Under the microscope, medullary thyroid carcinoma is made up of spindle-shaped cells as they’re long and skinny, like a spindle that’s used to spin fibers into thread. C-Cells in the tumor make excessive amounts of calcitonin which deposits between the C-Cells. As the calcitonin deposits, the resulting clumps of protein stick together and form fibrous deposits called amyloid around the C-cells, which can be seen with Congo red stain.