title: 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 degrees =accumulated daily temperature from hatching). Sporadic thyroid follicles are first detected in larvae at 142 D degrees (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 degrees (23 days post-hatch), and lactotrophs (prolactin producing cells) at 134 D degrees (25 days post-hatch). Scarce thyrotrophs are evident after detection of the first thyroid follicles (142 D degrees ), but coincident with a phase in which follicle number and activity increase (260 D degrees ). 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 degrees , 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.
authors.affiliation: Fish Endocrinology Laboratory, Department of Zoology/Zoophysiology, Göteborg University, Box 463, 40530 Göteborg, Sweden. ingibjorg.einarsdottir@zool.gu.se
authors.forename: Ingibjörg Eir
authors.initials: IE
authors.lastname: Einarsdóttir
authors.forename: Nadia
authors.initials: N
authors.lastname: Silva
authors.forename: Deborah M
authors.initials: DM
authors.lastname: Power
authors.forename: Heiddis
authors.initials: H
authors.lastname: Smáradóttir
authors.forename: Björn Thrandur
authors.initials: BT
authors.lastname: Björnsson
mesh_terms: D000818:Animals; D029941:Fish Proteins / Q000378:metabolism; D005412:Flatfishes; D006023:Glycoproteins / Q000378:metabolism; D013006:Growth Hormone / Q000378:metabolism; D007814:Larva / Q000254:growth & development / Q000378:metabolism; D008675:Metamorphosis, Biological / Q000502:physiology*; D010902:Pituitary Gland / Q000033:anatomy & histology / Q000166:cytology / Q000254:growth & development*; D010907:Pituitary Hormones / Q000378:metabolism; D011388:Prolactin / Q000378:metabolism; D013961:Thyroid Gland / Q000033:anatomy & histology / Q000254:growth & development* / Q000378:metabolism; D013972:Thyrotropin / Q000378:metabolism; D013974:Thyroxine / Q000378:metabolism; D014284:Triiodothyronine / Q000378:metabolism
chemical_list: D029941:Fish Proteins; D006023:Glycoproteins; D010907:Pituitary Hormones; C064448:somatolactin protein, fish; D014284:Triiodothyronine; D011388:Prolactin; D013972:Thyrotropin; D013006:Growth Hormone; D013974:Thyroxine