Activity of the pituitary gland in embryo and larval stages of coho salmon, Oncorhynchus kisutch

Prolactin-dependent modulation of organogenesis in the vertebrate: Recent discoveries in zebrafish

The scientific literature is replete with evidence of the multifarious functions of the prolactin (PRL)/growth hormone (GH) superfamily in adult vertebrates. However, little information is available on the roles of PRL and related hormones prior to the adult stage of development. A limited number of studies suggest that GH functions to stimulate glucose transport and protein synthesis in mouse blastocytes and may be involved during mammalian embryogenesis. In contrast, the evidence for a role of PRL during vertebrate embryogenesis is limited and controversial. Genes encoding GH/PRL hormones and their respective receptors are actively transcribed and translated in various animal models at different time points, particularly during tissue remodeling. We have addressed the potential function of GH/PRL hormones during embryonic development in zebrafish by the temporary inhibition of in vivo PRL translation. This treatment caused multiple morphological defects consistent with a role of PRL in embryonic-stage organogenesis. The affected organs and tissues are known targets of PRL activity in fish and homologous structures in mammalian species. Traditionally, the GH/PRL hormones are viewed as classical endocrine hormones, mediating functions through the circulatory system. More recent evidence points to cytokine-like actions of these hormones through either an autocrine or a paracrine mechanism. In some situations they could mimic actions of developmentally regulated genes as suggested by experiments in multiple organisms. In this review, we present similarities and disparities between zebrafish and mammalian models in relation to PRL and PRLR activity. We conclude that the zebrafish could serve as a suitable alternative to the rodent model to study PRL functions in development, especially in relation to organogenesis.

Developmental ontogeny of prolactin and its receptor in fish

Prolactin (PRL) is a member of a family of structurally similar proteins which includes growth hormone (GH) and somatolactin (SL) in teleost fish. The genes encoding these proteins are expressed principally in the pituitary gland and sequence analysis reveals they share considerable similarity. GH, PRL, and SL bring about their physiological action by binding to specific receptors localised in the membrane of cells in target tissue. The PRL receptor (PRLR) and GH receptor (GHR) have been identified in a number of teleosts but the SL receptor remains to be characterised. On hormone binding, receptors dimerise, and signal transduction occurs via the JAK/STAT signalling pathway. The principal action of PRL in fish is freshwater osmoregulation, although it has also been implicated in reproduction, behaviour, growth, and immunoregulation. The role of PRL in early development and metamorphosis is well established, respectively, in mammals and amphibians, although its role in fish is not so well known. Studies have shown that PRL mRNA and protein are restricted to the developing pituitary gland in fish embryos and larvae. PRLR mRNA and protein is also present in fish embryos and has a widespread tissue distribution in larvae. The levels of PRLR and PRL mRNA vary throughout embryonic and early larval development. The potential role of PRL in fish embryos and larvae is considered in relation to their physiological status.

Ontogeny of osmoregulation in postembryonic fish: a review

Salinity and its variations are among the key factors that affect survival, metabolism and distribution during the fish development. The successful establishment of a fish species in a given habitat depends on the ability of each developmental stage to cope with salinity through osmoregulation. It is well established that adult teleosts maintain their blood osmolality close to 300 mosM kg(-1) due to ion and water regulation effected at several sites: tegument, gut, branchial chambers, urinary organs. But fewer data are available in developing fish. We propose a review on the ontogeny of osmoregulation based on studies conducted in different species. Most teleost prelarvae are able to osmoregulate at hatch, and their ability increases in later stages. Before the occurrence of gills, the prelarval tegument where a high density of ionocytes (displaying high contents of Na+/K+-ATPase) is located appears temporarily as the main osmoregulatory site. Gills develop gradually during the prelarval stage along with the numerous ionocytes they support. The tegument and gill Na+/K+-ATPase activity varies ontogenetically. During the larval phase, the osmoregulatory function shifts from the skin to the gills, which become the main osmoregulatory site. The drinking rate normalized to body weight tends to decrease throughout development. The kidney and urinary bladder develop progressively during ontogeny and the capacity to produce hypotonic urine at low salinity increases accordingly. The development of the osmoregulatory functions is hormonally controlled. These events are inter-related and are correlated with changes in salinity tolerance, which often increases markedly at the metamorphic transition from larva to juvenile. In summary, the ability of ontogenetical stages of fish to tolerate salinity through osmoregulation relies on integumental ionocytes, then digestive tract development and drinking rate, developing branchial chambers and urinary organs. The physiological changes leading to variations in salinity tolerance are one of the main basis of the ontogenetical migrations or movements between habitats of different salinity regimes.

Immunocytochemical and ultrastructural characterization of mammosomatotrope-, growth hormone-, and prolactin-cells from the gilthead sea bream (Sparus aurata l., Teleostei): an ontogenic study

Growth hormone (GH), prolactin (PRL), and mammosomatotrope (MS) cells of gilthead sea bream, Sparus aurata, a teleost fish, were studied in specimens from hatching to 15 months (adults) using conventional electron microscopy and an immunogold method using anti-tilapia GH sera and anti-chum salmon PRL serum. MS cells, immunoreactive to both anti-GH sera and anti-PRL sera, had been first identified in fish in a previous study in newly hatched larvae and in older larvae and juvenile specimens of Sparus aurata by light microscopic immunocytochemistry. In the present work, MS cells reacted positively to immunogold label only in older larvae and juveniles and their secretory granules immunoreacted with both GH and PRL antisera or with only one of them. MS cells were ultrastructurally similar to the PRL cells, with which they coincided in time. This is the first report on the ultrastructural characterization of MS cells in fish. In adults, the secretory granules of GH cells (immunoreactive to anti-GH serum) were mainly round, of variable size, and had a homogeneous, highly electron-dense content. Irregularly shaped secretory granules were also present. PRL cells (immunoreactive to anti-PRL serum) were usually observed in a follicular arrangement; they showed few, small, and mainly round secretory granules with a homogeneous and high or medium electron-dense content. Some oval or elongated secretory granules were also observed. GH and PRL cells that showed involutive features were also found. In newly hatched larvae, GH, PRL, and MS cells could not be distinguished either by their ultrastructure or by the immunogold labeling of the secretory granules. In 1-day-old larvae, presumptive GH and PRL cells were observed according to their position in the pituitary gland. In 2-day-old larvae, a few cells showed some of the ultrastructural features described for GH and PRL cells of adults. During development, the number, size, and shape of the secretory granules in both cell types clearly increased and the organelles developed gradually. Some GH cells were found undergoing mitosis.

Metamorphosis in the summer flounder, Paralichthys dentatus: thyroidal status influences gill mitochondria-rich cells

Metamorphosis in the summer flounder (Paralichthys dentatus) is mediated by thyroid hormones (TH) and is accompanied by changes in gill mitochondria-rich cells (MRCs) and in salinity tolerance. Altered thyroid status during larval development and metamorphosis in this species influences salinity tolerance, though the influence of any hormone on MRCs of larval marine teleosts is not known. This study characterized the effect of altered thyroid status on MRC intracellular membranes, mitochondria size and ultrastructure, immunoreactive (ir)-Na(+),K(+)-ATPase, and cell size and density during metamorphosis in summer flounder. Inhibition of metamorphosis with thiourea (30 ppm) (TU, an inhibitor of TH synthesis) inhibited changes in MRCs, producing large "larval" type MRCs with weak reactivity to osmium; large, electron-lucent mitochondria; and weak ir-Na(+),K(+)-ATPase. Replacement of TH with TU + thyroxine-Na salt (100 ppb) rescued the fish from developmental inhibition, producing smaller "juvenile" type MRCs with strong reactivity to osmium; smaller, electron-opaque mitochondria; and strong ir-Na(+), K(+)-ATPase. The findings suggest that TH are necessary for MRCs to change from larval to juvenile form during metamorphosis.

Presence of Na-K-ATPase in mitochondria-rich cells in the yolk-sac epithelium of larvae of the teleost Oreochromis mossambicus

The purpose of this study is to provide biochemical evidence for the functions of the mitochondria-rich cell (MR cell) in the yolk-sac epithelium of the developing larvae of tilapia Oreochromis mossambicus. Western blotting with the antibody (6F) raised against avian Na-K-ATPase alpha1 subunit demonstrated the presence of Na-K-ATPase in yolk-sac epithelium of tilapia larvae and about 1. 46-fold more of the enzyme in seawater larvae than in freshwater ones. The yolk-sac MR cells were immunoreacted to the antibody (alpha5) against the alpha subunit of avian Na-K-ATPase and were double-labeled with anthroylouabain and dimethylaminostyrylethyl-pyridiniumiodine, suggesting the existence and activity of Na-K-ATPase in these cells. Binding of 3H-ouabain in the yolk sac of seawater larvae was much higher than in that of freshwater larvae (4.183+/-0.143 pmol/mg protein versus 1.610+/-0. 060 pmol/mg protein or 0.0508+/-0.0053 pmol/yolk sac versus 0. 0188+/-0.0073 pmol/yolk sac). These biochemical results are further evidence that yolk-sac MR cells are responsible for a major role in the osmoregulatory mechanism of early developmental stages before the function of gills is fully developed.

Early organization of the pituitary gland in Sparus aurata L. (Teleostei). An ultrastructural study

The cell organization of the pituitary gland and the relationship between neurohypophysis and adenohypophysis in early developmental stages of the gilthead sea bream, Sparus aurata, were studied by electron microscopy. In newly hatched larvae, the pituitary gland was embedded in the ventral floor of the diencephalon and separated from the hypothalamus by a continuous basal lamina. Elongated mesenchymal cells next to the ventral surface were observed. At this stage, there was no neurohypophysis and the adenohypophysis consisted of undifferentiated endocrine cells with small scarce secretory granules and a few stellate cells, with no distinctive zonation. An incipient neurohypophysis was present in 1-day-old larvae. The first evagination of the neurohypophysis into the adenohypophysis were observed in 2-day-old larvae and developed progressively with age, being deeper in the caudal zone. Two regions in the adenohypophysis, one anterior--the presumptive pars distalis--and one posterior--the presumptive pars intermedia--were found in 2-day-old larvae. Three regions (rostral and proximal pars distalis and pars intermedia) were clearly distinguishable in 4-day-old larvae. The ultrastructural features of the pituitary endocrine cells varied during gland differentiation, with the secretory granules gradually increasing in number and size, accompanying organelle development. Nevertheless, even in the oldest larvae studied (65 days), undifferentiated cells similar to those in the earliest stages were observed. The first blood vessels appeared in the neurohypophysis around 16 days after hatching. During early development, the pituitary gland progressively emerged from the ventral floor of the brain. By 16 days, the principal pattern of the pituitary gland architecture appeared to be established.

Investigations into the development of the pituitary gland-thyroid tissue axis and distribution of tissue thyroid hormone content in embryonic coho salmon (Oncorhynchus kisutch) from Lake Ontario

Total organism content of L-thyroxine (T4) and triiodo-L-thyronine (T3) were measured in the early developmental stages of a stock of Lake Ontario coho salmon from the egg to the yolk absorption stage. Whole organism T4 levels were constant between the egg and pre-hatch embryo stages, but fell progressively during yolk absorption. T3 levels were low from egg to eye-pigment appearance, but then increased prior to hatch and fell again during the post-hatch yolk absorption period.When expressed as ng/tissue, T4 content of the body compartment rose progressively between days 67 and 87 post-fertilization, whilst T4 content of the yolk compartment fell progressively during the same period; the pattern was not evident for tissue T3 content. When expressed as ng/g dry weight of tissue, the inverse relationship was found for T4, and T3 content of the body and yolk compartments decreased progressively and increased progressively, respectively during the same period, suggesting that thyroid hormones were selectively retained in the yolk compartment.Intensely "immunostained" (using anti-human β-TSH antibody) thyrotropic cells were present in small numbers in the pars distalis of the embryonic pituitary at the eye-pigment appearance stage, and the numbers increased markedly until the pre-hatch period.Administration of either bovine thyrotropic hormone (bTSH) or ovine growth hormone (oGH) had no effect on thyroid hormone content of larvae challenged during the yolk absorption period, suggesting that the thyroid tissue was not responsive to exogenous bTSH challenge at this time, and that oGH-sensitive 5'-monodeiodination was either not present or at levels that were too low to cause an elevation in total T3 content, or that the substrate levels were insufficient to permit a measureable increase in whole body T3 content.

Immunocytochemistry of somatotrophs, gonadotrophs, prolactin and adrenocorticotropin cells in larval sea bream (Sparus auratus) pituitaries

The chronological appearance of endocrine cells in the pituitary of sea-bream (Sparus auratus) larvae was studied using antisera against salmon prolactin, trout growth hormone, salmon gonadotropin and N-terminal human adrenocorticotropin. The larval pituitary (1-12 days after hatching) was oval in shape and was composed of a dense mass of cells with few neurohypophysial fibres. By 60 days after hatching it began to resemble the adult and was divisible into a distinct rostral pars distalis containing prolactin and adrenocorticotropin cells; a proximal pars distalis containing somatotrophs and gonadotrophs and a pars intermedia. Cells immunoreactive with antisera against growth hormone were observed immediately after hatching (2 days post-fertilization). Weakly staining prolactin cells were observed 2 days later in the region corresponding to the rostral pars distalis. Cells immunoreactive with antigonadotropin and anti-adrenocorticotropin sera were observed in the pituitary 6 and 8 days after hatching, respectively. All the cell-types studied were immunoreactive from the time they were first identified until the final samples 90 days after hatching.

Chronological appearance of the different hypophysial hormones in the pituitary of sea bass larvae (Dicentrarchus labrax) during their early development: an immunocytochemical demonstration

Antisera raised against chum salmon prolactin (PRL), rainbow trout growth hormone (GH), mammalian adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and luteinizing hormone (LH) were used to study the chronological appearance of immunoreactivity for PRL, GH, ACTH, TSH, LH, and melanocyte-stimulating hormone (MSH) in the pituitary of sea bass larvae (Dicentrarchus labrax) during the first 26 days after hatching. The anti-ACTH gives positive immunostaining in the ACTH cells as well as in the MSH cells; however, the two cell types can easily be distinguished by their different localization in the pituitary: ACTH in the rostral pars distalis, MSH in the pars intermedia. The first day after hatching cells immunoreactive for TSH, GH and ACTH could already be noticed, ACTH reacted strong in the pars intermedia but very weak in the rostral pars distalis. Cells immunopositive for PRL became visible between Days 9 and 15. With anti-LH, no positive reaction could be obtained during the first 26 days after hatching.

Changes in tissue and blood concentrations of thyroid hormones in developing chum salmon

The changes in tissue and blood concentrations of thyroxine (T4) and triiodothyronine (T3) were examined during development of the chum salmon (Oncorhynchus keta). Extraction methods previously established for tissue T4 were also validated for tissue T3, by parallel displacement curves to T3 standard in the radioimmunoassay and by the same elution patterns of immunoreactivity in a HPLC system. The T3 concentration of the eggs just after fertilization (4-9 ng/g) was lower than the T4 concentration (5-15 ng/g). Both T4 and T3 concentrations in the whole body decreased steadily during yolk absorption, primarily due to the decline of the hormone content in the yolk. Both T4 and T3 were detected in blood plasma at later stages of yolk absorption, and the plasma levels increased toward the end of yolk absorption. At the end of yolk absorption, when the larvae emerge from the gravel bed, a transient increase in whole body concentrations of T4 and T3 was observed. Plasma levels of T4 were always greater than the T3 levels. Thyroid follicles began to develop during the early stages of yolk absorption. These findings suggest important roles of maternal thyroid hormones for developing salmon embryos during yolk absorption.

Changes in thyroid hormone levels in eggs and larvae and in iodide uptake by eggs of coho and chinook salmon,Oncorhynchus kisutsch andO. tschawytscha

Developmental profiles of thyroxin (T4), triiodothyronine (T3) and radioactive iodide uptake were established for eggs and T4 and T3 profiles were established for larvae (whole-body, yolk-only and body-only) of coho and chinook salmon. T4 and T3 were consistently present in all samples. In eggs, hormone levels remained fairly constant in all cohorst for at least the first three weeks of incubation, but then fluctuated in both directions in some sample groups. Large increases in T4 (from 9 ng/g to 245 ng/g) were seen in 1985 chinook eggs 28 days after fertilization. Radioactive iodide uptake (which was used as a possible indicator of thyroxinogenesis) increased at least 10-fold in both 1986 coho and chinook eggs from 23-30 days after fertilization. T4 (62 ng/g) and T3 (393 ng/g) were found in the bodies of 28-day-old 1986 chinook embryos. In whole larvae, hormone levels varied depending upon the cohort studied. In general, initial body-only concentrations of both T4 and T3 decreased as body weight increased, but before yolksac resorption was completed, both thyroid hormone content and concentration increased (except for chinook T3). T4 and T3 content in larval yolk stayed constant as yolksac size decreased, resulting in increased thyroid hormone concentration in the yolksac. All of these data suggest that the initial source of thyroid hormones in coho and chinook salmon eggs is maternal, but that by approximately 3-4 weeks after fertilization, the developing embryos begin to produce their own thyroid hormones. After hatching, increases in tissue T4 and T3 concentration coupled with constant T4 and T3 content in diminishing yolksacs suggest that larvae also produce their own thyroid hormones; yolksac content then may reflect both the original maternal hormones and the larva-producted hormones.

Putative role of adenohypophysis in the osmoregulation of tilapia larvae (Oreochromis mossambicus; Teleostei): an ultrastructure study

Ultrastructure of the secretory cells in the adenohypophysis of tilapia (Oreochromis mossambicus) larvae hatched in fresh water or sea water was compared. Adenohypophysis of newly hatched larvae of tilapia is a short columnar body attached to the ventral floor of the diencephalon. The adenohypophysis is at its early differentiation stage, i.e., various types of secretory cells are still undistinguishable. Only part of the cells in the putative rostral pars distalis look like typical endocrine cells, containing well-developed rough endoplasmic reticulum, Golgi apparatus, and numerous secretory granules. The average size of secretory granules in freshwater-hatched larvae is significantly larger than those in seawater-hatched larvae (12,936 +/- 2854 nm2, N = 11 vs 3375 +/- 810 nm2, N = 10), suggesting some role of adenohypophysis in the osmoregulation of the early developmental stage of teleosts.

Effect of osmotic pressure on prolactin release in rainbow trout: in vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

Thyroid hormones in blood plasma of developing salmon embryos

Blood plasma thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) concentrations in developing embryos of chum (Oncorhynchus keta), coho (Oncorhynchus kisutch), chinook (Oncorhynchus tschawytscha), and Atlantic salmon (Salmo salar) were measured by radioimmunoassay between hatching and completion of yolk-sac absorption. Blood sampling was initiated when approximately 50% of the yolk that was present at hatching had been absorbed. At this time blood plasma levels of T4 (7-9 ng/ml) and T3 (1-5 ng/ml) were similar in all species. In embryos of each salmon species blood plasma levels of T4 increased to maximal values (9-16 ng/ml) when both differentiation of the abdominal body wall and absorption of the yolk sac were completed. Blood plasma levels of T3 generally decreased to low or nondetectable values during this time. There was either a trend toward a decrease or a stasis of levels of T4 and at the same time in some species an increase in levels of T3 in the blood plasma following the completion of yolk-sac absorption and the onset of exogenous feeding. These results demonstrate that thyroid hormones are present in the blood of developing teleost embryos. This finding is discussed in light of the possible role of thyroid hormones in control of the embryonic and larval development of fishes.

Plasma and pituitary prolactin levels in rainbow trout during adaptation to different salinities

The development of a highly specific radioimmunoassay for salmonid prolactin (PRL) using chinook salmon PRL allowed us to study plasma and pituitary PRL profiles in large sedentary rainbow trout (Salmo gairdneri) transferred from fresh water to seawater and vice versa. Plasma osmotic pressure and chloride levels were also measured for 3 weeks following change of salinity. Within 1 day after transfer to full seawater we observed a plasma PRL decrease, which stayed significantly lower (3-5 ng/ml) than the fresh water control group (10-15 ng/ml) during the entire experiment. Pituitary PRL content showed an initial abrupt increase, but after 3 weeks in seawater pituitary PRL content had decreased to the same level as in the fresh water control group. On the contrary, transfer from seawater to fresh water was followed within 1 day by a rise in plasma PRL levels, which stayed high (10-15 ng/ml) after 3 weeks in fresh water. Simultaneously, pituitary PRL content decreased significantly. These results may indicate an important role of PRL in fresh water adaptation of sedentary rainbow trout.