Histology of thyroid tissues (a) and immunohistochemistry for calcitonin (b) in thyroid tissue-organotypic culture. (a) At 40 days in culture, viable thyroid follicles enclosed by thyrocytes contain colloid substance in their lumens (F). H&E stain. (b) At 30 days in culture, thyroid follicles consisting of both thyrocytes and calcitonin-positive C cells (arrowheads) are clearly maintained.