Developmental endocrinology of the dipnoan, Neoceratodus forsteri

American Zoologist, Dec 1997 by Joss, Jean M P, Rajasekar, P Sylvia, Raj-Prasad, R Ashni, Ruitenberg, Kirsty

At 16 weeks of age and again at 36 weeks and 40 weeks of age, groups of four juvenile lungfish were pretreated for four days with 0.1% methimazole, a potent thyroid hormone inhibitor, (added to the water in which they were kept). Following methimazole treatment, these and four untreated lungfish of the same age and size range were injected intraperitoneally with 1 Tic of I125 and left for a further 24-hour period. After this time each fish was humanely killed and separated into lower jaw (containing thyroid tissue), upper head, anterior body (containing liver) remaining body and tail. The total c.p.m. per mg tissue was determined and the average value for tail and upper head subtracted from the total activity of the lower jaw to correct for background radioiodide uptake of nonthyroidal tissue. The thyroidal activity was then expressed as a percentage uptake of the injected dose per mg of thyroid tissue. There was no significant change (Student t-test) in the uptake of iodine between any of the methimazole-treated groups or the control group at 16 or 36 weeks but the thyroids of the control lungfish at 40 weeks increased uptake by approximately 50% (Fig. 4). This increase in iodine uptake is reflected in the increased follicle cell height observed at this stage. This may be due to stimulation by thyrotropin, since thyrotropes can be identified in the pituitary at this age, but the negative feedback relationship between thyroid hormones and thyrotropin is not yet established because the methimazole-treated lungfish (with decreased thyroid function) did not show greater uptake of iodine than did the controls.

Thyroid hormone receptors (TR) in liver

In most vertebrates examined, there are two types of thyroid hormone receptors, TRa and TR[. Looking in the amphibian, Xenopus laevis, Yaoita and Brown (1990) have shown that the expression of the genes coding for these receptors begins soon after the embryo hatches. TRa expression increases through the premetamorphic stage of development, is maximal at prometamorphosis and thereafter declines to low levels in the adult. Expression of TRy, on the other hand, is barely detectable during premetamorphosis, rises during prometamorphosis, reaches a peak at metamorphic climax and drops to 10% of its peak in the adult. Although thyroid receptors have not yet been characterised in lungfish, a study was undertaken to immunocytochemically differentiate between TRa and TRbeta in liver cells at the time when the thyroid was increasing in activity at 40 weeks. Antibodies specific for human TRa and TRbeta (Affinity Bioreagents) were employed at a dilution of 1:100 with a Vectastain ABC immunocytochemistry kit. Figure Sa, b shows that the nuclei of liver cells from 40-week-old lungfish immunostain predominantly with the antibody to human TRa.

CONCLUSIONS

The development of the pineal, pituitary and thyroid in the lungfish, N. forsteri, over the first twelve months post hatching, is equivalent to that of a premetamorphic amphibian. While the thyroid gland does appear to increase in activity, as judged by follicle cell height, at two distinct periods during the first year of development, neither of these was shown to correlate with a mature pituitary-thyroid axis in which the thyroid hormones are capable of influencing the secretion of thyrotropin from the pituitary. Moreover, thyroid receptors in the liver were predominantly of the alpha-type, characteristic of premetamorphosis in Xenopus.