Electron-microscopic studies of iodine-binding and peroxidase activity in the endostyle of the larval amphioxus (Branchiostoma lanceolatum)

Summary The asymmetric endostyle in the larval amphioxus (Branchiostoma lanceolatum) was examined by light-and electron-microscopic cytochemistry (peroxidase; incubation in diaminobenzidine) and autoradiography (incubation in ¹²⁵I^-). Compared to the adult the same cellular zones were also found in the larval endostyle, with the exception of zone 1, which was absent. The corresponding adult and larval zones had a similar morphology. All cells in zones 5a, 5b, and 6 were reactive for peroxidase. A reaction product was also present in the lateral 2 to 3 cell rows of zone 3. The dense reaction product was located on the inner surface of membranes of the rough endoplasmatic reticulum, Golgi apparatus and vesicles, and multivesicular bodies as well as on the outer surface of the luminal plasma membrane. An incomplete row of granule-containing, peroxidase-negative cells was located between zones 5b and 6. After incubation of larvae in sea water containing ¹²⁵I^-, autoradiographic grains were selectively concentrated over the lumen at the apical surface of all peroxidase-positive zones. The highest grain density occurred in relation to zone 5a, which in the adult has been recognized as the iodination center. Few grains were located over the cytoplasm. Methimazole, an inhibitor of peroxidase, abolished the cytochemical reaction and the appearance of autoradiographic grains. The observations indicate that iodination in the larval endostyle takes place extracellularly and is catalyzed by peroxidase bound in the plasma membrane.