Smooth muscle hamartoma of the lungs in a Wistar Hannover rat

Abstract

Hamartomas are tumor-like masses comprising disorganized normal tissue elements. To date, spontaneous hamartomas have been reported in several organs and tissues in rodents but not in the lungs. Here, we report the first case of a hamartoma in the lungs of a 108-week-old female Wistar Hannover rat. Grossly, a white spot, 7 mm in diameter, was observed on the costal surface of the left lung. Histopathologically, the nodular lesions adjacent to the bronchioles comprised mature smooth muscle cells. The lesion was not encapsulated and spread along the alveolar walls and ducts without compression of the surrounding tissue. In the nodules, elastic fibers enclosed small lumens lined with factor VIII-related antigen-positive endothelial cells. This structure suggested that the nodule mimicked an artery. Moreover, structural abnormalities were observed within the bronchioles and arterioles owing to the increased number of smooth muscle cells in the surrounding tissues. These features suggested that this was a case of tissue malformation rather than a neoplasm, leading to the diagnosis of a smooth muscle hamartoma of the lung.

Keywords: Wistar Hannover; lung; malformation; mooth muscle hamartoma; rat.

Fig. 1.

The white spots in the lungs were stained with hematoxylin and eosin. A nodular lesion comprising bundles of eosinophilic spindle cells is observed adjacent to the bronchioles. The nodule is not encapsulated and spread along the alveolar walls and ducts (arrowheads) without compression of the surrounding tissues and merges into the lamina propria and muscle layer of the bronchioles (arrow) (A). Spindle cells have short-to-long fusiform mature nuclei and abundant eosinophilic cytoplasm, which often includes vacuoles. The cells show no atypia of nuclei (B). Bundles of spindle cells concentrically enclosing another bundle are observed in some alveolar walls (C). The wall of the arteriole near the nodule is partially thickened by the accumulation of spindle cells (arrow) (D). Bars indicate 200 μm (A), 50 μm (B), and 20 μm (C and D).

Fig. 2.

Elastica Van Gieson (EVG) staining reveals the circular arrangement of elastic fibers in the nodule (A). The elastic fibers are also observed in the bundles of spindle cells concentrically enclosing another one (B). The spindle cells are increased between elastic fibers and epithelial cells of the bronchi which connect with the nodule (C). The spindle cells are also increased in the sub-endothelium of the arteriolar wall resulting in partial thickening of it (D). All figures show EVG staining. All bars indicate 20 μm.

Fig. 3.

Immunohistochemistry of the nodule. All spindle cells are positive for α-SMA (A). There is a lumen lined by factor VIII-related antigen-positive endothelial cells (arrow) inside of the elastic fiber ring (between arrowheads). Insertion shows the same region stained with EVG staining (the same as Fig. 2A) (B). PCNA-positive cells were few in any constituent cells of the nodule including the spindle cells (C). Bars indicate 200 μm (A) and 50 μm (B and C).