Experimental study on the activation of growth hormone-secreting cells during larval development of Nile tilapia, Oreochromis niloticus

The present experiments were designed to determine the effect of the exogenous treatment of Oreochromis niloticus females with l-thyroxin (T4) on the activation of growth hormone (GH)-secreting cells during larval rearing, and its subsequent effect on larval growth and survival. The ontogeny of GH-secreting cells was investigated immunocytochemically in the developing O. niloticus larvae, from control and T4-treated spawners. A few numbers of GH-immunoreactive cells were observed early one day after hatching. The number and immunoreactivity of GH-secreting cells increased with larval development. The injection of females O. niloticus with thyroxin (1 or 10 μg T4/g BW) greatly enhanced the immunoreactivity of GH-secreting cells in the pituitary gland of larvae as indicated by the quantitative and qualitative changes; the increase of both number and size, and strong immunoreactivities of GH-secreting cells, during the rearing period for the larvae produced from T4-treated females. Thus, thyroxin directly improved O. niloticus larval growth, since a marked increase in both, the length and weight of larvae occurred during the experimental period. In addition, larvae from treated females also gave a significantly higher survival rate than that of the control.It could be concluded that exogenous T4 in maternal circulation was transferred into oocytes and larvae. The transferred thyroid hormone appears to play a main role in the synthesis and secretion of GH during the early development of larvae and may confer a distinct advantage for the growth of the offspring of Nile tilapia, O. niloticus.