BIOCHEMICAL AND FUNCTIONAL-CHANGES DURING THE BOVINE FETAL THYROID DEVELOPMENT

To establish biochemical and functional relations during thyroid devel opment, the activity of thyroid peroxidase (TPO), nicotinamide adenine dinucleotide phosphate (NADPH)-cytochrome c reductase and monoamine o xidase (MAO) in a particulate fraction and the iodide transport and or ganification in slices of bovine fetal thyroid were examined throughou t gestation. The cytochemical localization of TPO, H2O2 generating sit es and MAO was also studied. Fetal glands were grouped in stages I to V according to increasing developmental features; adult tissues were a lso analyzed. TPO activity in each of the fetal stages was higher than in the adult; a marked increase was observed in stages IV and V. Iodi de transport (T/M) was similar in stages I to V and the adult. Iodide organification in fetal thyroids showed a similar pattern to that of T PO activity. When compared with the adult, at midgestation (stages II to III), a lower iodination coexisted with a higher TPO activity. The activity of NADPH-cytochrome c reductase and MAO, two enzymes previous ly proposed to participate in thyroid H2O2 generation, did not paralle l the level of iodide organification. Cells from stages II to V exhibi ted a positive cytochemical reaction for TPO in the rough endoplasmic reticulum (RER) and the perinuclear cisternae (PC). In stages IV, V, a nd adult, TPO was occasionally found in apical vesicles and microvilli , whereas H2O2 was detected within the RER and the PC. MAO reaction wa s positive in adult, but not in fetal thyroid. These results indicate that a high TPO activity accompanied the onset of the organification p rocess during fetal thyroid development. The level of iodination was a ssociated with the presence of TPO at a proper site rather than to the level of TPO activity. Evidence against a role of NADPH-cytochrome c reductase and MAO in the iodide organification was obtained.