Silicotic nodule

Silicotic nodules are concentrically layered nodules (confluent in this case) of hyalinized fibrous tissue. The cellular rim is a zone of activity. It has some black pigment in this case. Polarized light shows faint, milky birefringence of squarish crystals of silicon dioxide (silica). These are to be distinguished from brightly birefringent crystals of silicates, such as talc.

In silicosis, nodules are first found in regional lymph nodes and then in the lung. The lesion of silicoproteinosis (acute silicosis) is illustrated elsewhere.

Perivascular granulomas, intravenous drug user

This periarterial collection of epithelioid cells (arrows) represents a reaction to silicate crystals that are inject intravenously. The crystals may either cause intravascular thrombosis or may traverse the vessel wall to form perivascular granulomas, as in this case. The artery is at the lower left.

When viewed by polarized light, the perivascular collections of crystals have plate-like or needle-like (arrow) forms.

See also Case 20