Note the central necrosis, the surrounding palisading granulomatous reaction, and outer fibrosis. In Caplan's syndrome in coal miners, the necrotic centers contain considerable black pigment, and the granulomatous reaction is not so prominent. In contrast to the rheumatoid nodule, the simple silicotic nodule, shown here in a lymph node, is composed of concentric layers of collagen. There is no central necrosis. The palisading granulomatous reaction of a rheumatoid nodule is absent.
Polarized light reveals birefringent silica crystals.
If these nodules aggregate into large masses as in progressive massive fibrosis, there may be central necrosis and cavitation. Further, central necrosis and cavitation may occur in smaller nodules that become infected with mycobacteria, making the distinction from Caplan's nodules more difficult. The simple coal macule of coal miner's pneumoconiosis consists of an expansion of the interstitium around airways by macrophages filled with black pigment. It lacks necrosis and granulomatous inflammation of Caplan's nodules. In the picture, note the black pigment around the bronchiole as well as the irregularly-shaped coal macule at the upper right.