Development of the upper lip.

Objectives: To affirm and reanalyze George L. Streeter’s “mergingtheory”ofupper-lipdevelopmentinprimatesby observing progressive embryologic stages in facial developmentusingscanningelectronmicroscopy(SEM)andto further understand upper-lip development. Design: The study was conducted at the California RegionalPrimateResearchCenter,Davis.Twentyprimateembryos (Macaca fascicularis) and 2 fetuses were examined with SEM. The development of the frontonasal prominence,maxillaryprominence,medialnasalprominence,and lateral nasal prominence were sequentially observed. The contribution of these prominences to the formation of the upper lip and nose were carefully analyzed. Results: The maxillary prominence and medial nasal prominence form the upper lip, whereas the lateral nasal, medial nasal, and maxillary prominences form the nose. There is fusion of the maxillary prominence with the medial nasal prominence. This fusion has not been previously described. This has resulted in a modification of the current theory of upper-lip development into one we refer to as the “dynamic fusion theory.” Conclusions: The dynamic fusion theory explains the merging process of the mesenchymal and ecotodermal layersofthefacialprominencesthatcontributetotheupper-lip formation. The dynamic fusion theory of facial prominence movement details the interaction between epithelial layers: both epithelial layers must fuse properly to avoid cleft-lip deformities.