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Notes: The three dimensional morphology of the thyroid tissue in juvenile rainbow trout Oncorhynchus mykiss was determined using serial transverse and longitudinal sections of lower jaws stained with haematoxylin and eosin. The thyroid tissue comprised interconnecting tubular units not the follicles that are widely described in the literature. Some of these tubules were subdivided by branching, giving the appearance of follicles in the sections. The thyroid tissue was aggregated into two major concentrations of tissue, associated with the first and second branchial arteries, with the majority of the tissue located dorsal and lateral to the aorta. These observations in juvenile rainbow trout suggest that thyroid tissue of some species of fishes may not be follicular, as previously assumed. The physiological and evolutionary implications have yet to be determined.

Notes: The ability of developing rainbow trout Oncorhynchus mykiss embryos to compensate for elevated oocyte triiodothyronine (T3) content and whether elevation of oocyte T3 content within a physiologically meaningful range affects growth rates of the embryo or the expression of genes encoding for thyroid hormone receptors α(TRα) and β(TRβ) were examined. Oocytes were immersed in ovarian fluid alone (control) or T3-enriched ovarian fluid prior to fertilization and water hardening, to induce a dose-dependant increase in oocyte T3 content of c. 3 (control), c. 30 (LT3) or c. 110 ng egg−1(HT3). To examine the interaction of embryo somatic growth with altered thyroid state more effectively, the embryos were reared at two ambient temperatures (8·5 and 5·5°C ) to induce different growth rates. A significant decline in whole embryo T3 content was measured in the T3-treatment groups reared at both water temperatures by 3 weeks post-fertilization (dpf), and may have reflected the action of outer ring monodeiodinase, which was present in microsomes prepared from embryos 23 dpf. Whole embryo T3 levels in the HT3 group, however, remained higher than controls until phase 2 of development [the onset of endogenous thyroid hormone (TH) release]. This suggested that the embryos exerted some control over their response to exogenous TH, but that there was a limit to the level of control exerted by the embryonic tissues. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the presence of mRNA encoding for the two TR isoforms as early as 26 dpf, and quantitative real-time RT-PCR (qPCR) was used to examine the effect of elevated oocyte T3 content on the expression of these TR genes in embryos raised at 8·5 and 5·5° C, and sampled at similar developmental stages prior to the onset of embryonic TH synthesis, to ensure that the oocyte T3 was the only source of TH exposure to the embryo. There was a suppression of the TRα gene expression in the control 5·5° C group relative to the control 8·5° C group. In addition, both TRα and TRβ mRNA accumulation was lower, relative to the controls, in the LT3 treatment group reared at 8·5° C suggesting a suppressive effect of the lower level of T3 treatment on the TR gene expression. Conversely, there were no differences from controls in the HT3 treatment group, possibly indicating that this level of exposure overrides the down-regulating capacity of the embryo. Similar patterns were seen for TRα and TRβ mRNA accumulation in embryos reared at 5·5° C, but because of the temperature suppressed level of TRα mRNA in the controls, significant affects of the LT3 treatment were only found for TRβ. There were no measurable effects of T3 treatment on oocyte fertility or embryo somatic growth for either temperature treatment group, nor was somatic growth hormone content (measured only in the 8·5° C treatment group) apparently related to in ovo T3 levels. The results suggest that altered in ovo T3 levels, within the ranges used here, do not induce marked affects on embryo development, probably because of the ability of the embryo to maintain the integrity of its TH milieu.

Notes: Abstract. The development of the thyroid tissue in rainbow trout (Oncorhynchus mykiss) embryos between 20 and 88 days postfertilization (dpf) was studied using traditional histological methods in combination with immunostaining using antibodies raised against l-thyroxine (T4) and triiodo-l-thyronine (T3). In 20 dpf embryos, the thyroid primordium appeared as a simple tube aligned with the dorsal surface of the ventral aorta. The tubular primordium becomes progressively bifurcated at each end and follicles are formed by pinching off balls of cells from the tips of the bifurcations. The secretory activity of the thyroid tissue appears first as a synthesis phase, evident at 30 dpf, characterized by T4 and T3 immunostaining present only in the follicle lumen. A later hormone release or secretory phase was first evident for T4 immunostaining at 43 dpf and for T3 immunostaining at 46 dpf and was characterized by the presence of immunostaining both within the follicle lumen and also in the cytoplasm of some (but not all) thyrocytes. This stage was also coincident with a marked increase in the number of pituitary thyrotrophs. The proportion of embryos exhibiting immunostained thyrocytes increased progressively from 43 to 52 dpf, at which stage all embryos had T4- and T3-immunostained thyrocytes.