Critical study of endogenous peroxidase activity: its role in the morphofunctional setting of the thyroid follicle in the rat fetus.

Endogenous peroxidase is associated with thyroglobulin iodination. It first appears at 17 day of pregnancy in rat fetal thyroid and seems to be synthesized within the perinuclear cistern and the rough endoplasmic reticulum. Then, the enzyme is transferred by vesicles up to the follicular lumen where thyroglobulin iodination takes place. The different steps of this process can occur within a very short time, since peroxidase activity is already located in the apical plasma membrane of follicles formed as early as 17 days of pregnancy, which is consistent with autoradiographic data showing iodinated thyroglobulin at 17 days. It can be concluded that the follicle is functional as soon as it is formed.