ANALYSIS OF THE TOPOGRAPHIC DISTRIBUTION OF THYROID ACTIVITY IN A TELEOST FISH: CARASSIUS CARASSIUS L.

ABSTRACT Combined autoradiography and X-ray photography has demonstrated that the tissues of the caudal part of the lower jaw and the pronephric region have a considerably larger uptake of radioiodine than other tissues in the immature crucian carp, Carassius carassius L. It is known from previous studies that thyroid follicles occur in these regions. The pronephric thyroid differs from the mandibular thyroid with regard to radioiodine turnover. On an average, the pronephric thyroid has a larger uptake than the lower jaw thyroid. The relative uptakes varied considerably in the population investigated. The ratio mandibular uptake/total thyroid uptake has been determined at several sampling intervals. The means of these ratios show no significant trend with time, a finding which strongly supports the view that the two regions are variably sized parts of a physiologically homogeneous thyroid gland. The analysis is based on a mathematical model of iodine turnover. The method of comparing complex regression functions (sums of exponential functions) should be widely applicable. The correlation between the radioiodine content of either region and total thyroid content is low. Thus, it is necessary to study simultaneously all follicle-carrying tissues in fish with more than one aggregation of thyroid elements. The net radioiodine turnover in the crucian carp indicates a comparatively high degree of thyroid activity and considerably higher than that reported for C. auratus L. This phenomenon is discussed.