A new Odontogenic tumour has been officially recognized by the World Health Organization (WHO) as seen in the latest edition of the Classification of Head and Neck Tumours of 2017. Only 7 cases of the Primordial Odontogenic tumour has been reported so far (6 in 2014 and 1 in 2016). The tumour represent a kind of primordial ectomesenchymal odontogenic proliferation with features closely resembling those of the dental papilla. As these develop during childhood or early adolescence, the presence of unerupted teeth may be the result of their displacement during tumour growth. 
The WHO Classification of Head and Neck Tumours was updated this year. This book reflects the views of a working group that convened for a consensus and editorial meeting at the lnternational Agency for Research on Cancer, Lyon, 14-16 January 2016. It includes a list of each tumoral and cyst entities of the head and neck, including a recopilation of all the knownledge available of each tumour, describing the diagnostic criteria, pathological features, and associated genetic alterations in a disease-oriented manner. Among this new actualization, we have a new odontogenic tumour called "Primordial odontogenic tumour". 
Reported for the first time in 2014 by Mosqueda-Taylor A, Pires FR. AguirreUrizar JM, Carlos-Bregni R, de la Piedra-Garza JM, Martínez-Conde R, Paes de Almeida O and de Barros Dias BS. from the Autonomous Metropolitan University of Xochimilc (Mexico), Rio de Janeiro's University of the state (Brasil) and Vasco's Country University(Spain). They discovered 6 cases five involving the posterior region of the mandible, and one the posterior maxilla. There were three female and three male patients, whose ages ranged from 3 months to 18 years (mean, 11.5 years), no pain was referred, cortical expansion with displacement of teeth was noted. All lesions appeared as well-defined radiolucencies closely related to unerupted teeth; five showed an apparent pericoronal position in a dentigerous relationship, whereas the other one appeared to envelop completely an unerupted lower third molar and root resorption of neighbouring teeth.  
Representative radiographic images of Primordial Odontogenic Tumour. Taken from Mosqueda-Taylor A (2014). 
Macroscopically, The crown of the adjacent tooth was embedded in the tumour in 3 of the reported cases; in the other 4 cases, the associated teeth were easily detached from the tumours. All lesions were well circumscribed and salid, forming multilobulated whitish masses, with no cystic spaces on sectioning. 
Histopathology results shows a loose fibrous tissue containing variable numbers of fusiform and stellate fibroblasts, with minimal collagen production. Some areas have the appearance of cell-rich mesenchymal tissue. A characteristic finding is that the entire periphery of the tumour is covered by columnar or cuboidal epithelium, which in sorne areas shows scant, superficial layers of fusiform cells surrounded by a thin fibrous capsule. 
The lesion described as a solid tumour forming large, well-defined radiolucent lesions in close association with the crowns of unerupted teeth, was not considered a hyperplastic dental follicle or an adenomatoid odontogenic hamartoma, because of their great size. Microscopically, the main differential diagnoses should include ameloblastic fibroma, central odontogenic fibrom , odontogenic myxoma, and a hyperplastic dental follicle. 
(Left Picture) Microscopic findings of primordial odontogenic tumour: loose and myxoid-appearing fibrous tissue covered over its entire surface by columnar epithelium and a fibrous capsule (H&E) - (Right Picture) Macroscopic appearance showing the molar tooth embedded within the tumour. Taken from Mosqueda-Taylor A (2014). 
You can download Mosqueda-Taylor et al. Original article by clicking here
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Mosqueda-Taylor A, Pires F R, Aguirre-Urızar J M, Carlos-Bregni R, de la Piedra-Garza J M, Martınez-Conde R, Martınez-Mata G, Carre~no-Alvarez S J, da Silveira H M, de Barros Dias B S & de Almeida O P.Primordial odontogenic tumour: clinicopathological analysis of six cases of a previously undescribed entity .Histopathology: 2014 65, 606–612. 
EI-Naggar A.K., Chan J.K.C., Grandis J.R., Takata T., Slootweg P.J. (Eds): WHO Classification of Head and Neck Tumours (4th edition). IARC: Lyon 2017. 
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