Objectives: Teeth are formed through tissue interaction between dental epithelium and mesenchyme. However, maxillary incisor development seems to be more complex than the rests of the dentition because of the site where it develops. The maxillary incisor is composed of two thickening dental epithelia; the medial and lateral primary dental laminae (M-PDL and L-PDL); which align medio-laterally along the facial process. The primary dental laminae consequently fuse to form single dental lamina at the boundary between the medial nasal process (MNP) and maxillary process (MP) which fuse simultaneously. Methods: In order to investigate the contribution of MP to maxillary incisor development, MNP and MP epithelial of rat embryos were labeled with different fluorescent dye (diI and diO) prior to the fusion of the processes. The embryos were then cultured wholly for 30 hours and followed by maxillary organ culture for 18 hours. The labeled epithelia were then traced and taken photographs with fluorescent microscope. Results: The results show that the MNP epithelium contributes to both the M-PDL and the medial part of L-PDL whereas the MP epithelum contributes to the lateral part of L-PDL. To clarify whether lack of contribution of MP disrupts normal formation of maxillary incisor, we blocked fusion of MP to MNP by inserting aluminum foil between the processes. Iinhibition of the fusion between the processes disrupts the formation of the single dental lamina and results in separation of M-PDL and L-PDL in MNP. Conclusions: We conclude that development of maxillary incisor requires contribution of the maxillary process. Inhibition of the facial processes fusion not only keeps MP epithelium from contributing to L-PDL formation but also disrupts the fusion of M-PDL and L-PDL. This study was supported by the Thailand Research Fund # TRG4580022 and Grant-in-aid for scientific research from MEXT Japan.