Ultrastructural characteristics of thyroid medullary carcinoma

17 thyroid medullary carcinomas (TMC) were studied electron microscopically (EM). Two cell groups were revealed regardless of their histological structure: EM differentiated (with signs of C-cell differentiation) and non-differentiated (without these signs). There is no correlation between histological structure of the tumor and the degree of its EM differentiation. In patients who lived 5 years maximum after the diagnosis there were different histological types but EM the tumors had relatively monomorphous structure with domination of non-differentiated cells (more than 70%), high content of dark cells (50% and more) and low content of desmosomes (less than 30%). The prognosis is more favourable the higher is content of EM differentiated cells, the better desmosomes are developed and the lower is content of dark cells. The scope of surgery and local metastases do not influence the prognosis while distant metastases (stage IV) deteriorate the prognosis.