[Ultrastructural morphogenesis and functional differentiation of the dogfish thyroid follicle (author's transl)]

From the 20 mm stage, in the cells of the dogfish embryo thyroid, some Golgi vesicles get the ability to trap iodine. At subsequent stages, electrondense intracytoplasmic cavities with microvilli are observed in the Golgi field; their membranes also trap iodine. These cavities increase in volume, draw toward the cellular periphery and export their contents in an intercellular space previously limited by tight junctions. The same events occur in several adjacent cells, which constitute the first follicular lumen. The last stage of follicular individualization is basement membrane formation. Arguments are given which support the hypothesis of the Golgi origin of intracytoplasmic cavities with microvilli. Iodine organification just as thyroid hormones synthesis and secretion occur at 25 mm stage, when cells are not grouped in follicle. It is concluded that cells containing intracytoplasmic cavities with microvilli are fully functional units.