[A] Microscopic view of normal thyroid tissue with hyperplastic C-cells (arrows) dispersed between the follicles. C-cell hyperplasia (CCH) was defined as clusters of intrafollicular atypical C-cells (more than 50 per "low-power field" at 100× magnification) that lead to partial or complete obliteration of the follicular space. [B] C-cells are positive for immunohistochemical staining for calcitonin (dark areas). [C] Microscopic view of medullary thyroid cancer (MTC). Malignant C-cells have broken through the basement membrane and invaded the interstitium, which has led to stromal fibrosis (arrows). MTC lesions are composed of solid nests of epithelial cells with poorly defined cell borders. [D] Occasionally there is deposition of intensely eosinophilic material, amyloid, which can be stained with Congo-red (arrow). Amyloid is derived from calcitonin.