Histopathology, classification, and grading of gliomas

Neoplastic transformation occurs in all glial cell types of the human nervous system, producing a wide variety of clinico‐pathological entities and morphological variants. Astrocytomas are most common and span an unusually wide spectrum, ranging from the slowly growing juvenile pilocytic astrocytoma to the highly malignant glioblastoma multiforme. Diffusely infiltrating astrocytomas of the cerebral hemispheres show an inherent tendency for progression towards a more malignant phenotype. This change is morphologically categorized in histologic grading schemes (e.g., WHO Grade II to IV) and is associated with the sequential acquisition of genetic alteration, including mutation in the p53 and homozygous deletion of the p16 tumor suppressor genes. Loss of heterozygosity on chromosomes 10 and 19q as well as amplification of the EGF receptor are largely restricted to malignant gliomas and thus considered late events in astrocytoma progression. Gliomas often show phenotypic expresson of different glial cell lineages (e.g., oligoastrocytoma). Recent studies sugeest that the occurrence of mixed glomas is not indicative of a polyclonal orgin but rather reflects altered gene exprssion leading to a charge in the balance of growth factors influencing glioma defferentiation. © 1995 Wiley‐Liss, Inc.