View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Universidade do Algarve Anat Embryol (2006) 211: 47–60 DOI 10.1007/s00429-005-0055-z ORIGINAL ARTICLE Ingibjo¨rg Eir Einarsdo´ttir Æ Nadia Silva Deborah M. Power Æ Heiddis Sma´rado´ttir Bjo¨rn Thrandur Bjo¨rnsson Thyroid and pituitary gland development from hatching through metamorphosis of a teleost flatfish, the Atlantic halibut Accepted: 9 September 2005 / Published online: 8 December 2005 Ó Springer-Verlag 2005 Abstract Fish larval development, not least the specta- pophysis (NH), pars distalis (PD) and pars intermedia cular process of flatfish metamorphosis, appears to be (PI). The first sporadic endocrine pituitary cells are ob- under complex endocrine control, many aspects of served at the yolk sack stage, somatotrophs (growth which are still not fully elucidated. In order to obtain hormone producing cells) and somatolactotrophs (so- data on the functional development of two major matolactin producing cells) are first observed at 121 D° endocrine glands, the pituitary and the thyroid, during (23 days post-hatch), and lactotrophs (prolactin pro- flatfish metamorphosis, histology, immunohistochemis- ducing cells) at 134 D° (25 days post-hatch). Scarce try and in situ hybridization techniques were applied on thyrotrophs are evident after detection of the first thy- larvae of the Atlantic halibut (Hippoglossus hippoglos- roid follicles (142 D°), but coincident with a phase in sus), a large, marine flatfish species, from hatching which follicle number and activity increase (260 D°). through metamorphosis. The material was obtained The somatotrophs are clustered in the medium ventral from a commercial hatchery. Larval age is defined as region of the PD, lactotrophs in the anterior part of the day-degrees (D°=accumulated daily temperature from PD and somatolactotrophs are scattered in the mid and hatching). Sporadic thyroid follicles are first detected in posterior region of the pituitary. At around 600 D°, larvae at 142 D° (27 days post-hatch), prior to the coinciding with the start of metamorphosis, somatolac- completion of yolk sack absorption. Both the number totrophs are restricted to the interdigitating tissue of the and activity of the follicles increase markedly after yolk NH. During larval development, the pituitary endocrine sack absorption and continue to do so during sub- cells become more numerous. The present data on thy- sequent development. The larval triiodothyronine (T3) roid development support the notion that thyroid hor- and thyroxine (T4) content increases, subsequent to yolk mones may play a significant role in Atlantic halibut absorption, and coincides with the proliferation of thy- metamorphosis. The time of appearance and the sub- roid follicles. A second increase of both T3 and T4 occurs sequent proliferation of pituitary somatotrophs, lacto- around the start of metamorphosis and the T3 content trophs, somatolactotrophs and thyrotrophs indicate at further increases at the metamorphic climax. Overall, which stages of larval development and metamorphosis the T3 content is lower than T4. The pituitary gland can these endocrine cells may start to play active regulatory first be distinguished as a separate organ at the yolk sack roles. stage. During subsequent development, the gland be- comes more elongated and differentiates into neurohy- Keywords Flatfish Æ Pituitary Æ Thyroid Æ Metamorphosis Æ Hippoglossus hippoglossus I. E. Einarsdo´ttir (&) Æ B. T. Bjo¨ rnsson Fish Endocrinology Laboratory, Department of Zoology/Zoophysiology, Go¨ teborg University, Introduction Box 463, 40530 Go¨ teborg, Sweden E-mail: [email protected] The Atlantic halibut (Hippoglossus hippoglossus) is the Tel.: +46-31-7733684 Fax: +46-31-7733807 largest existent flatfish species with adult individuals being able to reach body weights well over 200 kg. This N. Silva Æ D. M. Power marine teleost is a batch spawner and each female Centro de Cieˆ ncias do Mar, Universidade do Algarve, spawns several batches of eggs over a period of Campus de Gambelas, 8000-139 Faro, Portugal approximately 4 weeks. Egg and larval growth and H. Sma´rado´ttir development are both temperature- and time-dependent Fiskeldi Eyjafjarðar Ltd., Glera´rgata 34, IS-600 Akureyri, Iceland and both factors are included in the definition of age as 48 day-degrees (D° = accumulated daily temperature (°C) the teleost species. Thyrotrophs are detected before from hatching). The hatched larvae are relatively small, hatching in salmonids (Naito et al. 1993; Saga et al. pelagic and symmetrical. The yolk sack period lasts for 1993), at hatching in sea bream (Sparus aurata) (Ayala 240–260 D° (46–50 days post-hatch), depending on the et al. 2003), and shortly after hatching in Japanese temperature, after which the larvae start to feed (first flounder and sea bass (Dicentrarchus labrax) (Miwa and feeding) on live zooplankton (Olsen et al. 1999). At Inui 1987b; Cambre et al. 1990) whereas, in the ayu around 600–650 D° (79–84 days post-hatch), the larvae (Plecoglossus altivelis), thyrotrophs appear as late as start to undergo a major morphological restructuring, 50 days after hatching (Saga et al. 1999). Unexpectedly, the metamorphosis. During this process, the body depth in Japanese eel (Anguilla japonica), Anguilla obscura and of the larvae increases notably and the left eye starts to Anguilla bicolor TSH immunoreactive cells are first seen migrate over to the right side. Thus, the larvae become just prior to the onset of metamorphosis, the transition cranially asymmetrical. At the same time, the skin pig- from leptocephalus to glass eel, while active T4 pro- mentation also becomes asymmetrical, with a dark right ducing cells are earlier observed in the leptocephalus (upper) side and a white left (under) side. The larvae stage. Thus, it has been assumed that early on in eel descend to the bottom to continue life as a benthic ontogeny, TH production is independent of TSH regu- flatfish. The metamorphic process takes 2–3 weeks and lation (Ozaki et al. 2000). metamorphosed halibut, older than 800 D° (around Other pituitary hormones that may also be candidate 100 days post-hatch) are classified as juveniles. regulators of larval development are growth hormone Hormones play an important role in regulating (GH), prolactin (PRL) and somatolactin (SL), due to developmental processes. The importance of the thyroid their pluripotent actions in juvenile and adult fish. GH is hormones (THs), thyroxine (T4) and triiodothyronine produced in somatotrophs and, in teleosts, it is the (T3), in vertebrate development is well established principal growth-promoting hormone, affecting various (Hadley 1992; Tata et al. 1993; Power et al. 2001). In aspects of energy metabolism, food conversion and fish, prior to the maturation of the larval thyroid gland, appetite, while also playing an important role in osmo- fish eggs and, subsequently, the larval yolk sack contain regulation as well as in reproduction and immune significant amounts of THs of maternal origin (Kobuke function (Peter and Marchant 1995; Bjo¨ rnsson 1997; et al. 1987; Tagawa and Hirano 1987; Brown et al. 1988; Perez-Sanchez 2000; Bjo¨ rnsson et al. 2002). PRL, pro- Greenblatt et al. 1989; Tagawa et al. 1990a, b). This duced in lactotrophs, is a versatile hormone, having an source of THs is likely to be of importance for the amazing array of functions in different vertebrate classes physiological regulation of growth, development and (Bole-Feysot et al. 1998). In freshwater and euryhaline osmoregulation in larvae prior to the development of teleosts, including some flatfishes, a major function ap- functional endogenous thyroid follicles. In fish, THs are pears to be osmoregulation (Gutt 1985; McCormick also involved in the transition from the larval to the 2001; Wada et al. 2004). Other functions of PRL in juvenile stage, the most dramatic manifestation of which vertebrates include the regulation of growth, develop- is flatfish metamorphosis. Elevated endogenous TH ment, metabolism, stimulation of endocrine glands, concentrations have been observed during metamor- behavior, reproduction and immune function (Bole- phosis in the Japanese flounder (Paralichthys olivaceus) Feysot et al. 1998; Forsyth and Wallis 2002). SL, pro- and the summer flounder (Paralichthys dentatus) (Tag- duced in somatolactotrophs, was first discovered in the awa et al. 1990a, b; Schreiber and Specker 1998)andTH pituitary of the flounder (Ono et al. 1990) and has, treatments stimulated flatfish metamorphosis (Inui and subsequently, been shown to be involved in the regula- Miwa 1985; Miwa and Inui 1987a; Solbakken et al. tion of adiposity and gonadal function in salmonids and 1999) and the transformation from larvae to juvenile in sparid fish (Company et al. 2001), to have a hypercal- round fish (Reddy and Lam 1992; de Jesus et al. 1998; cemic action in rainbow trout (Oncorhynchus mykiss)in Solbakken et al. 1999; Deane and Woo 2003). In con- fresh water (Kakizawa et al. 1993), to alter transepi- trast, exposure to inhibitors of TH synthesis retarded the thelial transport of inorganic phosphate in monolayer classical metamorphosis, as observed in flatfish (Inui and cultures of flounder (Pleuronectes americanus) renal Miwa 1985; Miwa and Inui 1987a) as well as the larval– proximal tubuli epithelium (Renfro 1997), to be involved À juvenile transition observed in other teleost species in the retention of HCO3 ions during acidosis in rain- (Dales and Hoar 1954; Okimoto et al. 1993). bow trout (Kakizawa