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Most granular cell tumours (GCTs) are benign proliferations of purported Schwannian derivation, showing immunoreactivity for Schwann cell-related antigens. Due to incomplete agreement on the precise nature of GCTs (reactive vs neoplastic), we performed an immunohistochemical study with the alkaline phosphatase/anti-alkaline phosphatase (APAAP) technique on 30 GCTs. The aim was to evaluate their growth patterns and the possible relationships of granular cells with other nerve sheath-related cell types (i.e., Schwann and perineurial cells, and dendritic cells displaying CD34/vimentin immunoreactivity). An expansive growth pattern was detected in five cases, a pseudoinfiltrative growth pattern in nine cases and a mixture of the above in the remaining 16 cases. Besides immunoreactivity for S-100 protein, neuron-specific enolase, vimentin, CD57, CD68, MAC 387, alpha-1-antitrypsin and alpha-1-antichymotrypsin in granular cells, we documented intimate architectural relationships between granular cells, Schwann and perineurial cells, and a third type of CD34/vimentin-positive nerve sheath-related cell in most GCTs. These results suggest that GCTs are heterogeneous lesions. Some of them show a pseudoinfiltrative growth pattern and retain close relationships with the normal components of the nerve sheath. In other lesions, granular cells grow in an expansive fashion and constitute the predominant cell component of the tumour. These architectural and immunophenotypic differences may reflect a different nature of GCTs: they may initially represent reactive or hamartomatous lesions that subsequently acquire truly neoplastic potential. [ABSTRACT FROM AUTHOR]