What is the typical appearance of cells in a medullary thyroid carcinoma?

In medullary thyroid carcinoma, the hallmark characteristic is the presence of sheets of cells within an amyloid stroma. This type of cancer arises from parafollicular C cells (also known as C-cell carcinoma) that produce calcitonin. During histological examination, medullary thyroid carcinoma displays nests or sheets of polygonal or spindle-shaped cells. The stroma is often infiltrated with amorphous deposits of amyloid, which is derived from the misfolding of calcitonin or other peptide hormones. The presence of amyloid is a key diagnostic feature and can be confirmed using special stains like Congo red, which highlights the amyloid deposits, giving the tissue a characteristic apple-green birefringence under polarized light.

This distinctive combination of cellular arrangement and amyloid presence is what distinguishes medullary thyroid carcinoma from other types of thyroid neoplasms, contributing to its diagnosis and the understanding of its behavior.