Since Bailey and Cushing (1926), all brain tumor classifications have
been called histogenetic. The nosographic position that the tumor types
progressively acquired in the classification systems derived from the
resemblance of tumor cells to those of the cytogenesis, modified
whenever new information became available from different biological
research fields and especially from molecular genetics. Classically, on
the basis of the rough correspondence between the mature/immature aspect
of tumor cells and the benign/malignant biological behavior of the
tumors, the histological labels contained a prognostic significance. The
supposed origin of the tumors was thus a factor for prognosis. Later on,
with the concept of anaplasia (Cox, 1933; Kernohan et al., 1949) new
criteria were introduced for establishing the malignancy grades of
tumors. Immunohistochemistry and later molecular genetics further
refined the prognostic diagnoses, substantially increasing the
opportunities to recognize the cell origin of tumors, beside revealing
the pathogenetic mechanisms. Prognoses became more accurate, as required
by the greater and more targeted possibilities of therapy.