Beginning around the fourth week of gestation, the future face starts as a series of swellings surrounding the primitive mouth opening, or stomodeum. The development of the face is a complex biological process involving the careful assembly of multiple components during the first weeks of embryonic life. This intricate process of tissue integration determines the shape of the nose, jaw, lips, and palate.
The Five Primary Building Blocks
The basic template of the face is established by five distinct mesenchymal swellings, known as facial prominences, that appear early in the fourth week of development. These prominences construct the face, each contributing a specific region to the final structure. The central, unpaired structure is the frontonasal prominence, which forms the forehead and the upper part of the nose.
Flanking the stomodeum are four paired prominences derived from the first pharyngeal arch. The two maxillary prominences form the upper cheeks and the lateral parts of the upper jaw and lip. Beneath them, the two mandibular prominences grow to create the lower jaw and the lower lip.
Development of the Midface and Upper Jaw
The formation of the midface is the most complex part of this embryonic assembly, starting with the frontonasal prominence. On the lower portion of this prominence, a pair of thickenings, called nasal placodes, appear during the fifth week. These placodes soon sink inward, creating the nasal pits, which are the primitive nostrils.
The tissue surrounding each nasal pit organizes into two ridges: the medial nasal prominence toward the midline and the lateral nasal prominence on the outside. The two medial nasal prominences grow toward each other, eventually fusing in the midline to form the central structure of the upper face, known as the intermaxillary segment. This merged structure gives rise to the philtrum of the upper lip, the four upper incisor teeth, and the primary palate.
Simultaneously, the paired maxillary prominences expand, growing medially toward the nasal structures. The maxillary prominence on each side fuses with the lateral nasal prominence to form the cheeks and the sides of the nose. The maxillary prominences also merge with the intermaxillary segment, completing the upper lip and the upper jaw structure by the end of the seventh week.
Formation of the Lower Face
The development of the lower face is a simpler process compared to the midface. The two mandibular prominences, which originate from the first pharyngeal arch, are positioned below the stomodeum. These paired structures grow toward the midline and meet directly.
By the fifth week of gestation, the two mandibular prominences merge completely along the center line. This fusion creates the entirety of the lower lip, the chin, and the underlying jawbone, or mandible. The bone of the mandible develops through a process called intramembranous ossification around a temporary, rod-shaped cartilage.
This transient structure, called Meckel’s cartilage, acts as an initial scaffold for the developing lower jaw. Although the bulk of the cartilage eventually regresses or forms ligaments, its presence provides temporary support for the forming bone of the mandible.
When Fusion Fails
The precise timing and coordination of these fusion events are important, and a failure at any point can lead to a facial anomaly. The most common of these is cleft lip, which results from a lack of fusion between the maxillary prominence and the medial nasal prominence on one or both sides. This defect typically occurs between the sixth and seventh weeks of embryonic life.
A cleft palate is a separate, though often related, condition that involves the roof of the mouth. The secondary palate forms later, from two palatal shelves that grow inward from the maxillary prominences. These shelves must elevate and fuse along the midline, a process that happens between the eighth and twelfth weeks. If the palatal shelves fail to meet and fuse, a cleft palate results, leaving a direct opening between the mouth and nasal cavity.