Fetal mesenchymal hamartoma of kidney. A tumor of secondary mesenchyme.

The ultrastructure of a fetal mesenchymal hamartoma of the kidney shows undifferentiated mesenchymal cells, whose morphological characteristics conform to those of secondary mesenchyme. In contrast to primary mesenchyme or mesoblast, which may form epithelial structures, secondary mesenchyme is incapable of doing so. While Wilms' tumor may be considered a tumor of mesoblast, the fetal mesenchymal hamartoma is believed to originate from secondary mesenchyme or some of its more mature derivatives.