Ultrastructure of osteoblastoma.

The ultrastructure of three cases of osteoblastoma is described. The osteoblasts resemble normal osteoblasts with a few exceptions: irregular, indented nuclei, and occasional mitochondria with curved cristae and electron-lucent areas. The osteocytes and osteoclasts basically resemble their normal counterparts. There are also osteoprogenitor cells in different stages of maturation, some containing glycogen. The ultrastructure of an additional case histologically classified as aggressive osteoblastoma is described. It is essentially similar to the previous three cases of typical osteoblastoma. The only exception is the presence of osteoblasts with electron-lucent nuclei and less prominent organelles than the typical osteoblastoma cells have markedly indented and multilobed nuclei, dilated rough endoplasmic reticulum and lipid droplets. However, these differences from osteoblastoma cells are not pathognomonic. The final diagnosis of osteoblastic tumors rests at the light microscopy level.