Not in the classification: odontogenic hamartomata and allied lesions: can we find clarity at the interface of development and neoplasia?

Odontogenic lesions can present the diagnostic pathologist with uncertainty due to the range of clinical presentations and histological features. Whilst most of the tooth-forming apparatus involutes on completion of tooth formation, small remnants of both the epithelial and mesenchymal components remain in many individuals, both within the jaws and the overlying gingival mucosa. These developmental remnants can give rise to a range of lesions which span from reactive/inflammatory lesions through hamartomata to those which are neoplastic in nature. This varied etio-pathogenesis does not result in discrete diagnostic entities, but rather a range of overlapping clinical presentations and histological features, which makes definitive diagnosis challenging. This spectrum of lesions and their interrelationships have long been the subject of discussion in the oral pathology literature, and because many of the issues raised regarding our understanding of the biological potential of these lesions remain unresolved, this has contributed to persistent ambiguity in their diagnostic features. In this paper, we will outline the literature published relating to two poorly defined and understood epithelial and mesenchymal odontogenic lesions, which commonly arise in the gingiva and in relation to the crown of unerupted teeth, respectively. We aim to provide clearer diagnostic criteria for two poorly described entities, the odontogenic gingival epithelial hamartoma (OGEH) and the odontogenic follicular hamartoma (OFH), whilst acknowledging the spectrum of lesions which exist and where diagnostic certainty cannot be assured. Additionally, we briefly review the recent progress in understanding the inter-relationship of developing odontoma and ameloblastic fibro-odontoma with a view to suggesting helpful features which will allow for greater diagnostic certainty in this area.