Establishment and characterisation of single cell-derived embryonic stem cell lines from the gilthead seabream, Sparus aurata.

An important bottleneck in fish aquaculture research is the supply and maintenance of embryos, larvae, juvenile and adult specimens. In this context, cell lines represent alternative experimental models for in vitro studies that complement in vivo assays. This allows us to perform easier experimental design and sampling and avoid the sacrifice of animals. Embryonic stem (ES) cell lines have attracted increasing attention because they have the capability to proliferate indefinitely and could be differentiated into any cell type of the organism. To minimise cell heterogeneity and increase uniformity of in vitro studies results, in this manuscript we report the development and characterisation of two single cell-derived ES cell lines (monoclonal) from the morula stage embryos of the gilthead seabream, Sparus aurata, named as SAEC-A3 and SAEC-H7. Both cell lines have been passaged for over 100 times, indicating the establishment of long-term, immortalised ES cell cultures. Sequence analyses confirmed the seabream origin of the cell lines, and growth analyses evidenced their high viability and proliferating activity, particularly in culture medium supplemented with 10-15% fetal bovine serum and 22 °C. Both cell lines showed the ability to generate embryoid bodies and show different sensitivity and response to all-trans retinoic acid. The analysis of epithelial (col1α1) and neuronal (sox3) markers in differentiated cultures revealed that SAEC-A3 tended to differentiate towards epithelial-like cells whereas SAEC-H7 tended to differentiate towards neuronal-like cells. Both cell lines were efficiently transfected with pDsRed2-ER and/or pEGFP-N1 plasmids, indicating that they could represent useful biotechnological tools. Daily expression of pcna showed significant expression rhythms, with maximum levels of cell proliferation during the day-night transition. Currently, these cell lines are being successfully used as experimental models for the study of cellular metabolism, physiology and rhythms as well as for toxicological, pharmacological and gene expression analyses.

Current knowledge on the melatonin system in teleost fish.

Melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. Investigations aiming at understanding how melatonin mediates the effects of photoperiod on crucial functions and behaviors have been very active in the last decades, particularly in mammals. In fish a clear-cut picture is still missing. Here we review the available data on (i) the sites of melatonin production in fish, (ii) the mechanisms that control its daily and annual rhythms of production and (iii) the characterization of its different receptor subtypes, their location and regulation. The in vivo and in vitro data on melatonin effects on crucial neuroendocrine regulations, including reproduction, growth, feeding and behavioral responses, are also reviewed. Finally we discuss how manipulation of the photic cues impact on fish circannual clock and annual cycle of reproduction, and how this can be used for aquaculture purposes.

Differential expression of three different prepro-GnRH (gonadotrophin-releasing hormone) messengers in the brain of the european sea bass (Dicentrarchus labrax).

The expression sites of three prepro-gonadotrophin-releasing hormones (GnRHs), corresponding to seabream GnRH (sbGnRH: Ser(8)-mGnRH, mammalian GnRH), salmon GnRH (sGnRH: Trp(7)Leu(8)-mGnRH), and chicken GnRH-II (cGnRH-II: His(5)Trp(7)Tyr(8)-mGnRH) forms were studied in the brain of a perciform fish, the European sea bass (Dicentrarchus labrax) by means of in situ hybridization. The riboprobes used in this study correspond to the three GnRH-associated peptide (GAP)-coding regions of the prepro-GnRH cDNAs cloned from the same species (salmon GAP: sGAP; seabream GAP: sbGAP; chicken GAP-II: cIIGAP), which show little oligonucleotide sequence identity (sGAP versus sbGAP: 42%; cIIGAP versus sbGAP: 36%; sGAP versus cIIGAP: 41%). Adjacent paraffin sections (6 mm) throughout the entire brain were treated in parallel with each of the three anti-sense probes and the corresponding sense probes, demonstrating the high specificity of the hybridization signal. The results showed that both sGAP and sbGAP mRNAs had a broader expression in the olfactory bulbs, ventral telencephalon, and preoptic region, whereas cIIGAP mRNA expression was confined to large cells of the nucleus of the medial longitudinal fascicle. In the olfactory bulbs, both the signal intensity and the number of positive cells were higher with the sGAP probe, whereas sbGAP mRNA-expressing cells were more numerous and intensely stained in the preoptic region. Additional isolated sbGAP-positive cells were detected in the ventrolateral hypothalamus. These results demonstrate a clear overlapping of sGAP- and sbGAP-expressing cells in the forebrain of the European sea bass, in contrast to previous reports in other perciforms showing a clear segregation of these two cell populations.

Immunohistochemical localization of glucocorticoid receptors in the forebrain of the rainbow trout (Oncorhynchus mykiss).

The distribution of glucocorticoid receptor-expressing cells was studied in the forebrain of the rainbow trout by means of antibodies produced against a fusion protein made of the NH2-terminal fragment of the rainbow trout glucocorticoid receptor fused in frame with glutathione-S-transferase. The results indicate that glucocorticoid receptor-expressing cells are located in many brain regions from the telencephalon to the spinal cord, with the highest density in the neuroendocrine component of the brain, the preoptic region and the mediobasal hypothalamus, and in the periventricular zone of the optic tectum. In virtually all cases, the labeling was located in the nucleus of the cells, although on very rare occasions, a slight labeling of the cytoplasm was detected. Concerning the preoptic region, the most striking feature was the high density of glucocorticoid receptors in the magnocellular preoptic nucleus, known to contain corticotrophin-releasing factor (CRF)-, vasotocin-, and isotocin-expressing cells. Colocalization experiments showed that 100% of the CRF-immunoreactive neurons in the preoptic nucleus express glucocorticoid receptors. In the mediobasal hypothalamus, the highest expression was found in the nucleus lateralis tuberis and parts of the nucleus recessus lateralis. Concerning the pituitary, the glucocorticoid receptor was consistently found in the rostral pars distalis, with the exception of the prolactin cells, and in the proximal pars distalis, which in trout contains thyrotrophs, gonadotrophs, and somatotrophs. In the hindbrain, expression of glucocorticoid receptors were localized mainly in the periventricular regions.

The GnRH system in the European sea bass (Dicentrarchus labrax).

The cDNA sequences encoding three GnRH forms, sea bream GnRH (sbGnRH), salmon GnRH (sGnRH) and chicken GnRH II (cGnRH II), were cloned from the brain of European sea bass, Dicentrarchus labrax. Comparison of their deduced amino acid sequences to the same forms in the gilthead sea bream, Sparus aurata, and striped bass, Morone saxatilis, revealed high homology of the prepro-cGnRH II (94% and 98% respectively), and prepro-sGnRH (92% to both species). The sbGnRH exhibited dissimilar identities, with high homology to the striped bass (93%), and lower homology (59%) to the gilthead sea bream. Two transcript types were identified for the GnRH-associated peptide (GAP)-sGnRH as well as for the GAP-cGnRH II, which suggests a possible alternative splicing followed by the addition of an early stop codon. In order to obtain antibodies specific for the three GnRH precursors, recombinant GAP proteins were produced. The differential expression of the three GnRHs previously reported in the brain by means of in situ hybridization, using riboprobes corresponding to the GAP-coding regions, was fully confirmed by immunocytochemistry using antibodies raised against the recombinant GAP proteins, indicating that the transcripts are translated into functional proteins. Moreover, this approach allowed us to follow, for the first time, the specific projections of the different cell groups: sGAP fibers are distributed mainly in the forebrain with few projections reaching the pituitary, sbGAP fibers are mainly present in the preoptic area, mediobasal hypothalamus and predominantly project to the pars distalis of the pituitary, whereas cGnRH II fibers have a widespread distribution primarily in the posterior brain, and do not project to the pituitary. These new tools will be extremely useful to study further the development, regulation and functional significance of three independent GnRH systems in the brain of vertebrate species.

Localization of tyrosine hydroxylase-immunoreactivity in the brain of the Senegalese sole, Solea senegalensis.

The localization of catecholamines in the brain of the Senegalese sole was determined by immunohistochemical techniques using antibodies against tyrosine hydroxylase. Although the general pattern of distribution of catecholamines is consistent with that reported in other teleosts, some remarkable differences are observed. The most rostral tyrosine hydroxylase immunoreactive (TH-ir) cells were identified in the olfactory bulbs, in which a clear asymmetry in the number and location of TH-ir perikarya and fibers was observed. The number of TH-ir cells is manifestly higher in the right olfactory bulb, especially in the internal cell layer. TH-ir fibers are also much more abundant in the right bulb, principally in the glomerular and internal cell layers. Other TH-ir cell masses were identified in the ventral telencephalon, preoptic area, caudoventral hypothalamus, posterior tuberculum, synencephalon, isthmic region and rhombencephalon. Surprisingly, no ir cell bodies were identified in the ventromedial thalamic nucleus, which exhibits a large number of TH-ir cells in other teleosts. The presence of TH-ir fibers in the brain of sole is particularly evident within and around the nuclei in which immunoreactive cells are found. However, other zones such as the dorsal telencephalon, posterior commissure, optic tectum, torus semicircularis, reticular formation or inferior olive also displayed TH-ir fibers. TH-ir axons also enter the infundibulum, reaching the proximal pars distalis of the adenohypophysis. The distribution of TH-ir cells and fibers is compared with that observed in other teleosts and is discussed in a comparative context.

Distribution of serotonin in the brain of the Senegalese sole, Solea senegalensis: an immunohistochemical study.

We report the distribution of serotonin immunoreactive (5-HT-ir) structures in the brain of the adult Senegalese sole, Solea senegalensis, using the streptavidin-biotin-peroxidase complex immunohistochemical method. We have found a wide distribution of immunoreactive fibers throughout the entire brain. 5-HT-ir cell bodies appeared restricted to some periventricular nuclei associated with the diencephalic recesses, and in the rhombencephalic reticular formation and inferior olivary region. Specifically, cerebrospinal fluid-contacting serotoninergic cells were found within the pars dorsalis and pars ventralis of the nucleus recessus lateralis, in the paraventricular organ and in the nucleus recessus posterioris. In the brainstem, 5-HT-ir perikarya appear within the superior and inferior raphe, the nucleus reticularis superioris, the nucleus interpeduncularis and the inferior olive. Although positive fibers were not found in the neurohypophysis, a few 5-HT-ir cells were identified in the adenohypophysis. This distribution is compared with those found in other fishes and discussed in the context of putative roles of 5-HT as a neuroendocrine factor and neurotransmitter in the Senegalese sole.

Characterization of neuropeptide Y expression in the brain of a perciform fish, the sea bass (Dicentrarchus labrax).

The distribution of neuropeptide Y (NPY) gene expression was mapped in the brain of the sea bass (Dicentrarchus labrax) by in situ hybridization with 35S-UTP labeled cRNA probes. Gene expression was mainly detected within the forebrain, although NPY mRNA transcripts were also localized in the tectum and tegmentum mesencephali and posterior brain. New NPY-expressing nuclei were found in the dorsal and ventral telencephalon, preoptic area, tuberal hypothalamus, synencephalon, tegmentum mesencephali and posterior brain. The profuse NPY gene expression within the main neuroendocrine areas of the teleost fish further supports a physiological role in the control of the pituitary secretion. In addition, NPY gene was expressed within the primary visual, olfactory and gustatory circuits of teleost which, subsequently, project to hypothalamic feeding center in teleost fish. Our results extend the NPY-expressing areas known in teleost species.

The torus longitudinalis in the gilthead seabream: an undescribed fiber tract link with the valvula cerebelli.

In this paper, we present a brief anatomical description of the torus longitudinalis and the valvula cerebelli in a percomorph fish, the gilthead seabream (Sparus aurata) based on the analysis of serial brain sections stained by paraldehyde fuchsin, Groat's hematoxylin-picroindigocarmin and cresyl violet. The existence of a small undescribed fiber tract directly bridging the ventral torus longitudinalis and the granular layer of the rostral valvula cerebelli is also reported. This small fiber tract was observed in its integrity on a few transverse sections providing that the angle of sectioning was appropriate. The existence of the anatomical link between the TL and the VC described in this paper might sustain the role of the TL as part of an ascending cerebello-tectal circuit, at least in gilthead seabream. However, these fibers might also represent fibers of passage originating from other brain structures.

A morphological study of the brain of Solea senegalensis. I. The telencephalon.

In this paper we present an anatomical description of the telencephalon of Solea senegalensis based on cresyl violet and haematoxilin-eosin-stained serial transverse sections. This work was conducted as a basis for the precise localization of neuroendocrine territories in the brain of a species with growing interest in marine aquaculture. The external asymmetric morphology of Senegalese sole is correlated with the asymmetry of the forebrain. The right olfactory nerve and bulb are larger than the contralateral ones and this asymmetry is also extended to the cerebral hemispheres. The olfactory bulb comprises an outer olfactory nerve fiber layer, a glomerular layer, an external cellular layer, a secondary olfactory fiber layer and an internal cellular layer. The telencephalic hemispheres can be divided in area dorsalis and area ventralis, consisting of eleven and eight cell masses, respectively. The area dorsalis comprises five subareas: a pars medialis (Dm), subdivided into four nuclei termed Dml to Dm4; a pars dorsalis (Dd); a pars lateralis (D1), which consists of dorsal (Dld), ventral (Dlv) and posterior (Dlp) subdivisions; a pars centralis (Dc); and more caudally, a pars posterioris (Dp), which is very prominent in this species. A nucleus taenia (NT) was observed in the transitional region between area dorsalis and area ventralis. The area ventralis consists of pars dorsalis (Vd), pars ventralis (Vv), pars supracommissuralis (Vs), pars postcommissuralis (Vp), pars lateralis (V1), pars centralis (Vc), pars intermedia (Vi) and nucleus entopeduncularis (E). A periventricular organ, that we have termed lateral septal organ (LSO), was observed in the ventral telencephalon, medial to Vv.

Histochemical aspects of the yolk-sac and digestive tract of larvae of the Senegal sole, Solea senegalensis (Kaup, 1858).

Histochemical distribution of glycoproteins, carbohydrates and proteins rich in different amino acids were studied using histological and histochemical procedures, in Senegal sole, Solea senegalensis (Kaup, 1858) larvae from hatching until day 15. Glycogen, proteins and glycoproteins were detected in the yolk-sac of the larvae at hatching and during the yolk-resorption. The epithelial digestive system (brush border, enterocytes and goblet cells) contained neutral and acid mucins (carboxylated and/or sulphated). Glycogen was observed in the cytoplasm of the digestive absortive cells (enterocytes) and in the liver (hepatocytes) on day 3-4 posthatching. Protein reactions, and specially those that showed proteins rich in arginine, tyrosine and tryptophan, were very intense in the zymogen granules of the pancreatic cells. Oesophageal and intestinal goblet cells contained glucose N-acetyl and sialic acid residues, but the mucin content of these mucous cells did not show affinity towards Con-A, suggesting the absence of glycoproteins with Mannose and/or glucose residues. WGA showed a very intense positivity in the microvilli of the digestive epithelium of the larvae and positive granules for both lectins, specially for Con-A, were detected in the cytoplasm of the anterior intestinal enterocytes.

Immunocytochemical distribution of cytochrome P4501A (CYP1A) in developing gilthead seabream, Sparus aurata.

CYP1A is a major inducible enzyme in the metabolism of xenobiotic substrates. In this paper we investigate by means of immunohistochemistry, the tissue distribution of constitutive cytochrome P4501A (CYP1A) during the period of endogenous nutrition (from hatching until day 4) in developing gilthead seabream, Sparus aurata larvae. For this purpose, a polyclonal antiserum (BN-1, Biosense Laboratories) directed against conserved piscine CYP1A sequences was used on paraffin-embedded sections from seabream larvae. From hatching onward, CYP1A immunoreactivity was observed in the following tissues and cells: syncytial, oil-globule envelopes and matrix of the yolk-sac, kidney (epithelia of renal tubules), cardiac muscle cells, skin epidermal cells, troncal musculature, enterocytes of different intestinal regions, goblet cells of the bucco-pharyngeal region, gill epithelial cells and the endothelia of the vascular system of various tissues (especially from liver and brain). Moreover, eye (retina), olfactory epithelium and some positive nerve fibers located in the proximity of the olfactory bulbs and running ventrally toward the posterior brain were strongly CYP1A immunoreactive. In general, the intensity of immunostaining increased with larval development.

Immunocytohistochemical characterization of pituitary cells of the bluefin tuna, Thunnus thynnus L.

In this paper we report the first complete mapping of the pituitary in a tuna species. The various different adenohypophysis cell types of the bluefin tuna, Thunnus thynnus L. have been identified and located using different antisera against mammalian and piscine hormones and various histochemical techniques: PAS, Alcian Blue pH 2.5 and lectins -ConA and WGA(Neutral and Acidic Glycoproteins); Bromophenol Blue (Proteins) and Tioglycollate-Ferric-Ferricianide-FeIII (-S-S- groups). Prolactin (PRL) and adrenocorticotrophic (ACTH) cells were located in the rostral pars distalis (RPD) of the pituitary, while the proximal pars distalis (PPD) displayed gonadotrophic (GTH), thyrotrophic (TSH), somatotrophic (GH) and also a few PRL cells. Moreover, somatolactin (SL) and melanotrophic (MSH) cells were identified inside the pars intermedia (PI). Interestingly, some SL-immunoreactive fibers were also detected in the neurohypophysis. Some GTH cells were also located on the exterior surface of the PI. Glycoproteins containing mannose (Man) and/or glucose (Glc); N-acetyl-glucosamine (GlcNAc) and/or sialic acid sugar residues, as well as -S-S- groups, were observed in GTH, TSH and SL cells. The Bromophenol Blue technique stained amphiphilic SL, acidophilic GH cells and weakly ACTH cells. GH and ACTH cells were unreactive to PAS, Alcian Blue, Tioglycollate-Ferric-Ferricianide-FeIII and lectin (Con A and WGA) techniques. Finally, PAS reaction was positive in amphiphilic SL cells, which were PbH unreactive, while MSH and ACTH cells were stained with PbH technique.

Histochemical study of skin and gills of Senegal sole, Solea senegalensis larvae and adults.

A battery of horseradish peroxidase-conjugated lectins (Con A, WGA and DBA), as well as conventional histochemical techniques (PAS, saponification, Alcian Blue pH 0.1, 1, 2.5, chlorhydric hydrolisis, neuraminidase, Bromophenol blue, Tioglycollate reduction and Ferric-ferricyanide-FeIII) were used to study the content and distribution of carbohydrates, proteins and glycoconjugate sugar residues on the skin and gills of Senegal sole, Solea senegalensis larvae and adults: During larval development of Solea senegalensis (from hatching until day 45 posthatching), epidermal sacciform, as well as branchial and epidermal chloride cells were unreactive with all cytochemical tests performed in this paper. Mucous or goblet cells of the corporal skin and gills containing strongly sulphated acid glycoproteins were evident on days 15-20 of larval development, as well as in epidermal and branchial mucous cells of adult specimens, which also contained GlcNAc and/or sialic acid. In adult specimen, the proteic content was higher in branchial mucous cells than in epidermal cells. In larvae, variable amounts of glycoproteins containing sialic acid, GlcNAc, GalNAc, Man and/or Glc residues were observed in epithelial cells and/or cuticle. GlcNAc and/or sialic acid sugar residues were only weakly detected in glycoproteins of some epidermal and branchial mucous cells of larvae by day 45, because from hatching until metamorphosis, lectin reactions (WGA, Con A and DBA) were negative in mucous cells.

Regional sex differences in spine density along the apical shaft of visual cortex pyramids during postnatal development.

Dendritic spines from the apical shaft of layer V pyramids were counted on Golgi-stained sections of the monocular subfield of the primary visual cortex of 10-, 20-, 40- and 60-day-old male and female rats. Dendritic segments located in layer IV and at 100-300 microns from the soma had a significantly higher spine content in 10-day-old females when compared to males. This sex difference was extended to outer dendritic segments with increasing age, and became restricted to dendritic segments of outer layers (II-III) located at 400-550 microns from the perikaryon in 40-day-old rats. Sex differences in spine content finally disappeared by day 60. These results show the existence of specific laminar and temporal sex differences in the development of dendritic spines in the apical shaft of visual cortex pyramids.

Developmental sex differences and effect of ovariectomy on the number of cortical pyramidal cell dendritic spines.

Dendritic spines in the apical shafts of layer V pyramidal neurons were quantitatively assessed in Golgi preparations of visual cortex from male and female rats, 10-60 postnatal days old. The number of dendritic spines increased in both sexes from days 10 to 20. However, we found that 10-day-old females had a significantly greater number of dendritic spines than males of the same age. The absolute difference further increased with increasing age, reaching a maximum at 20 days. Then, the number of dendritic spines progressively decreased in females between days 20 and 60, showing a certain tendency to increase in males during the same period. As a result of this different developmental trend, the number of spines was similar in 60-day-old male and female rats. Ovariectomy of females, at day 30, prevented the decrease in the number of dendritic spines. These results indicate that spine development in the rat visual cortex is dependent on sex, suggesting a possible influence of sex steroids on pyramidal cell maturation.

Binding characteristics and daily rhythms of melatonin receptors are distinct in the retina and the brain areas of the European sea bass retina (Dicentrarchus labrax).

Melatonin is synthesized, with a circadian rhythm, in the pineal organ of vertebrates, high levels being produced during the scotophase and low levels during the photophase. The retina also produces melatonin, although in the case of the European sea bass, its secretion pattern appears to be inverted. In the study described here, radioreceptor assay techniques were used to characterize the melatonin binding sites, their regional distribution and their daily variations. Brain and retina membrane preparations were used in all the binding assays and 2-[125I]iodomelatonin ([125I]Mel) as radioligand at 25 degrees C. The specific binding of [125I]Mel was seen to be saturable, reversible, specific and of high affinity. In all the tissues assayed, the power of the ligands to inhibit [125I]Mel binding decreased in the following order: melatonin>>4-P-PDOT>luzindole> or =N-acetylserotonin, which points to the presence of Mel1-like receptors. The inhibition curves of 4-P-PDOT suggested the presence of two different binding sites in the brain areas, but only one type of site of low affinity in the neural retina. No daily variations in [125I]Mel binding capacity (Bmax) or affinity (Kd) were detected in the brain areas, while a clear rhythm in Kd melatonin receptor affinity and Bmax binding capacity was observed in the retina. Kd and Bmax retinal rhythms were out of phase with the lowest Kd and the highest Bmax occurring at scotophase. This result suggests that retinal melatonin is a paracrine factor able to control receptor desensitization during photophase when ocular melatonin is higher in this species.

Aging reverts to juvenile conditions the synaptic connectivity of cerebral cortical pyramidal shafts.

Quantitative analysis of the total number and distribution of dendritic spines along the apical shafts of layer V cerebral cortical pyramids has been performed on aging rats (90-120 to 1,135 days old) and on rats during the period of early and late development (10-80 days). As expected from previous work, present results show that the total number of dendritic spines along the shafts increase from 10 to 80 days, after which it starts to gradually decrease until the last age studied (1,135 days). The quantitative analysis of the effect of aging on the relative decrease of dendritic spines shows that this decrease starts being homogeneous along the whole length of the apical shafts and that from a certain age onwards, estimated according to present results in 400 days, this effect is significantly more pronounced in layers IV and III-II than in deep layers. Furthermore, the comparison made between the distribution of dendritic spines along the apical shafts of pyramidal neurons of old and young animals has shown that aging produces a regression of this distribution to juvenile conditions.

Daily locomotor activity and melatonin rhythms in Senegal sole (Solea senegalensis).

The daily locomotor and melatonin rhythms of the Senegal sole, a benthonic species of increasing interest in aquaculture, are still unknown, despite the fact that such knowledge is of prime importance for optimising its production. The aim of the present research was therefore to investigate the daily rhythms of locomotor activity and melatonin in the Senegal sole. For this purpose, the individual locomotor activity rhythms of fish were registered using a photocell. Plasma and ocular melatonin rhythms were studied in animals reared in circular tanks placed in earth under an LD 12:12 light regime and 16-18 degrees C temperature range (spring equinox). Blood and eye samples were taken every 3 h during a complete 24-h cycle. The impact of a light pulse in the middle of the dark period (MD) on plasma melatonin was also studied. Locomotor activity was mainly nocturnal, with 84.3% of the total activity occurring during darkness. The levels of plasma melatonin were higher at night (55 pg/ml) than during the day (2 pg/ml), while ocular melatonin levels appeared to be arrhythmic. Both weight and melatonin content were found to be significantly higher in the left eye in relation to the right eye. A light pulse in MD provoked a significant decrease in plasma melatonin levels. In summary, photoperiod is a key factor in synchronizing locomotor activity and melatonin rhythms in the Senegal sole, whose nocturnal habits should be taken into account for their rearing by aquaculture.

Cytoarchitectonic study of the brain of a perciform species, the sea bass (Dicentrarchus labrax). I. The telencephalon.

A cytoarchitectonic analysis of the telencephalon of the sea bass Dicentrarchus labrax, based on cresyl violet-stained serial transverse sections, is presented. Rostrally, the brain of the sea bass is occupied by sessile olfactory bulbs coupled to telencephalic hemispheres. The olfactory bulbs comprise an olfactory nerve fiber layer, a glomerular layer, an external cellular layer, a secondary olfactory fiber layer, and an internal cellular layer. Large terminal nerve ganglion cells are evident in the caudomedial olfactory bulbs. We recognized 22 distinct telencephalic nuclei which were classified in two main areas, the ventral telencephalon and the dorsal telencephalon. The ventral telencephalon displays four periventricular cell masses: the dorsal, ventral, supracommissural, and postcommissural nuclei; and four migrated populations: the lateral, central, intermediate, and entopeduncular nuclei. In addition, a periventricular cell population resembling the lateral septal organ reported in birds is observed in the ventral telencephalon of the sea bass. The dorsal telencephalon contains 13 nuclei, which can be organized into five major zones: the medial part, dorsal part, lateral part and its ventral, dorsal, and posterior divisions, the central part, and posterior part. Based on histological criteria, two cell masses are recognized in the ventral division of the lateral part of the dorsal telencephalon. The nucleus taenia is found in the caudal area of the dorsal telencephalon, close to the ventral area. This study represents a useful tool for the precise localization of the neuroendocrine territories and for the tracing of the neuronal systems participating in the regulation of reproduction and metabolism in this species.