Morphogenesis of the human upper lip. (A) SEM facial view of a Stage 13 human embryonic head. (B) SEM micrograph of the right nasal pit of a late Stage 15 human embryo. (C) Enlarged detail of the lower nasal pit shown in B. The boundary between the maxillary and lateral nasal processes is clearly marked by the rounded cells at the surface. Rounded cells also appear at the contact site between the medial and lateral nasal processes. (D) Lateral view of a Stage 17 human embryonic head. The maxillary process is puffed laterally and wedges between the medial and lateral nasal processes. (E) SEM micrograph of a Stage 18 human embryonic head (facial view). (F) Enlarged detail view of the left nostril of the embryo shown in E. Arrowhead points to distinct epithelial bridges in the lower part of the slit-shaped nostril, which continue to fuse and reduce the nostril. All panels are from (original figure numbers 4, 15, 17, 27, 46, and 52, copyright of Springer-Verlag Berlin Heidelberg 1985), with kind permission of Springer Science and Business Media. fnp, frontonasal prominence; lnp, lateral nasal process; man, mandibular process; max, maxillary process; mnp, medial nasal process. Scale bars in B, C, and D are 100 μm. Scale bar in E and F are 1 mm and 10 μm, respectively.