ABSTRACT
The thyroid gland of ten albino rats aged 4 weeks [with high blood level of thyroid stimulating hormone] were studied ultrastructurally in an attempt to elucidate the functional activity of the gland and the pathway of thyroid secretion. The ultrastructural features of the thyroid follicular cells showed two patterns of activity indicative of synthesis and secretion. The main part of thyroglobulin synthesis was localised in ribosomes, transported to cisternae of endoplasmic reticulum, then migrated to the Golgi zone and came out of this zone in small membrane-bound vesicles to be released into the colloidal lumen. Based on the observations of this work, it was possible to recognize two pathways of of hormone release within the active processing and secretory cells the major pathway includes the formation of colloid droplets from the follicular lumen, this is followed by intracellular digestion of colloid by lysosomes and subsequent release of thyroid hormone; the other minor pathway of the secretion takes place within the follicular cells and both hormone synthesis and iodine-protein-binding are intracellular processes involving formation of small vesicles within the Golgi apparatus. The results of this work point out to the involvement of the apical microvilli and the specialization of the basal plasma membrane in concentration of iodide. In conclusion, two stages in the secretory cycle of thyroid follicular cells have been recognized. The synthesizing cell represents the first stage, where the precursors of the thyroid hormone were formed while the active processing and secretory cell represented the second stage, where packing and different secretory pathways were established