Basic Morphology

As proposed by Shimada et al. in their original classification (Shimada et al. 1984), pNTs are classified into four basic morphological categories (Shimada et al. 1999a): neuroblastoma (Schwannian stroma-poor); ganglioneuroblastoma, intermixed (Schwannian stro-

ma-rich); ganglioneuroma (Schwannian stroma-dominant); and ganglioneuroblastoma, nodular (composite, Schwannian stroma-rich/stroma-domi-nant and stroma-poor). Within each category one or more subtypes are recognized (see below). The first three categories and their subtypes are based on morphological changes according to the maturational sequence. The Shimada system distinguishes biologically favorable pNTs with a potential of age-appropriate maturation and biologically unfavorable pNTs without such a potential (see Chap. 4). In the last category, the tumor is composed of clearly distinct multiple clones, representing different states of maturation or maturational arrest. Among these categories, ganglioneuroma is not considered a separate entity, but rather as a fully mature form of tumor constituting the end of the biological continuum for all the pNTs, a model which postulates that ganglioneuromas are neuroblastomas at a later time in their development.

To date, there is no clear distinction in molecular characteristics between pNTs with a potential of regression and pNTs with a potential of maturation. In fact, during the maturational sequence of pNTs, the vast majority of neuroblastic cells probably die before or after reaching a certain degree of maturation (cellular death during the process of tumor maturation, comparable to cellular death seen in the process of normal organogenesis). By contrast, Schwannian stroma, once developed and established, are believed by many to constitute a persistent and dominant component in pNTs.

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