Thyroid gland cancer. Light micrograph of a section through a thyroid gland medullary carcinoma. The cells of the malignant (cancerous) tumour resemble those of bone marrow. It arises in the cells (C cells, dark brown) of the thyroid gland. Medullary carcinomas cause the C cells to produce calcitonin (thyrocalcitonin) hormone, which lowers the levels of calcium and phosphate in the blood. Medullary carcinomas are often associated with tumours of other organs (multiple endocrine neoplasia, MEN). Anti-calcitonin antibody stain. Magnification: x55 when printed 10 centimetres wide.