Pulmonary hamartoma. An ultrastructural study

Three so‐called pulmonary hamartomas were studied by electron microscopy. Histologically, the hamartomas included one case with mainly mature cartilage (chondroma), one with loose myxoid fibrous tissue and rare areas of mature cartilage, and one with only loose myxoid fibrous tissue. Ultrastructurally, a common feature in all three cases was the presence of stellate, undifferentiated mesenchymal cells. These cells were sparse in the chondroma, and abundant in the other two cases. Areas of transition between undifferentiated mesenchyme and cartilage were present in two cases (the condroma and the myxoid fibrous tumor with areas of mature cartilage). Unusual stellate smooth muscle cells were present in areas of loose myxoid tissue. The epithelial components revealed continuity and morphologic identity to terminal bronchiolar and alveolar epithelium. A distinct basement membrane was always present. These observations support the concept that pulmonary hamartomas represent a histologic spectrum of benign mesenchymal neoplasms, which originate in peribronchial connective tissue and incorporate respiratory epithelium as they expand.