Many aspects of development, differentiation and metabolism in vertebrates are regulated by small biologically active molecules such as thyroid hormones and retinoids. They act via a family of intracellular receptor proteins which act directly on target genes. Thyroid hormones are passed on to the eggs by broodfish prior to spawning (Kobuke et al., 1987; Tagawa and Hirano, 1987; Brown et al., 1987; Greenblatt et al., 1989, Tagawa ef al., 1990) and may provide the necessary physiological regulation for growth, development and osmoregulation in larvae prior to the functional development of their own endocrine glands. There is little doubt that thyroid hormones are essential for fish development, for example, in the metamorphosis of the flounder (Miwa et al. 1988; Yamano ef al. 1994). Also, treatment of larvae from several species with thyroid hormone has beneficial effects on development and survival (Brown and Nunez, 1994) and it has recently been shown that thyroid hormone receptors are abundant in sea bream larvae (Llewellyn et al., 1996). However, the investigations into thyroid hormones so far have centred on hormonal measurements or treatments and have not attempted to identify the tissues, which respond to these hormones during development by detecting the presence of specific receptors in those tissues. Growth hormone (GH) is one of the major hormones regulating growth in vertebrates. However, despite evidence that GH cells are present (Power and Canario, 1992; Cambre et al., 1990) and GH is synthesised in recently hatched fish (Funkenstein et al., 1992) and that, in the Japanese flounder, GH expression increases throughout metamorphosis, the functional importance of GH in fish larval development remains to be ascertained.
The prime aim of the project is to establish the importance of thyroid hormones and growth hormone (GH) in the normal and abnormal development of two marine species. These are the Atlantic halibut (Hippoglossus hippoglossus) a cold-water fish with considerable potential as a new aquaculture species and the sea bream (Sparus aurata), a warm-water fish cultivated in the Mediterranean and Portugal, but for which problems of larval culture elevate production costs and reduce its economic potential.
This will be investigated by:
- The determination of whole body T4, T3 growth hormone (GH) and IGF1 profiles during development of the larvae and young fish.
- The determination of the stages at which tissues become responsive to thyroid hormones through the production of thyroid hormone receptor (THR) and GH by the production of IGF1.
- The experimental manipulation of the levels of GH, thyroid hormones, and THRs and the determination of the effect of this on development.
- The generation of a model system for abnormal development, based on the actions of retinoic acid, a regulator of vertebrate development already implicated in the production of abnormal larvae in vertebrates. The use of this model system to determine morphological and molecular aspects of abnormal development.
Brown CL and Nunez JM 1994 Hormone actions and applications in embryogenesis. Perspectives in Comparative Endocrinology. Nat Res Council Canada. pp.-339
Brown CL, Sullivan CV, Bern HA and DickhoffWW 1987 Occurrence of thyroid hormones in early developmental stages ofteleost fish. TransAm Fish Soc Symp 2, 144-150.
Cambre M., Mareels G., Corneillie, S., Moons, L., Ollevier, F. and Vandesande, F. 1990 Chronological appearance of the different hypophysial hormones in the pituitary of Sea bass larvae (Dicentrarchus labrax) during their early development: an immunocytochemical demonstration. Gen. Comp. Endocrinol. 77, 408-415
Funkenstein, B., Tandler, A. & Cavari, B 1992 Developmental expression ofthe growth hormone gene in the gilthead sea bream Sparus aurata. Mol. Cell. Endocrinol. 87, R7-R9
Greenblatt M, Brown CL, Lee M, dauder S and Bern HA 1989 Changes in thyroid hormone levels in eggs and larvae and in iodine uptake by eggs of coho and chinook salmon, Oncorhynchus kisutch and O. tshawytscha. Fish Physiol Biochem 6, 261-278.
Kobuke LJ, Specker JL and Bern HA 1987 Thyroxine content of eggs and larvae of coho salmon Oncorhynchus kisutch. JExp Zool 242, 89-94.
Llewellyn, L., Ramsurn, V.P., Sweeney, G.E., Wigham, T. and Power, D.M. (In Press) Expression of thyroid hormone receptor during early development of the sea bream (Sparus aurata). Proc 18th Conf of European Comp Endocrinologists, Rouen, 1996.
Miwa S, Tagawa M, Inui I and Hirano T 1988 Thyroxine surge in metamorphosing flounder larvae. Gen Comp Endocrinol 70, 158-163.
Power, DM & Canario AVM 1992 Immunocytochemistry of somatotrophs, gonadotrophs, prolactin and adrenocorticotropin cells in larval sea bream (Sparus aurata) pituitaries. Cell TissueRes. 269, 341-346
Tagawa M, Miwa S, Inui Y, de Jesus EG and Hirano T 1990 Changes in thyroid hormone concentrations during early development and metamorphosis of the flounder. Zool Sci 7, 93-96.
Yamano K, Takano-Ohmuro H, Obinata T and Inui Y 1994 The effect of thyroid hormone on developmental transition of myosin light chains during flounder metamorphosis. Gen Comp Endocrinol 93, 321-326.
Funding SchemeCSC - Cost-sharing contracts