Thyroid and pituitary gland development from hatching through metamorphosis of a teleost flatfish, the Atlantic halibut

Abstract

Fish larval development, not least the spectacular process of flatfish metamorphosis, appears to be under complex endocrine control, many aspects of which are still not fully elucidated. In order to obtain data on the functional development of two major endocrine glands, the pituitary and the thyroid, during flatfish metamorphosis, histology, immunohistochemistry and in situ hybridization techniques were applied on larvae of the Atlantic halibut (Hippoglossus hippoglossus), a large, marine flatfish species, from hatching through metamorphosis. The material was obtained from a commercial hatchery. Larval age is defined as day-degrees (D°=accumulated daily temperature from hatching). Sporadic thyroid follicles are first detected in larvae at 142 D°(27 days post-hatch), prior to the completion of yolk sack absorption. Both the number and activity of the follicles increase markedly after yolk sack absorption and continue to do so during subsequent development. The larval triiodothyronine (T 3 ) and thyroxine (T 4 ) content increases, subsequent to yolk absorption, and coincides with the proliferation of thyroid follicles. A second increase of both T 3 and T 4 occurs around the start of metamorphosis and the T 3 content further increases at the metamorphic climax. Overall, the T 3 content is lower than T 4 . The pituitary gland can first be distinguished as a separate organ at the yolk sack stage. During subsequent development, the gland becomes more elongated and differentiates into neurohypophysis (NH), pars distalis (PD) and pars intermedia (PI). The first sporadic endocrine pituitary cells are observed at the yolk sack stage, somatotrophs (growth hormone producing cells) and somatolactotrophs (somatolactin producing cells) are first observed at 121 D°( 23 days post-hatch), and lactotrophs (prolactin producing cells) at 134 D°(25 days post-hatch). Scarce thyrotrophs are evident after detection of the first thyroid follicles (142 D°), but coincident with a phase in which follicle number and activity increase (260 D°). The somatotrophs are clustered in the medium ventral region of the PD, lactotrophs in the anterior part of the PD and somatolactotrophs are scattered in the mid and posterior region of the pituitary. At around 600 D°, coinciding with the start of metamorphosis, somatolactotrophs are restricted to the interdigitating tissue of the NH. During larval development, the pituitary endocrine cells become more numerous. The present data on thyroid development support the notion that thyroid hormones may play a significant role in Atlantic halibut metamorphosis. The time of appearance and the subsequent proliferation of pituitary somatotrophs, lactotrophs, somatolactotrophs and thyrotrophs indicate at which stages of larval development and metamorphosis these endocrine cells may start to play active regulatory roles.

Thyroid gland development in Rachycentron canadum during early life stages

“…In fish, the thyroid is not macroscopically recognized but present scattered thyroid follicles (Gorbman et al 1983). Anatomically the thyroid tissue in teleost fishes is composed by several follicles, which can be found near the ventral aorta and to the arterial bulb (Santamaría et al 2004, Delgado et al 2006 and in the insertion of the branchial arch (Einarsdóttir et al 2006), as was also observed for cobia in the present study.…”

“…The thyroid follicle appearance during the transition of the endogenous feeding to the exogenous is essential for the synthesis of its own THs, because during the embryonary development and during the yolk sac absorption the THs are from maternal origin (Tagawa and Hirano 1987, 1990, Tanaka et al 1995, Yamano 2005, Delgado et al 2006. It was observed by Einarsdóttir et al (2006) in Hippoglossus hippoglossus that with larval development, the number of thyroid follicles increase and the colloid volume decreases. The authors attributed this phenomenon as indicative of the thyroid tissue activity.…”

“…Evidence of high follicle activity for Paralichthys olivaceus and Solea senegalensis include an increasing number and size of the follicles, the change of cell shape from ovoid to cubic or cylindrical, and the presence of reabsorption vesicles in the colloid. These characteristics of high cell activity were observed during the metamorphosis period, coinciding with increasing levels of plasmatic THs (Delgado et al 2006, Einarsdóttir et al 2006.…”

Regulation of troponin T expression during muscle development in sea breamSparus auratusLinnaeus: the potential role of thyroid hormones

“…Then, the upper phase was removed, lyophilised, reconstituted in assay buffer (0.01·mol·l -1 PBS, pH·7.6) and assayed. Assays for both T3 and T4 were highly specific and reproducible and were performed as previously described (Einarsdóttir et al, 2006) under equilibrium conditions, using T2777 anti-T3 (<0.01% cross reactivity with T4; SigmaAldrich) and T2652 anti-T4 polyclonal sera (~3% cross reactivity with T3; Sigma-Aldrich). Results are presented as means ± s.e.m.…”

Effect of different kinds of live feed on thyroid structure of marine fish larvae coral trout Plectropomus leopardus (Lacepède, 1802)

“…The lack of colloidal mass inside thyroid follicles indicates that the colloidal mass was in use or, in another word, indicates the ongoing activity of thyroid follicles [20]. The colloid inside follicles contains thyroid hormone.…”