs. 5A-D; 6A-D). The ir-βFSH and ir-βLH cells show a dispersed distribution, closer to the interdigitations of the NH and interspersed with ir-GH cells that are dorso-ventrally located, more distant to the NH (Figs. 5, 6). Moreover, in P. axelrodi, ir-βFSH and ir-βLH fibers are also observed in the NH (Figs. 5A, C). In the negative control, no ir-GH, ir- βFSH or ir-βLH cells were detected (data not show).

FIGURE 5| Microphotographs of sagittal sections in the pituitary gland of Aphyocharax anisitsi showing the location of βFSH- A., βLH- C. and GH- E. ir cells of the PPD. Gray box shows the area of detail microphotographs. Details of βFSH- B., βLH- D. and GH- F. ir cells of the PPD. Black star: neurohypophysis; FSH: follicle-stimulating hormone; GH: growth hormone; LH: luteinizing hormone; PI: pars intermedia; PPD: proximal pars distalis; RPD: rostral pars distalis.

FIGURE 6| Microphotographs of sagittal sections in the pituitary gland of Paracheirodon axelrodi showing the location of βLH- A. and βFSH- C. ir cells of RPD and PPD and GH- E. ir cells of PPD. Gray box shows the area of detail microphotographs. Details of βLH- B. and βFSH- D. ir cells of RPD and PPD and GH- F. ir cells of the PPD. Black star: neurohypophysis; FSH: follicle-stimulating hormone; GH: growth hormone; LH: luteinizing hormone; PI: pars intermedia; PPD: proximal pars distalis; RPD: rostral pars distalis.

Pars intermedia (PI). In both species, ir-SL cells (Figs. 3F, G; 4F, G) are spherical, acidophilic (Figs. 1A, B; 2D), PAS-positive (Figs. 1E, F; 2F, H), and are close to the neurohypophyseal fibers that penetrate deeply into the PI. In P. axelrodi specifically, ir-SL cells are observed in the PPD too. Ir-MSH cells (Figs. 3C, E; 4C, E) are basophilic with H-E (Figs. 1A, B; 2D) and PAS-negative (Figs. 1E, F; 2F, H). Ir-ACTH and ir-MSH cells were recognized by the antibody against ACTH, since it recognizes a common region shared by both peptides, which derive from the same precursor molecule, proopiomelanocortin (POMC). These MSH cells show an elongated shape and are close to the neurohypophyseal fibers interspersed with ir-SL cells. In the negative control, no ir-SL or ir-ACTH cells in the negative control were detected.

Finally, Fig. 7 shows a camera lucida drawings of sagittal sections of the pituitary gland, obtained by histological and immunohistochemical analysis, showing the distribution of adenohypophyseal cells of P. axelrodi (A) and A. anisitsi (B).

FIGURE 7| Camera lucida drawings of sagittal sections of the pituitary gland, obtained by histological and immunohistochemical analysis, showing the distribution of adenohypophyseal cells of P. axelrodi A. and A. anisitsi B.. RPD: rostral pars distalis; PPD: proximal pars distalis; PI: pars intermedia; NH: neurohypophysis. () prolactin cells; () adrenocorticotropin cells () growth hormone; ( ) gonadotropin cells; () somatolactin cells; (+) melanotropin cells.

Discussion​

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The pituitary of P. axelrodi and A. anisitsi, is in the ventral zone of the hypothalamus, and its structural and cellular characteristics coincide with that reported in other teleost species (Pandolfi et al., 2001; Borella et al., 2009; Kawauchi et al., 2009; Zohar et al., 2010; Honji et al., 2013; Norris, Carr, 2013). According to the scientific literature, the pituitary of both species is the leptobasic type, which is characterized by a neurohypophysis which has a well-developed pituitary stalk, and a globular adenohypophysis (Van Oordt, Peute, 1983).

Although the distribution of adenohypophyseal cells in P. axelrodi and A. anisitsi is mostly like that described in many teleost species, in general, the identification and detailed distribution of the various types of ADH cells has been investigated in a few reophilic species of South America as: the silver-plated, Salminus hilarii Valenciennes, 1850 (Honji et al., 2013), the golden, Salminus brasiliensis (Cuvier, 1816) (Jesus et al., 2014) both of the Characiform order and the tropical catfish, Steindachneridion parahybae (Steindachner, 1877) (Honji et al., 2015) from the Siluriform order. Generally, the morphological aspects, obtained by histological and immunochemical analyzes, are like those described for S. hilarii (Honji et al., 2013), chanchita, Cichlasoma dimerus (Heckel, 1840) (Pandolfi et al., 2001; ²⁰⁰⁶) and arapaima, Arapaima gigas (Schinz, 1822) (Borella et al., 2009) among others (Cerdá-Reverter, Canosa, 2009; Kawauchi et al., 2009), with some slight differences that we will detail below.

In both species, ir-ACTH and ir-PRL cells were observed disposed in compact groups in the rostral pars distalis (RPD). In P. axelrodi, particularly, ir-βFSH and ir-βLH cells were also found in the RPD. Ir-PRL cells have previously been demonstrated in the RPD of many other teleosts (Batten, 1986; Parhar et al., 1998; Segura-Noguera et al., 2000; Rodríguez-Gómez et al., 2001; Sánchez Cala et al., 2003), where they were arranged as a compact mass, like in P. axelrodi and A. anisitsi. This is different than described for A. gigas when the ir-PRL cells are scarce, arranged in thin strands, and weakly immunostained with the employed antiserum (Borella et al., 2009). PRL is important for the survival of freshwater teleost fishes, since it causes an increase in the concentration of ions and maintains sodium levels in plasma (Pandolfi et al., 2001; Kawauchi et al., 2009; Zohar et al., 2010; Honji et al., 2013; Norris, Carr, 2013). Several studies have shown that PRL also influences fish reproductive cycling, as PRL may be involved in spermatogenesis, vitellogenesis and ovulation, beyond its important osmoregulatory function in fish (Edery et al., 1984; Sandra et al., 2000; Cavaco et al., 2003; Whittington, Wilson, 2013). In teleosts, ir-ACTH cells are typically distributed in palisade at the RPD, between PRL cells and branches of neurohypophysial tissue (Schreibman et al., 1973; Van Oordt, Peute, 1983; Borella et al., 2009), as seen in P. axelrodi and A. anisitsi. ACTH stimulates the interrenal gland to produce cortisol that is related with several physiological processes: stress response, metabolism, osmoregulation, among others (Pandolfi et al., 2001; Laiz-Carrión et al., 2003; Borella et al., 2009; Kawauchi et al., 2009; Honji et al., 2013).

Within the proximal pars distalis (PPD), ir-GH, ir-βFSH and ir-βLH cells were found dorsoventrally distributed, for both species. Ir- βFSH and ir-βLH cells abundant in an area close to the fibers of the neurohypophysis with a round or oval shape, these characteristics agree with the typical described for several teleosts (Van Oordt, Peute, 1983; Agulleiro et al., 2006). βFSH cells were characterized with both heterologous antibodies, anti-chum salmon and anti-mummichog βFSH, obtaining a weaker immunostaining with the first one. The processes of ovulation and secretion of gonadal steroids, depend mostly βLH, although βFSH in fish has also a steroidogenic function (Levavi-Sivan et al., 2010). Likewise, βFSH stimulates the early follicular development and the preparation of the gonads for the posterior actions of βLH (Pandolfi et al., 2006; Borella et al., 2009; Zohar et al., 2010; Norris, Carr, 2013).

Even if in A. anisitsi no immunostaining was observed in RPD, in P. axelrodi ir-βFSH and ir-βLH cells are observed in the dorsal part of this region, as already reported in A. gigas (Borella et al., 2009). This localization in the RPD constitutes a novel result, since in other teleost species such as C. dimerus and Bagrus bayad (Fabricius, 1775) gonadotropins are found both in the dorsal and ventral portions of PPD, as well as around the PI (Pandolfi et al., 2006; Mousa et al., 2012). Moreover, in P. axelrodi, ir-βFSH and ir-βLH fibers are also observed in the NH. This coincides with the reported distribution in C. dimerus, C. dimerus suggested that, even if the precise role of brain-derived FSH and LH is not clear, the abundance of immunopositive fibers in the NH could imply a neuromodulatory function together with a possible role in the control of pituitary cells activity (Pandolfi et al., 2006). More detailed studies are necessary to confirm the pattern of distribution of βFSH and ir-βLH cells in P. axelrodi.

On the other hand, previous studies demonstrated that antiserum raised against human-β-TSH selectively cross-react with the TSH-producing cells of several teleost species (García-Hernández et al., 1996; Segura-Noguera et al., 2000), although a weak immunoreactivity to this antiserum was observed in some teleosts too (Nozaki et al., 1990; Yan, Thomas, 1991). In this work, we could not detect ir-TSH cells with the antibody used (data not shown). Considering that TSH has been shown in other teleost species before, the lack of immunolabelling in our experiments is probably due to the specific antibody chosen for this study and it will be interesting to describe the localization of TSH in P. axelrodi and A. anisitsi using other antibodies in future studies.

Finally, ir-GH cells were larger than the former with oval shape, some of them with vacuoles in the cytoplasm, this feature agree with other teleosts (Rendon et al., 1997; Vissio et al., 1997; Parhar et al., 1998; Segura-Noguera et al., 2000; Rodríguez-Gómez et al., 2001; Sánchez Cala et al., 2003). These cells were frequently found in teleosts surrounding the neurohypophysial processes of the PPD (Van Oordt, Peute, 1983), as seen in P. axelrodi and A. anisitsi. For example, this arrangement was not observed in A. gigas pituitary, as there are no NH branches penetrating the pars distalis (Borella et al., 2009). GH has the effect of increasing cellular metabolic rates, and it also induces the liver cells to produce somatomedins. These, in turn stimulate the mitotic indexes of chondrocytes of the epiphyseal plate and therefore promotes the lengthening of the bones and, consequently, growth (Pandolfi et al., 2001; Borella et al., 2009; Honji et al., 2015).

In the pars intermedia (PI), ir-MSH and ir-SL cells were found surrounding the neurohypophyseal branches. In the pars intermedia, the identified ADH cells release somatolactin (SL) and melanocyte stimulating hormone (MSH). On both species, the ir-SL cells present in the PI of pituitary gland were distributed mainly in the peripheral areas of this region, forming cords or clusters surrounding the neurohypophysial tissue and blood capillaries, apparently intermingled with the MSH cells (Batten, 1986; Cambré et al., 1986; Quesada et al., 1988; Vissio et al., 1997; Rodríguez-Gómez et al., 2001). However, in P. axelrodi specifically, ir-SL cells are observed in the PPD too. Similar to results showed for S. hilarii (Honji et al., 2013), where is observed a cross-reaction of antisera against chum salmon SL with some cells in PPD region. According to Margolis-Kazan et al. (1981) and Batten (1986) the SL and gonadotropins (βFSH and βLH) hormone can contain antigenically similarity, which might be difficult to remove during biochemically purification.

The SL function in teleost fishes is not yet fully established, since mode of action seems to vary between species. For example, in salmonids, SL has been reported to be involved in sexual maturity and smoltification (Rand-Weaver et al., 1992; Bhandari et al., 2003; Borella et al., 2009; Onuma et al., 2010). In other species, however, it has been associated with in the adaptation to background and photoperiod changes, in metabolism, stress response and reproductive physiology (Zhu, Thomas, 1997; Mousa, Mousa, 1999; Rand-Weaver et al., 1992; Vissio et al., 2002; Honji et al., 2013). On the other hand, α-MSH participates in the adaptation to skin color and in the stimulation of ACTH release during stress (Lamers et al., 1992; Laiz-Carrión et al., 2003; Borella et al., 2009).

There are three reproductive dysfunctions in teleost fish are frequently observed in many species when breeding in captivity, according to Zohar, Mylonas (2001) and Mylonas et al. (2010). First, female vitellogenesis and male spermatogenesis fail completely when broodstocks are maintained in captivity. Second, cultured females fail to spawn at the end of the reproductive cycle (oocytes undergo normal vitellogenesis, final maturation, and ovulation; however, ovulated eggs are not released to the water). Third, vitellogenesis appears to progress normally in cultured females, but oocytes fail to reach final maturation, resulting in neither ovulation nor spawning. The second case of dysfunction could be the case of P. axelrodi specifically, since gonadally mature females and males are normally found, however, ovulated eggs or sperm are not released to the water.

Nevertheless, before planning the manipulation of ADH hormone secretion for this species, which will enhance fish reproduction in captivity, it is first necessary to establish the morphophysiological parameters of cell types the pituitary gland and specially that involved in the reproductive process. So, this is the first morphological and immunohistochemical study of the pituitary of P. axelrodi and A. anisitsi. Overall, the pituitary components of P. axelrodi and A. anisitsi presented very similar histological and immunohistochemical characteristics. The results showed a pattern of distribution of pituitary cells similar to that of other teleost species, with only slight differences in the distribution of βFSH and SL for P. axelrodi.

Acknowledgments​

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We would like to thank Dr. A. Shimizu for the generous contribution of mummichog and chum salmon βFSH – βLH antibodies and chum salmon PRL, SL, GH antibodies. We also thank Dra. Maria Florencia Scaia for her contribution in the revision of the English of the manuscript. This work was supported by the following Grants: PICT-2016/0086 (Agencia de Promoción Científica y Técnica), UBACyT 2016/0038 (Universidad de Buenos Aires).

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Authors

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Laura Rincón Camacho¹, Andrea G. Pozzi¹, Eliane G. de Freitas², Akio Shimizu³ and Matías Pandolfi¹

^[1]    Laboratorio de Neuroendocrinología y Comportamiento de Peces y Anfibios, Departamento de Biodiversidad y BiologíaExperimental (DBBE), FCEN, UBA. Ciudad Universitaria, Intendente Güiraldes 2160, C1428EHA. Ciudad Autónoma de BuenosAires, Argentina. (LRC) laura.rinconc27@gmail.com; (AGP) pozziag@gmail.com; (MP) matiaspandolfi4@gmail.com

^[2]    Departamento de Zoologia e Botânica, Instituto de Biociências Letras e Ciências Exatas (IBILCE), Universidade EstadualPaulista (UNESP). São José do Rio Preto, Brazil. elianegfreitas@gmail.com

^[3]    National Research Institute of Fisheries Science, Fisheries Research Agency, Kanazawa, Yokohama, Japan. aneko@affrc.go.jp

Authors’ Contribution

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Laura Rincón Camacho: Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing (original draft), Writing (review & editing).

Andrea G. Pozzi: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Writing (original draft).

Eliane G. de Freitas: Funding acquisition, Investigation, Methodology, Writing (original draft).

Akio Shimizu: Resources.

Matías Pandolfi: Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Writing (original draft).

Ethical Statement​

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All procedures were conducted in accordance with international standards, Guide for Care and Use of Laboratory Animals (NRC, 2011) on animal welfare as well as being compliant with local regulations (CICUAL, Comision Institucional para el Cuidado y Uso de Animales de Laboratorio), Protocol #75b.

Competing Interests

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The authors declare no competing interests.

How to cite this article

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Camacho LR, Pozzi AG, Freitas EG, Shimizu A, Pandolfi M. Morphological and immunohistochemical comparison of the pituitary gland between a tropical Paracheirodon axelrodi and a subtropical Aphyocharax anisitsi characids (Characiformes: Characidae). Neotrop Ichthyol. 2020; 18(1):e190092. https://doi.org/10.1590/1982-0224-2019-0092

Copyright​

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This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Distributed under

Creative Commons CC-BY 4.0

© 2020 The Authors.

Diversity and Distributions Published by SBI

Accepted January 5, 2020 by Bernardo Baldisserotto

Submitted August 15, 2019

Epub April 20, 2020