Fig. 3.5. a A honeycomb, some still filled with baby bees, corresponding to the nuclei of an oligodendroglioma. b "Honeycomb" pattern of an oligodendroglioma. S-4362-81, H&E.

neuroectodermal or not needs to be proven by other studies.

Many well differentiated tumors frequently consist of more than one type of cell, just as a family contains old and young persons, male and female, tall and short, even skinny and obese. This is especially true in gliomas, as astrocytes (fibrillary, gemistocytic or protoplasmic), oligodendroglia and even ependymocytes share the same progenitor. When a diagnosis of a particular type of glioma is made, it does not necessarily imply a pure culture of that particular type of glia cell, only a majority of that type of cell. When the proportion of other types becomes significant, a diagnosis of mixed glioma can be made, although a determination of the necessary proportion is quite arbitrary and subjective - usually at least 25%. This is another area where controversies arise.

Patterns of Cellular Arrangement

Each tumor appears to be different. In order to classify tumors into groups, one has to find some common denominator for each group. As we mentioned above, we first use the morphology of the tumor cells to estimate the lineage of the cell and tumor. However, although the histological variations on normal cells are very limited, those of tumor cells are quite marked. No matter how we combine the shape, size and degree of staining of the nucleus and cytoplasm and the shape, size and number of cell processes, we can make only a limited number of normal cell types, which can resemble the tumor cells to various degrees. Not all tumors are diffusely infiltrating or consist of randomly arranged or packed cell masses. Tumors belonging to a similar lineage have a tendency to show a particular pattern formed by groups of cells. These patterns are not specific and frequently overlap among different groups but are helpful when combined with cell morphology. The patterns are not necessarily present in the whole tumor but are often found only in small foci, which one has to search for.

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