Assessment of thyroid function in adult medaka (Oryzias latipes) and juvenile rainbow trout (Oncorhynchus mykiss) using immunostaining methods

The effect of injected bovine TSH on the pattern of anti-T(4) and anti-T(3) immunostaining of the thyroid tissue was examined in rainbow trout (Oncorhynchus mykiss) and medaka (Oryzias latipes) to determine if the previously reported immunostaining of the cytoplasm of thyrocytes is due to the presence of colloid pinocytotic vesicles and is thus indicative of thyroid hormone release. We hypothesized that the number of immunostained thyrocytes should increase following a TSH challenge, and this should parallel other indicators of increased thyroid activity. In medaka, immunostained thyrocytes were only found following the TSH challenge, and were most marked after 24 to 72 hours; the immunostaining was associated with large colloid-filled cytoplasmic vesicles. In trout, the number and staining intensity of immunostained thyrocytes were increased after the TSH challenge; the immunostaining was present throughout the cytoplasm of the thyrocytes. These findings support the working hypothesis that the immunostaining of the thyrocytes is associated with the pinocytosis of thyroglobulin by the thyrocytes in parallel with an increase in release of thyroid hormone, and that this investigational approach provides a reliable indicator of thyroid hormone release activity.