Preoptic gonadotropin-releasing hormone (GnRH) neurons innervate the pituitary in teleosts

In most teleosts, there are three groups of gonadotropin-releasing hormone (GnRH) neurons. In this study we addressed the question of GnRH neuronal innervation of the pituitary in the dwarf gourami and the tilapia using immunocytochemistry combined with biocytin tract tracing. Biocytin was applied to the pituitary attached to the brain in vitro. Similar results were obtained in both species. GnRH neurons retrogradely labeled with biocytin were observed only in the preoptic area. These results indicate that preoptic GnRH neurons innervate the pituitary. Negative labeling of biocytin in the terminal-nerve and midbrain GnRH neurons suggests that these two GnRH neuronal populations do not project to the pituitary. Biocytin-positive but GnRH-negative neurons were also observed in the preoptic area and the ventromedial parts of the hypothalamus, suggesting neuropeptidergic and aminergic innervation of the pituitary besides GnRH.

Histology and ultrastructure of the gut of the tilapia (Tilapia spp.), a hybrid teleost

The morphology of the intestine has been studied in a species of warm water fish, Tilapia spp., a hybrid teleost of notable economic importance. Light and electron microscope results show that the intestine is a relatively undifferentiated muscular tube lined with a simple columnar epithelium interspersed with goblet cells. The proximal region has a greater surface area, manifested by elongated mucosal ridges. The enterocytes are covered apically with uniform microvilli and exhibit the typical ultrastructural features of pinocytosis, namely extensive invaginations of the luminal plasma membrane and massive accumulation of vesicles in the apical cytoplasm. The distal intestine mucosa is thinner and less elaborately folded and consists of columnar cells with shorter and sparser microvilli. Their supranuclear cytoplasm contains abundant clear vacuoles. Numerous endocrine cells can also be seen. Regional cellular ultrastructural features are correlated with digestive functions.

Telencephalic ascending gustatory system in a cichlid fish, Oreochromis (Tilapia) niloticus

Central fiber connections of the gustatory system were examined in a percomorph fish Oreochromis (Tilapia) niloticus by means of the horseradish peroxidase (HRP), biocytin, and carbocyanine dye tracing methods. The primary gustatory areas in tilapia are the facial, glossopharyngeal, and vagal lobes of the medulla. The secondary gustatory nucleus (SGN) is a dumb-bell-shaped structure located in the isthmic region. In the SGN, there are two or three layers of neurons lining the ventromedial periphery of the nucleus and a molecular layer constituting of the major part of the nucleus. The SGN receives bilateral projections from the facial lobes and ipsilateral projections from the glossopharyngeal and vagal lobes. Ascending fibers originating from the SGN form the ipsilateral tertiary gustatory tract. A major part of the tract courses rostrally and terminates ipsilaterally in several diencephalic nuclei: the preglomerular tertiary gustatory nucleus (pTGN), the posterior thalamic nucleus, the nucleus diffusus lobi inferioris, the nucleus centralis of inferior lobe, and the nucleus recessus lateralis. The remaining small fiber bundle enters the medial and lateral forebrain bundles and terminates directly in two telencephalic regions; the area ventralis pars intermedia (Vi) and the area dorsalis pars posterior (Dp). Ascending fibers from the pTGN pass through the lateral forebrain bundle and terminate ipsilaterally in the dorsal region of area dorsalis pars medialis (dDm) of the telencephalon. Following biocytin injections into the dDm, small, round cells were labeled in the pTGN. After biocytin injections into the Vi and Dp of the telencephalon, retrogradely labeled cells were found in the ipsilateral SGN. The results show that the ascending fiber connections of the central gustatory system in the percomorph teleost tilapia are essentially similar to those of mammals. That is, the pathway from the primary gustatory areas (facial, glossopharyngeal, and vagal lobes) through the SGN and pTGN to the dDm in tilapia corresponds with the mammalian gustatory pathway from the solitary nucleus through the pontine taste areas (nucleus parabrachialis) and the thalamic relay nucleus (ventral posteromedial nucleus) to gustatory neocortices. In addition, the pathway from the primary gustatory areas through the SGN to the Vi and Dp in tilapia corresponds with the pathway from the solitary nucleus through the pontine taste areas to the amygdala in mammals.

Gonadotropin-releasing hormone (GnRH) innervation of the pituitary in a cichlid fish, Oreochromis niloticus: a brain lesion study

In most vertebrates, multiple gonadotropin-releasing hormone (GnRH) neuronal groups have been reported. In tilapia three GnRH neuronal groups (terminal nerve, preoptic, midbrain) have been reported. Which of the three GnRH cell groups regulate the pituitary is not well known. We performed brain lesions of each neuronal group and studied immunocytochemically the changes of GnRH fiber distribution in the pituitary. Lesions of the preoptic cell group resulted in almost complete absence of GnRH fibers in the neurohypophysis of the proximal pars distalis. After lesions of the terminal nerve GnRH cell group, no changes were observed in the distribution of GnRH fibers in the pituitary. Lesions of the midbrain cell group were unsuccessful because of high mortality. The present study indicates that the preoptic GnRH cell group is the main contributor of the pituitary innervation.

Ultrastructural study on the stomach of Tilapia spp (Teleostei)

An ultrastructural study has been made of gastric mucosa of a teleostean fish, Tilapia spp. The cytological features of the surface mucous cells, mucous neck cells, glandular cells and endocrine cells are described. The surface mucous cells, identified by their superficial localization, are characterized by apical granules. The mucous neck cells are distinguished by the appearance of their mucous granules and their localization between surface mucous cells and glandular cells. The gastric glands contain only one form of cell whose fine structure is similar to cells that secrete hydrochloric acid. Physiological implications of some ultrastructural features are also discussed.

Morphometry of the kidneys of fresh water percoid and ostariophysian teleosts

Kidneys of four fresh water teleost species representing percormorpha (Oreochromis niloticus and Micropterus salmoides) and Ostariophysi (Cyprinus carpio and Clarias mossambicus) were studied for quantitative structural characteristics using tissues fixed by perfusion. The renosomatic index (weight of kidney/weight of fish) was higher in the ostariophysian than percoid fish (p < 0.05) but values of volume of nephronic tissue/body weight were not significantly different. Results obtained by point counting morphometry showed that the kidneys of the percoid species were made up of 56.85-64.4% nephronic tissue, 30.45-32.47% blood vessels, 2.84-8.20% ureter and ureteral ducts and 2.07-2.48% connective tissue. The kidneys of the osariophysian species comprised of 35.27-36.71% nephronic tissue, 27.86-29.29% blood vessels, 1.59-4.20% ureter and its ducts, 29.07-29.51% interstitial cells and 2.70-3.60% connective tissue. The mean values for the volume proportions of the components of nephronic tissue were not significantly different in the percoids and ostariophysians and they were; renal corpuscles 3.84-6.54%, neck segments 0.50-1.11%, proximal segments 67.17-69.72%, distal segments 18.16-20.55%, and collecting tubule-collecting duct (CT-CD) system 5.66-7.41%. These results show that the quantitative structural characteristics of the kidneys of fresh water species from different orders are quite similar and emphasise the all persuasive influence of the environment on renal structure and function in teleosts.

Occurrence of axons with certain immunohistochemical markers in teleost gingiva and teeth

The occurrence of axons with certain neurochemical markers in gingiva and teeth in the lower jaw of the cichlid Tilapia mariae was examined by immunohistochemistry. It was found that gingival and pulpal axons can be labelled with antibodies against calcitonin gene-related peptide, substance P, tyrosine hydoxylase, neuropeptide Y, choline acetyl transferase and vasoactive intestinal polypeptide. It is concluded that both gingival and dental domains in T. mariae contain axons with sensory and autonomic chemical phenotypes, the occurrence and distribution of which are generally similar to the mammalian counterpart.

A method for mass harvesting islets (Brockmann bodies) from teleost fish

In certain species of fish, the insulin-producing tissue is uniquely located in separate structures called Brockmann bodies (BBs). Tilapia BBs have been shown to be a simple and inexpensive source of islet cells for xenotransplantation research. Each donor tilapia contains roughly 12-15 BBs, measuring from 0.3 to 5.0 mm in maximum dimension, in a triangular region of adipose tissue bounded by the liver, stomach, and spleen/gallbladder. At present, the larger BBs (usually 2-4) are harvested by microdissecting these "BB regions" using jeweler's forceps and microvascular scissors while being visualized with the aid of a dissecting microscope. It is a simple but time-consuming task that would not be applicable for harvesting massive amounts of BB tissue for large animal studies. Therefore, we have developed an easier and more efficient method of harvesting BBs based on a standard enzymatic method for isolating human adipocytes. BB regions are harvested from donor fish and pooled into a 50 mL plastic tube containing collagenase Type II (3 mg/mL) in Hank's balanced salt solution (HBSS); the tube is then placed in a 37 degrees C waterbath/shaker for roughly 15 min. The exact length of the digestion interval is determined by visual inspection of the tube to determine whether the BBs have been liberated. The digestion is then stopped by adding excess cold HBSS. The adipocytes float while the BBs and residual connective tissue (i.e., a few blood vessels, nerves, and bile ducts) form a pellet. The pellet is washed several times in HBSS and then placed in a culture dish. The BBs are easily handpicked with a siliconized pipette. Based on functional data and DNA content, this new method roughly doubles or triples our yield of BB tissue per donor fish. To determine whether BBs harvested in this manner functioned in a manner similar to those harvested by microdissection, we performed a series of transplants using mass-harvested BBs. Long-term normoglycemia was achieved in streptozotocin-diabetic nude mice and mean graft survival time was not altered in streptozotocin-diabetic euthymic balb/c mice. However, the total weight of donor fish required per recipient was decreased by 50% in both strains.

Apical structures of "mitochondria-rich" alpha and beta cells in euryhaline fish gill: their behaviour in various living conditions

BACKGROUND

One of the characteristic features of the two types (alpha and beta) of "mitochondria-rich" (chloride) cells in the gill epithelium of freshwater fishes is the presence in their apical region of tubulovesicular structures. A further analysis of the ultrastructural features of these apical elements as well as that of their modifications under various living conditions should help to understand better the respective rôle of both alpha and beta cells in these conditions.

METHODS

Atlantic salmon (Salmo salar) maintained in fresh water as well as tilapia (Oreochromis niloticus) maintained either in fresh water or in deionized water or in 20% saltwater were examined. Measurements of surface areas of apical structures in the various living conditions were also performed.

RESULTS

In the alpha cells of freshwater fishes, the apical structures consisted of isolated vesicles containing a filamentous material resembling that coating the apical surface. They were closely related to the apical plasma membrane and did not penetrate the region containing the tubular system. When fishes were transferred to deionized water, the number of the apical membrane folds increased significantly, as did the number and size of apical structures which became elongated. In saltwater-adapted fishes, the apical structures showed a tendency to collapse and took the appearance of flattened and slightly curved elements. These observations tended to indicate that in alpha cells the apical structures were extensions of the apical plasma membrane and thereby might be implicated in sodium uptake when fishes are placed in fresh or deionized water and in chloride excretion when they are transferred to salt water. In beta cells, the apical structures were usually separated from the apical plasma membrane by a zone rich in cytoskeleton elements. They penetrated deeply into the supranuclear region, where they intermingled with the elements of the tubular system. They consisted mainly of tubular elements that contained a material resembling that present in the trans tubular Golgi network from which they might originate. The apical structures remained unaltered in beta cells whatever the medium (fresh or deionized water) in which the fish was placed.

CONCLUSIONS

The alpha cells which are usually thought to be mainly involved in chloride excretion when fishes are transferred into seawater might also be implicated in sodium uptake in freshwater living conditions. The rôle of beta cells, in contrast, still remains to be established.

Development of the dorsal root ganglion in a teleost, Oreochromis mossambicus (Peters)

The precursor crest cells of the spinal dorsal root ganglia (DRG) in the tilapia, Oreochromis mossambicus, were analysed by HNK-1 antibody staining, scanning electron microscopy, and DiI labeling techniques. The ontogeny of the DRG was followed in the embryos and young fry of the fish. Neural crest cells which contribute to the formation of the DRG were observed to commence their migration in the trunk region after 40 hours postfertilization. They do not penetrate the somites but travel through the space between the neural tube and the somite. Crest cells destined to become the DRG accumulate at the midsomitic region where the ventral root exits. At 50 to 80 hours postfertilization, they differentiate and become bipolar sensory cells. The DRG continues to grow and develop right through hatching at 115 hours. During the early larval stages, crest cells accumulate around the ventral root and the DRG eventually fuses with the motor root, giving rise to a situation in which the DRG contains not only the sensory cells but also motor fibres. The mixed nature of the DRG was confirmed by HRP retrograde labeling. We believe that this is the first report in describing the formation of the DRG in a teleost.

Nodes of Ranvier and myelin sheath dimensions along exceptionally thin myelinated vertebrate PNS axons

The trigeminal alveolar branch in the lower jaw of the cichlid Tilapia mariae was examined by light and electron microscopy on single and serial sections, and by light microscopy on teased fibre preparations. The principal purpose was to find out if the exceptionally thin myelinated axons (d < 1 micron) present in this nerve possess true nodes of Ranvier, and to determine the dimensions of their myelin sheaths. This necessitated analysis of the whole size range of myelinated fibres, with respect to nodal and internodal morphology. The results show that the exceptionally thin myelinated fibres exhibit primitive nodal regions, with patches of axolemmal undercoating, and few Schwann cell processes in the node gap. This contrasts with the more complex nodal organization seen in larger trigeminal alveolar branch fibres. For the whole population of myelinated fibres the number of myelin lamellae increases rectilinearly with axon diameter, and sheath length increases with fibre diameter according to a logarithmic expression. The myelin sheaths of the exceptionally thin trigeminal alveolar branch fibres are composed of 10-20 lamellae, and extend 35-50 microns along the axon. These results show that the structural complexity of nodal regions in the trigeminal alveolar branch decreases with decreasing fibre size, that the exceptionally thin myelinated trigeminal alveolar branch fibres possess primitive nodes and that they have very short myelin sheaths. Our crude theoretical calculations suggest that these fibres might be capable of saltatory conduction.