Placental transmogrification of the lung is a histologic pattern frequently associated with pulmonary fibrochondromatous hamartoma.

Placental transmogrification of the lung is a term introduced to describe a peculiar histologic pattern characterized by formation of placental villuslike structures in the lung parenchyma. It has been reported to occur in association with bullous emphysema and lipomatosis. To study the relationship between placental transmogrification and pulmonary hamartomas. Reports of 38 cases of pulmonary hamartomas during 18 years (1982-1999) were reviewed. All histologic slides of these cases were examined for the presence of villuslike papillary projections and placenta-like structures. Hamartomas with prominent papillary projections or placenta-like structures were further investigated to assess the histogenesis and proliferation of epithelial and stromal cells. Immunohistochemical analysis was performed on paraffin-embedded tissue using monoclonal antibodies against Ki-67 and thyroid transcription factor 1 (TTF-1) and polyclonal antibodies against c-Kit antigen (a stem cell factor receptor/mast cell growth factor receptor) in conjunction with Leder stain for naphthol-ASD-chloroacetate esterase. Placental transmogrification was identified in 6 of 38 cases of pulmonary fibrochondromatous hamartomas. The histologic change consisted of an abundant myxoid or edematous fibroadipose stroma with a respiratory epithelial lining, resulting in papillary projections that resembled immature placental villi. Epithelium lining the papillary projections was positive for TTF-1 (70%-90%) and Ki-67 (3%-5%). In contrast, stromal cells were negative for TTF-1 with only rare cells immunoreactive for Ki-67. A number of stromal spindle cells and occasional cells in epithelium were c-Kit immunoreactive; however, concurrent Leder stain demonstrated that these c-Kit-positive cells were mast cells and not stem cells. Placental transmogrification is frequently associated with pulmonary fibrochondromatous hamartomas and may be induced by or associated with a proliferation of lining epithelial components in the hamartomas. The significance of numerous mast cells within stroma of placental transmogrification is unclear and their possible role in inducing stromal proliferation needs to be further evaluated.