Duodenal carcinoid with cytoplasmic whorls of microfilaments.

A unique ultrastructural feature in a duodenal carcinoid is described, viz., large juxtanuclear accumulations of microfilaments arranged in whorls. Similar microfilaments are found in the D1 cell of the gastro-entero-pancreatic endocrine system. Apparently identical whorl-like accumulations of filaments have been previously described in bronchial cells of animals treated with nitrosamines and in human giant cell carcinoma of the lung. We suggest that the D1 cell is the cell of origin for the described carcinoid as well as for the above-mentioned tumours of the lung.