Histogenesis of the so-called "intravascular bronchioloalveolar tumour".

Three cases of Dial and Liebow's intravascular bronchioloalveolar tumour are described, with emphasis on ultrastructure and histogenesis. The tumour cells are quite different from adjacent reactive alveolar cells and electron microscopy does not support the suggested alveolar cell origin. The tumour cells contain abundant microfilaments, moderate amounts of rough endoplasmic reticulum and varying numbers of Weibel-Palade bodies. With features of smooth muscle, myofibroblast and endothelial cell differentiation, a vascular origin from precursor mesenchymal cells such as the vasoformative reserve cells is suggested.