Minute pulmonary meningothelial‐like nodules and myxedema

Abstract

Minute meningothelial-like nodules (MMN)formerly known as minute chemodectomas are incidental microscopic findings in lungs. Often multiple, they seem to be reactive proliferations rather than neoplasms. Their ultrastructure and immunohistochemical reactions are consistent with a meningothelial nature. The function of the presumed normal pulmonary cell underlying them is unknown. MMN occur somewhat more frequently in relationship to thromboembolism, heart failure, and high altitude. It has been postulated that a tendency to pulmonary edema may be a common factor in their initiation, and that, like arachnoid granulations, they may be involved in the absorption and transfer of fluid. We have observed large numbers of MMN in the lungs of a human cretin in whom myxedema was a prominent feature. We have also observed them consistently in the lungs of lambs with a similar condition made hypothyroid in utero by means of maternal thyroidectomy. Thyroid hormone normally plays a role in interstitial fluid homeostasis. MMN formation may be secondary to an alteration in interstitial fluid in the lung caused by a lack of thyroid hormone. Alternatively, it may be secondary to matrix protein or other metabolic effects of hypothyroidism, or might even be a direct response to pituitary secretion. The observation of an unexpected link between cretinism and MMN provides a potential tool for the study of MMN, and more importantly, for investigation of the underlying cell and its possible role in an arachnoid-like function in the periphery of the pulmonary acinus.