Face

At the end of the fourth week, facial prominences consisting primarily of neural crest-derived mesenchyme and formed mainly by the first pair of pha-ryngeal arches appear. Maxillary prominences can be distinguished lateral to the stomodeum, and mandibular prominences can be distinguished caudal to this structure (Fig. 15.21). The frontonasal prominence, formed by proliferation of mesenchyme ventral to the brain vesicles, constitutes the upper border of the stomodeum. On both sides of the frontonasal prominence, local thickenings of the surface ectoderm, the nasal (olfactory) placodes, originate under inductive influence of the ventral portion of the forebrain (Fig. 15.21).

During the fifth week, the nasal placodes invaginate to form nasal pits. In so doing, they create a ridge of tissue that surrounds each pit and forms the nasal prominences. The prominences on the outer edge of the pits are the lateral nasal prominences; those on the inner edge are the medial nasal prominences (Fig. 15.22)

During the following 2 weeks, the maxillary prominences continue to increase in size. Simultaneously, they grow medially, compressing the medial nasal prominences toward the midline. Subsequently the cleft between the medial nasal prominence and the maxillary prominence is lost, and the two fuse (Fig. 15.23). Hence, the upper lip is formed by the two medial nasal prominences and the two maxillary prominences. The lateral nasal prominences do not participate in formation of the upper lip. The lower lip and jaw form from the mandibular prominences that merge across the midline.

Initially, the maxillary and lateral nasal prominences are separated by a deep furrow, the nasolacrimal groove (Figs. 15.22 and 15.23). Ectoderm in the floor of this groove forms a solid epithelial cord that detaches from the overlying ectoderm. After canalization, the cord forms the nasolacrimal duct; its upper end widens to form the lacrimal sac. Following detachment of the cord, the maxillary and lateral nasal prominences merge with each other. The nasolacrimal duct then runs from the medial corner of the eye to the inferior meatus of the nasal cavity, and the maxillary prominences enlarge to form the cheeks and maxillae.

The nose is formed from five facial prominences (Fig. 15.23): the frontal prominence gives rise to the bridge; the merged medial nasal prominences provide the crest and tip; and the lateral nasal prominences form the sides (alae) (Table 15.2, p. 389).

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Maxillary Prominences

Figure 15.21 A. Lateral view of an embryo at the end of the fourth week, showing position of the pharyngeal arches. B. Frontal view of a 4.5-week embryo showing the mandibular and maxillary prominences. The nasal placodes are visible on either side of the frontonasal prominence. C. Scanning electron micrograph of a human embryo at a stage similar to that of B.

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Figure 15.21 A. Lateral view of an embryo at the end of the fourth week, showing position of the pharyngeal arches. B. Frontal view of a 4.5-week embryo showing the mandibular and maxillary prominences. The nasal placodes are visible on either side of the frontonasal prominence. C. Scanning electron micrograph of a human embryo at a stage similar to that of B.

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