The natural history of pulmonary ossification

Abstract

Introduction: Pulmonary ossification (PO) is a rare pathologic finding, defined as a widespread heterotopic bone formation within the lungs. PO occurs in two forms. The first form is the nodular circumscribed type; the second form is the racemose or branching type. Males are more often affected than females. PO occurs more commonly in older patients, in the sixth decade. The most common site of occurrence is the basal lobes.

Material and Methods: Autopsy of a 65 year old man revealed advanced hepatic cirrhosis. Death resulted from fatal hemorrhage from ruptured esophageal varices. In addition we found widespread PO in both lungs. About 100 tissue samples were taken from the lungs, processed in a histokinetic tissue processor, impregnated with paraplast, and cut into 5 μ sections. The sections were stained as follows: Hematoxylin and Eosin, Verhoef's elastic stain, van Gieson Trichrome stain, Gordon and Sweet's Silver impregnation, Prussian Blue reaction, PAS reaction, Celestin blue, Hematoxylin, Ethanolic-Saffron, Alcian Green (pH 2.8, CHESA) stain reaction.

Results: The formation and growth of these bony structures occur as a type of osseous metaplasia of the connective tissue. Mesenchymal cells proliferate in the interstitial tissue and produce osteoid. Insofar as formation and growth take place in the absence of blood vessels, immature (woven) bone is formed. This is true for the nodular circumscribed type of PO. In the branching type of PO blood vessel proliferation is often observed, which results in the formation of mature (lamellar) bone. These mature bone structures often enclose an endosteum-lined marrow cavity which contains fat or red marrow. Finally, we also observed peculiar desmoid-like connective tissue proliferation, which occasionally showed transformation into bone.

Discussion: PO occurs in two forms. The nodular cirbumscribed type is composed of osseous and osteoid material without marrow elements. It is usually deposited within the lung septa. The second form is the racemose type. It consists of branching spicules of bone in the alveolar septa. This osseous tissue contains mostly marrow elements. As for the pathogenesis of PO, we believe in heterotopic bone formation resulting from connective tissue proliferation which subsequently undergoes osseous metaplasia. In addition to PO, we found peculiar desmoid-like connective tissue proliferation in the lungs, which occasionally showed transformation into bone. PO appears to be functionally unimportant.