Relative contributions of the facial processes to facial development: a microsurgical assay.

The facial processes, which consist of the medial nasal process (MNP), the lateral nasal process (LNP), and the maxillary process (MP), are basal components in facial morphogenesis, especially upper lip formation. To examine the relative role of each facial process in normal or abnormal facial development, rat embryos that had had a part of each facial process excised were cultured for 72 hr in vitro from gestational day 11.5 (plug day = day 0). At the termination of culture, although the epithelial wound was healed over, the defect was observed corresponding to excised region in form. Only in the MNP-excised group was cleft liplike malformation observed, but in other groups this malformation was absent or at a lower rate. This suggests that the medial nasal process in this stage plays a critical role in normal facial development as well as cleft lip formation.