FINE-STRUCTURAL LOCALIZATION OF ENDOGENEOUS PEROXIDASE IN THE ENDOSTYLE OF ASCIDIANS, CIONA-INTESTINALIS - PART OF PHYLOGENETIC STUDIES OF THE THYROID-GLAND

The fine structural localization of peroxidase activity in the endostyle of C. intestinalis was studied by cytochemistry at EM level. Only zone 7 cells in this organ are positive for this enzyme activity. The reaction product is recognized in the external surface of the apical plasma membrane, in the cisternae of the rough endoplasmic reticulum, of the nuclear envelope, the Golgi lamellae, small vesicles of the Golgi region and of the apical cytoplasm. By EM autoradiography of 125I, the main site of iodination of thyroglobulin-like protein is the apical plasma membrane region of the zone 7 cells. The possibility of iodination of protein taking place within the endostylar lumen near the zone 7 cell cannot be ruled out. The relationship between the fine-structural localization of peroxidase activity and the site of iodination of protein is discussed and this may lead to a better understanding of the evolution of the thyroid gland.