The ultrastructure of medullary carcinoma of the breast.

Three cases of medullary carcinoma complying with strictly defined criteria have been examined by electron microscopy. These carcinomas are characterised by the presence of light and dark tumour cells which exhibit prominent organelles and well-developed Golgi complexes but lack secretory activity. The essential stromal cellular infiltrate of macrophages, lymphocytes and plasma cells contains macrophage-lymphocyte clusters suggesting the existence of an immune response. Also present in the stroma are blood vessels lined by high-endothelial cells of the type said to facilitate lymphocyte migration. It is suggested that light and dark tumour cells with well-developed organelles but absent secretory activity, macrophage-lymphocyte clusters and stromal high-endothelial venules represent specific ultrastructure features of typical medullary carcinoma associated with good prognosis.