We have exammed the organic components of the nidus of an osteoid oste
oma in 4 patients using scanning electron microscopy. Different organi
sation of the osteoblastic cells and of the fibrous structures of the
osteoid matrix in different zones was demonstrated. These findings ind
icated a greater degree of immaturity in the centre, as compared with
the peripheral area where the process of differentiation was more evid
ent.