Thyroid hormones in eggs of various freshwater,marine and diadromous teleosts and their changes during egg development

Thyroid hormone concentrations in unfertilized eggs of 26 species of various freshwater, marine and diadromous teleosts were examined, together with changes in their concentrations during egg development in some species. Significant quantities of both thyroxine (T₄) and triiodothyronine (T₃) were found in eggs of all species examined. Mean T₄ and T₃ concentrations in eggs varied from 0.04 (marbled sole) to 15.00 ng/g (chum salmon), and from 0.07 (goldfish) to 9.95 ng/g (Pacific herring), respectively. T₄ concentrations were significantly greater than T₃ concentrations in eggs of most freshwater fishes, whereas T₃ concentrations were greater in seawater fishes. During the course of development, thyroid hormones in eggs decreased markedly before hatching. These findings suggest that thyroid hormones are consistently present in teleost eggs, and thus may play an important role in the egg development.     

Hepatic 5′-monodeiodinase activity in teleosts in vitro: A survey of thirty-three species

The in vitro hepatic 5′-monodeiodination of thyroxine (T₄) to triiodothyronine (T₃) in Oreochromis mossambicus, Channa striata, Clarias batrachus, Cyprinus carpio and Oxyeleotris marmorata was found to be time, pH and temperature dependent, and related to the amount of substrate (T₄) and homogenate introduced into the reaction vessel, in a manner which was consistent with Menton-Michaelis kinetics, and thus indicative of an enzyme-regulated process. Dithiothreitol introduced into the reaction vessel stimulated T₃ production in a dose-related manner. Hepatic 5′-monodeiodinase activity was also detected in a further 28 species of teleosts suggesting that the peripheral monodeiodination of T₄, which is well-documented in salmonids, is also widespread amongst other teleost fishes. All species examined exhibited evidence of enzymatic deiodination, but there were marked differences in K_m and V_max values between the species. There was no apparent phylogenetic or environmental relationships to explain the widely divergent K_m and/or V_max values, nor was there a correlation between K_m and V_max when the species were considered together.     

Propylthiouracil-induced hypothyroidism in coho salmon, Oncorhynchus kisutch: effects on plasma total thyroxine, total triiodothyronine, free thyroxine, and growth hormone

Thyroid hormones transiently increase during parr-smolt transformation in coho salmon, Oncorhynchus kisutch, and are believed to trigger morphological, physiological, behavioural, and neural changes. The effectiveness of propylthiouracil (PTU) to induce hypothyroidism in smolting coho salmon was determined by immersing coho salmon, Oncorhynchus kisutch, in 30 mg l–1 PTU from May 1, two weeks prior to the consistent annual total thyroxine (TT4) peak in mid-May, until the last sampling date. Plasma was obtained at two sampling dates from control and PTU -treated coho salmon: May 15, during the plasma TT4 peak; and May 26, after the TT4 peak. Radioimmunoassays were used to measure plasma TT4, total triiodothyronine (TT3), free thyroxine (FT4), and salmon growth hormone (sGH). The PTU -treatment inhibited the natural smoltification-related increases in plasma TT4, TT3 and GH levels compared with controls, but PTU-treatment did not affect these hormone levels when they were low. PTU -treatment increased FT4 and decreased TT3 and sGH levels in the May 26 sample. In the May 15 sample, FT4 levels were unaffected by PTU-treatment, whereas TT4 levels were decreased. These data demonstrate the ability of PTU to induce hypothyroidism in salmonids as shown by the decrease in TT4 and TT3. These data demonstrate that PTU treatment by immersion can induce hypothyroidism in salmonids as shown by: (1) the inhibition of the natural increases of TT4 and TT3; (2) the increase in FT4 levels corresponding to the lowered TT3 levels, suggesting an inhibition of thyroxine 5-monodeiodinase activity. We also show for the first time that PTU treatment can lower plasma GH levels in salmonids. This lowering of plasma GH level is associated with the decrease in TT3 levels and the increase in FT4 levels. The PTU induced lowering in GH levels may contribute to the observed changes in FT4 and TT3, since GH is known to increase thyroxine 5-monodeiodinase activity.     

半滑舌鳎(Cynoglossus semilaevis)黑色素富集激素基因的克隆和表达

采用同源克隆技术获得了佛罗里达红罗非鱼(Oreochromis sp.)前原黑色素浓集激素1 (prepro-melanin concentrating hormone 1,pmch1)基因cDNA全长序列,将其命名为flpmch1;并通过实时荧光定量PCR (qRT-PCR)分析天生黑斑佛罗里达红罗非鱼组织表达谱和低温胁迫后其各组织表达差异。结果表明,flpmch1 cDNA序列全长629 bp,开放阅读框411 bp,编码136个氨基酸,存在12个潜在磷酸化位点和1个长度为199 bp的CpG岛。Flpmch1与24个物种Pmch1氨基酸序列比对结果表明,Flpmch1与尼罗罗非鱼(O.niloticus)、奥利亚罗非鱼(O.aureus)、莫桑比克罗非鱼(O.mossambicus)、布氏新亮丽鲷(Neolamprologus brichardi)的Pmch1相似性最高。系统进化树分析显示硬骨鱼纲Pmch聚成一个大支,Flpmch1与尼罗罗非鱼Pmch1进化地位最接近。组织表达谱分析显示,flpmch1 mRNA在多个组织中均有表达,其中在脑中表达量最高,且粉白色皮肤中的表达量显著高于黑色皮肤(P<0.01)。低温胁迫后flpmch1 mRNA各组织表达量较对照组均呈下调表达。推断flpmch1可能参与了佛罗里达红罗非鱼天生黑斑的形成和过冬黑化过程。     

Histopathological assessment of gonadal tissue in wild fishes

Histology offers a powerful tool in the study of reproductive health of fishes. It is routinely used for sex verification, identifying stage of development, documenting presence of intersex, tumors, parasites and other abnormalities and quantifying atresia. It can also be used for more subtle changes such as thickness of the vitelline envelope at various stages, yolk appearance, necrosis of sperm, and Sertoli cell proliferation. Gonadal histology, in conjunction with hormone and vitellogenin measurements, morphological and fecundity studies, can provide insights into the effects of various environmental stressors on reproductive health. However, much research, both field and laboratory, is needed to understand cause and effect for observed changes and to understand the meaning of many of the histological observations made in field studies, in terms of reproductive success of fish populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

仿刺参性腺酶解过程风味变化

生物酶解是提高水产品蛋白质利用率的有效手段,且酶解过程往往伴随肽类、氨基酸、小分子挥发性成分等的生成或反应,进而引起酶解液风味变化。仿刺参(Apostichopus japonicus)性腺富含蛋白质和多种功效成分,具有良好的开发利用前景。为探究仿刺参性腺酶解过程中蛋白质和风味的变化规律,采用中性蛋白酶对其进行酶解,对酶解过程中可溶性蛋白质、氨基酸态氮、游离氨基酸组成、挥发性成分的变化情况进行了检测分析。结果显示,仿刺参性腺匀浆液中可溶性蛋白质和氨基酸态氮的初始含量分别为1.14和0.15 g/100 g,可溶性蛋白质在酶解前30 min内迅速增加,之后基本保持不变,氨基酸态氮含量在前90 min内随时间延长而逐渐增加,90 min后略有下降,90 min时水解度最大,达43.66%。随着酶解时间的延长,酶解液的鲜味有所增强,腥味减弱,甜味和苦味也略有增强。酶解后游离氨基酸含量显著增加(P<0.05),其中,呈鲜味的谷氨酸含量最高,其次为呈甜味的甘氨酸和丙氨酸。仿刺参性腺酶解过程气味发生明显变化,烃类种类和相对含量均明显增加,二甲基硫醚含量显著降低,这可能是酶解液腥味减弱的主要原因     

Thyroid hormones in brown trout (Salmo trutta) reproduction and early development

Gravid brown trout (Salmo trutta) females were injected with various doses of a synthetic gonadotropin-releasing hormone analog (GnRHa), given with or without an injection of triiodothyronine (T₃), in order to investigate the potential of T₃ (a) to enhance the stimulatory effect of GnRHa on ovulation, and (b) to enhance the growth and survival of the produced progeny. From the time the hormonal treatments were initiated until ovulation was detected 5–38 days later, endogenous plasma T₃ levels increased from an average of 3.6 to 11.6 ng ml⁻¹. Injection with 20 mg T₃ kg⁻¹ body weight, further elevated plasma T₃ levels at ovulation (16.0 ng ml⁻¹. Mean time to ovulation was reduced significantly in fish injected with 10 μg kg⁻¹ of GnRHa, whereas treatment with lower doses was ineffective. Injection with T₃ did not enhance the ovulatory response of brown trout to GnRHa. Unfertilized eggs obtained from T₃-injected females had a higher T₃ content, suggesting a transfer of T₃ from the maternal circulation into the oocytes. Maternal T₃ injection had no effect on egg fertilization rates, embryo survival to eyeing and hatching, or the prevalence of abnormal larvae at the time of hatching. Length and weight gain of the progeny during yolk absorption was also not influenced by maternal T₃ treatment. At the completion of yolk-sac absorption, progeny from females injected with T₃ had a higher prevalence of skeletal abnormalities than controls. The results suggest that in teleosts like brown trout, which have high endogenous circulating T₃ levels, treatment of females with T₃ does not enhance responsiveness to GnRHa and it has the potential for deleterious effects on their offspring.     

In vitro steroidogenesis by gonads of the Russian sturgeon, Acipenser gueldenstaedti Brandt

Ovaries and testes from the Russian sturgeon at different stages of sexual maturity were incubated with ;sup3;H-androstenedione or ³H-pregnenolone. The major metabolites of androstenedione in both sexes were testosterone and 5- and 5ß-androstanediols, but no evidence was found for the gonadal production of 11-oxygenated androgens. Pregnenolone was converted to 17-hydroxyprogesterone, androstenedione and testosterone together with reduced metabolites. 11-Oxygenated androgens were found in female serum by gas chromatography-mass spectrometry (GC-MS), in serum of both sexes by radioimmunoassay (RIA), and was detectable by RIA in interrenal but not gonadal tissue. The results suggest that sturgeon may differ from teleosts in that 11-oxygenation may take place in extragonadal tissues.     

Selenium supplementation changes glutathione peroxidase activity and thyroid hormone production in Senegalese sole (Solea senegalensis) larvae

Dietary selenium (Se) influences the glutathione peroxidase (GPx) activity and the thyroid hormone system in vertebrates, but these relations are poorly described in fish. The present study aimed to evaluate the effect of Se supplementation on Senegalese sole larvae, focusing on GPx activity and thyroid status. Two groups of larvae were fed on either control (C) or Se‐enriched live feed, from 2 days after hatch (DAH) until 34 DAH. Senegalese sole larvae fed Se‐enriched live feed significantly increased their whole‐body Se, compared to larvae from C group. The activity of GPx was higher in larvae from Se group, suggesting that the levels of this mineral were suboptimal in the control diet. TH levels were also higher in larvae fed Se‐enriched live feed, suggesting that Se may have a positive influence on the TH production in early life stages of marine fish larvae.     

Lipid class and fatty acid composition of phospholipids from the gonads of skipjack tuna

ABSTRACT:   Lipid class and fatty acid composition of phospholipids from the gonads of skipjack tuna were examined to evaluate effective utilization of the processing of by-products. The predominant phospholipids in the ovaries were phosphatidylcholine (PC; 47.9%), phosphatidylethanolamine (PE; 19.3%) and lyso-phosphatidylcholine (LPC; 19.1%). In contrast, those in the testes were PC (40.1%), PE (29.3%) and phosphatidylserine (PS; 9.6%). The percentage of docosahexaenoic acid (DHA) was markedly high at more than 50% in LPC of the ovaries, and PE and PS of the testes were also high. The percentages of DHA at sn-position 2 of the predominant phospholipids, except for PC in the testes, were more than 60%, in particular PE in the testes was remarkably high at 81.9%. After storage for 2 days at 5°C, the LPC content in the ovaries increased twofold and the DHA level of LPC was the same as before, though the contents of other phospholipids decreased.     

Dietary effects on thyroid hormones in the red drum,Sciaenops ocellatus

Juvenile red drum (Sciaenops ocellatus) were cultured at 25°C on a variety of diets and blood sampled over eight weeks to examine the relationship between growth and plasma thyroid hormone levels. Maximum growth rates were achieved on formulated experimental diets and a simulated natural shrimp diet. Associated with these maximal rates was a significant increase in triidothyronine (T₃), but no consistent change in thyroxine (T₄). Reduced rations of diets resulted in low growth rates associated with significantly lowered levels of T₃ but not T₄. To determine whether weight gain could be increased by application of exogeneous hormone, diets were supplemented with T₃ or T₄ at 2, 10, and 50 mg hormone/kg diet. Significantly elevated T₃ was induced by supplementation with 10 and 50 mg T₃/kg diet, although there were no indications of an anabolic effect of T₃ incorporation, and 50 mg T₃/kg diet was in fact associated with decreased weight gain. Incorporation of T₄ into diets had no effect on growth or T₃, and had effects on T₄ which were small and inconsistent, indicating that T₄ may not be effectively absorbed from the gut. No difference was found in response to hormone feeding between low (6 ppt) or high (35 ppt) water salinity. T₃ levels thus appear to closely parallel growth in fish on unsupplemented diets, whereas T₄ which were small and manipulation. Supplementation with T₃ is not an effective means of stimulating growth in red drum fed optimum diets. Whereas thyroid hormones may function to regulate intermediary metabolism in red drum, elevated endogenous thyroid hormone levels appear adequate to supply tissue needs during juvenile growth in culture.     

A preliminary study on tailoring of fillet iodine concentrations in adult Atlantic salmon (Salmo salar L.) through dietary supplementation

The present study was performed to assess to what degree supplemented dietary iodine (I) was retained in selected tissues, including the fillet of adult Atlantic salmon (Salmo salar) reared in sea water. Atlantic salmon weighing approximately 1.5 kg were randomly assigned to three net pens per treatment and fed moist pellets (based on minced saithe and herring) supplemented with 0, 40 or 80 mg iodine (as KI) kg^?1 on dry weight basis for 150 days. The iodine concentrations in the experimental feeds were analysed to be 10, 54 and 86 mg kg^?1 dry weight, respectively. Growth, mortality and blood haemoglobin concentration (Hb) were recorded. Iodine concentrations were measured in muscle, liver and kidney after 90 and 150 days of feeding by inductively coupled plasma‐mass spectrometry. In addition, plasma thyroxine (T₄) and triiodo‐thyronine (T₃) were determined. The weight gain during the period was approximately 1 kg for all treatments. There were no mortalities, and blood Hb levels were within normal ranges. The iodine concentration in muscle, liver and kidney were all affected by the dietary iodine level, despite wide intratreatment variation. After 150 days, fillets of fish fed 10, 54 and 86 mg I kg^?1 showed mean concentrations of 0.4, 0.5 and 0.9 mg I kg^?1 wet weight, respectively, whereas the iodine concentration in the liver and the kidney increased approximately three times in the dietary groups. Similarly, plasma T₄ and T₃ showed great variation within the treatments. No significant correlations were found between individual tissue iodine concentration and thyroid hormone concentration in any of the groups at any sampling time. This preliminary feeding experiment showed that fillet iodine in adult Atlantic salmon can be increased up to 1.4 mg I kg^?1 wet weight by dietary iodine 80 times the minimum requirement for salmonids, without impacting health, performance or plasma thyroid hormone status.     

Changes in plasma prolactin and growth hormone concentrations during freshwater adaptation of juvenile chum salmon (Oncorhynchus keta) reared in seawater for a prolonged period

Freshwater adaptability of chum salmon was examined in juvenile fish reared in seawater for 4 months. The fish, weighing about 40g, were transferred directly to fresh water in October, when their cohorts are migrating in the North Pacific Ocean. Plasma sodium concentration decreased from 167 mM in seawater to about 130 mM during the first 24h, and increased gradually during 2–7 days after the transfer. No immunoreactive prolactin (PRL) was detected in the plasma of the seawater-adapted fish nor during the first 24h in fresh water. Significant levels of PRL were detected after 2–3 days. The maximal level (2.6 ng/ml) was observed after 5 days and became undetectable again after 7 days; no significant correlation was seen between the changes in plasma sodium and PRL levels during the transfer. Plasma growth hormone levels were relatively constant, except for a significant decrease 12h after the transfer. Although plasma thyroxine levels were highly variable during the experiment, a significant decrease and an increase were observed 12h and 5 days after the transfer, respectively. The present study indicates that juvenile chum salmon retain hyperosmoregulatory ability even after prolonged rearing in seawater. Examination of turnover rates, rather than changes in plasma levels, seems to be essential to clarify the osmoregulatory roles of the hormones.     

Changes in alkaline phosphatase isoenzyme activity in tissues and plasma of Atlantic salmon (Salmo salar) before and during smoltification and gonadal maturation

Changes in tissue and plasma isoenzymes of alkaline phosphatase (ALP) were qualitatively and quantitatively determined for male and female Atlantic salmon parr, silvery parr, smolt, immature grilse, prespawning grilse and postspawning grilse using cellulose acetate electrophoresis, densitometry and spectrophotometry. Tissue ALP isoenzymes were isolated from intestine, kidney, bone, liver, and gonad and compared to plasma isoenzymes. Parr plasma displayed three isoenzymes from bone and liver (slow and fast). During smoltification, ALP activity increased in tissue extracts from liver, gonad, and kidney of males and females. Total plasma ALP activity also increased and was due to slow and fast liver isoenzymes. During ovarian development, total ALP plasma activity increased in females and was due mostly to liver isoenzymes and an incompletely identified isoenzyme or isoenzyme mixture (band 2). However, in males total ALP plasma activity did not increase during maturation and no band 2 was evident. In male and female maturing adult grilse, bone ALP activity declined and the isoenzyme band evident in parr plasma could not be detected. ALP activity declined in the plasma of postspawning males and females. In females this was due partly to the total clearance of band 2 from the plasma, together with lowered levels of liver isoenzymes. Treatment of postspawned grilse in February and March with triiodothyronine and thyroxine elevated plasma thyroid hormone levels and increased plasma ALP levels. In conclusion, plasma ALP isoenzyme activities change with physiological state, and knowledge of the conditions governing these changes is important when using these enzymes as a diagnostic tool.     

Changes in serum thyroxine and triiodothyronine concentrations during metamorphosis of the Southern Hemisphere Lamprey Geotria australis,and the effect of propylthiouracil,triiodothyronine and environmental temperature on serum thyroid hormone concentrations of ammocoetes

Serum thyroid hormone concentrations were measured during the seven stages of metamorphosis (1–7) of the southern hemisphere lamprey, Geotria australis. The respective mean concentrations ± SEM of serum thyroxine (T₄) and triiodothyronine (T₃) fell from 31.73 ± 4.09 and 5.06 ± 0.70 nM in large ammocoetes sampled in February, at the time when metamorphosis was initiated, to 4.54 ± 0.36 and 1.03 ± 0.12 nM at stage 5. Although there was a small, but significant, recovery of serum T₄ concentrations during stages 6 and 7, no such corresponding statistically significant rise occurred in serum T₃ concentrations. Serum thyroid hormone concentrations in ammocoetes sampled during the period when metamorphosis was taking place, exhibited a marked seasonal increase between February and May–June (late autumn/early winter); serum T₃ and T₄ concentrations peaked in May–June and were, respectively, > 2 fold and > 8 fold higher than those recorded for samples in late February (mid summer). By mid-July the serum T₄ and T₃ levels had declined from the peak values. Ammocoetes taken from streams at 16°C in June and acclimated to aquaria water at 25°C or 6°C had significantly lower serum T₃ and T₄ concentrations at the higher temperature, and also a lower serum T₄, but not T₃ concentration, at the lower temperature. Treatment of separate groups of ammocoetes with either propylthiouracil or T₃ for 70 days significantly depressed and raised respectively, the serum thyroid hormone and hepatic T₃ concentrations and caused significant changes in the body weight, but did not induce the onset of metamorphosis.     

Changes in GnRH levels in the brain and pituitary gland during migrations of sockeye salmon and chum salmon

Levels of two moleculer types of gonadotropin-releasing hormone (GnRH), salmon GnRH (sGnRH) and chicken GnRH–II (cGnRH–II) in the various brain regions and pituitary gland of sockeye salmon (Oncorhynchus nerka) and chum salmon (O. keta) during smoltification and spawning migration, respectively, were measured using specific time-resolved fluoroimmunoassay (TR-FIA) systems. Changes in sGnRH levels in different brain regions tended to be specifically synchronized with serum thyroid hormone or pituitary gonadotropin (GTH) levels during smoltification and spawning migration, respectively. In contrast, cGnRH–II levels did not show such synchronized changes. SGnRH and cGnRH–II in various brain regions might have different roles during smoltification and spawning migration of salmonid fishes.     

Thyroid hormone deiodinase systems in salmonids,and their involvement in the regulation of thyroidal status

The trout thyroid secretes L-thyroxine (T₄) which undergoes enzymatic deiodination in liver and other tissues. Based on mammalian studies, T₄ outer-ring deiodination (ORD) or T₄ inner-ring deiodination (IRD) could generate respectively 3,5,3′-triiodo-L-thyronine (T₃) or 3,3′,5′-T₃(rT₃), while subsequent T₃ORD or T₃IRD could generate respectively 3,5-diiodo-L-thyronine (T₂) or 3,3′-T₂, and rT₃ORD or rT₃IRD could generate respectively 3,3′-T₂ or 3′,5′-T₂. In practice, T₄ in trout undergoes hepatic ORD to produce T₃ but negligible IRD to produce rT₃, and T₃ in turn undergoes negligible ORD but modest IRD to produce 3,3′-T₂. T₄ORD, which is particularly important in converting T₄ to the biologically more potent T₃, also occurs in gill, muscle and kidney. At least two isozymes are involved: i) a high-affinity, propylthiouracil (PTU)-sensitive T₄ORD which displays ping-pong kinetics, requires thiol as a cofactor, and is present in liver, gill and muscle, and ii) a low-affinity, PTU-insensitive T₄ORD with sequential kinetics with a thiol cofactor, and is present in liver and kidney. Receptor-bound T₃ is derived primarily from the plasma for kidney, mainly from intracellular sources for gill and about equally from both plasma and intracellular sources for liver. Thus, the high-affinity T₄ORD may produce T₃ for local intracellular use while the low-affinity 5′-monodeiodinase may produce T₃ for systemic use. T₄ORD activity responds to nutritional factors and the physiologic state of the fish. Furthermore, T₃ administered orally for either 6 weeks or 24h reduces the functional level (V_max) of hepatic T₄ORD, and T₃ added to isolated hepatocytes also reduces activity, indicating direct T₃ autoregulation of T₄ORD to maintain hepatocyte T₃ homeostasis. However, T₃ administration also induces T₄IRD to produce biologically inactive rT₃ and induces T₃IRD to produce 3,3′-T₂. Thus, the trout liver has several iodothyronine deiodinase systems which in a coordinated manner regulate tissue T₃ homeostasis in the face of a T₃ challenge. It does this by decreasing formation of T₃ itself, by diverting T₄ substrate to biologically inactive rT₃ and by increasing the degradation of T₃. These deiodinases differ in many respects from any mammalian counterparts.     

Developmental changes in pituitary–thyroid axis, and formation of gonads in leptocephali and glass eels of Anguilla spp.

SUMMARY: Pituitary, thyroid gland and gonads in leptocephali of Japanese eel Anguilla japonica (19.8–32.6 mm in total length), A. obscura (45.0 mm), and A. bicolor pacifica (49.5 mm) and those in glass eels of the Japanese eel were histologically and immunohistochemically examined in order to observe the developmental changes of these endocrine organs in the Anguillidae. The pituitary, consisting of adenohypophysis and neurohypophysis in Japanese eel leptocephali over 22.5 mm, did not contain thyroid stimulating hormone (TSH) immunoreactive cells. Such cells were, however, detectable in the more developed pituitaries of leptocephali of A. obscura and A. bicolor pacifica and in those of glass eels. Conversely, thyroxine (T₄)-immunoreactive thyroid follicles could be detected in all specimens, both leptocephalic and glass eel. Only in glass eels, gonads were found in the body cavity, and these gonads harbored one or two primordial germ cells (PGC) per cross-section. Our results indicate that thyroid hormones (TH) production started prior to TSH production, and that TSH and TH are both secreted during the metamorphosis from leptocephalus to glass eel. Therefore, it is plausible that the TSH–TH axis is involved in the metamorphosis from leptocephalus to glass eel, but not in the early growth from preleptocephalus to leptocephalus.