Renal angiomyolipoma: an ultrastructural investigation of three cases with histogenetic considerations

Renal angiomyolipoma is a rare lesion composed of smooth muscle cells, adipose tissue and abnormal vessels. It is currently classified as a benign, non-epithelial renal tumor. It has a high incidence in patients suffering from tuberous sclerosis but is more frequently found as an isolated renal lesion. Three cases of renal angiomyolipoma, 2 of which underwent perfusion-fixation, were studied by electron microscopy to clarify the cellular composition of this lesion. In the smooth muscle cells abundant accumulation of glycogen was found, whereas the lipocytes disclosed normal ultrastructural features. However, a smaller number of smooth muscle cells also contained lipid, thus simulating an intermediate cell stage between adipose- and smooth muscle cells. The abnormal thickening of the subendothelial spaces contained collagen fibrils in a homogeneous matrix, fibroblast-like cells and non-specific vesicular structures. These findings suggest a secondary vascular damage, i.e. the thickened vessels may not be a primary, integral part of renal angiomyolipoma. Evidence of a common precursor cell of renal angiomyolipoma was not disclosed. It is concluded that renal angiomyolipoma is a hamartoma composed of mature adipose cells and smooth muscle cells displaying different degrees of abnormal differentiation.