Morphological and functional development of the thyroid tissue in rainbow trout (Oncorhynchus mykiss) embryos

The development of the thyroid tissue in rainbow trout (Oncorhynckus mykiss ) embryos between 20 and 88 days postfertilization (dpf) was studied using traditional histological methods in combination with immunostaining using a ntibodies raised against L-thyroxine (T-4) and triiodo-L-thyronine (T-3). I n 20 dpf embryos, the thyroid primordium appeared as a simple tube aligned with the dorsal surface of the ventral aorta. The tubular primordium become s progressively bifurcated at each end and follicles are formed by pinching off balls of cells from the tips of the bifurcations. The secretory activi ty of the thyroid tissue appears first as a synthesis phase, evident at 30 dpf, characterized by T-4 and T-3 immunostaining present only in the follic le lumen, A later hormone release or secretory phase was first evident for T-4 immunostaining at 43 dpf and for T-3 immunostaining at 46 dpf and was c haracterized by the presence of immunostaining both within the follicle lum en and also in the cytoplasm of some (but not all) thyrocytes. This stage w as also coincident with a mal ed increase in the number of pituitary thyrot rophs, The proportion of embryos exhibiting immunostained thyrocytes increa sed progressively from 43 to 52 dpf, at which stage all embryos had T-4- an d T-3-immunostained thyrocytes.