Early stages of synthesis of thyroglobulin (Tg), thyroxine (T4), and triiodothyronine (T3) in fetal rat thyroid. An immunoelectron microscopic study.

Immunoelectron microscopy was applied for detection of Tg, T4, and T3 in fetal rat thyroid gland during the early stages of synthesis of these compounds, in order to elucidate the subcellular aspects of onset of their production. Tg was detected in the perinuclear space, rER, and Golgi apparatus as early as the sixteenth day of gestation when no follicles had yet formed. When primitive follicles had formed after the seventeenth day of gestation, both T4 and T3, as well as Tg, appeared on the surface of microvilli and occasionally in their core portions. These findings provide evidence to suggest that follicles are essential to extracellular synthesis of T4 and T3 in fetal rat thyroid gland.