Comparative thyroidology: thyroid gland location and iodothyronine dynamics in Mozambique tilapia (Oreochromis mossambicus Peters) and common carp (Cyprinus carpio L.)
2007
SUMMARY In teleosts, the thyroid gland is mostly found in the subpharyngeal region.
However, in several species thyroid follicles are found in, for example,
heart, head kidney and kidney. Such heterotopic thyroid follicles are active,
and considered to work in concert with the subpharyngeal thyroid. In
Mozambique tilapia ( Oreochromis mossambicus ) thyroid activity is,
indeed, restricted to the subpharyngeal region; in common carp ( Cyprinus
carpio ) the functional endocrine thyroid is associated with renal
tissues. The subpharyngeal follicles of carp comprise only 10% of the total
thyroid tissue, and these follicles neither accumulate iodide nor synthesize
or secrete thyroid hormones to a significant degree. Although the shape and
size of carp subpharyngeal and renal follicles vary, the epithelial cell
height of the thyrocytes and thyroxine immunoreactivity do not differ, which
suggests that the activity of the carp subpharyngeal thyroid follicles is
dormant. Differences in thyroid physiology between the two fish species were
further assessed at the level of peripheral thyroid hormone metabolism. Carp
clears plasma of thyroid hormones faster than tilapia does. Furthermore, a
significant amount of conjugated thyroid hormones was observed in the plasma
of tilapia, which was preceded by the occurrence of thyroid hormone conjugates
in the subpharyngeal region and coincides with the appearance of conjugates in
the surrounding water. Apparently, plasma thyroid hormone conjugates in
tilapia originate from the thyroid gland and function in the excretion of
thyroid hormones. Our data illustrate the variability in teleostean
thyroidology, an important notion for those studying thyroid physiology.
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