Summary
Highlights
In the third week post-fertilization, the embryo is a flat, disc-shaped organism composed of three pluripotent germ layers: endoderm (inner), mesoderm (central), and ectoderm (outer). By week four, the embryo undergoes folding and begins to take on a more human-like form, resembling a shrimp.
The neural tube at the head end expands to form the primitive forebrain and a frontal prominence. Lateral to the neural tube, paraxial mesoderm segments into somidomirs and somites. A stomodeum, which will become the oral cavity, forms between the frontal prominence and the cardiac bulge. A buccopharyngeal membrane initially separates the stomodeum from the foregut but later disintegrates.
Six pharyngeal arches, symmetrical bumps of mesoderm, sprout from the primitive pharynx. Neural crest cells migrate to these arches, supporting the development of ectomesenchyme, crucial for craniofacial development. These arches are separated by external branchial grooves and internal pharyngeal pouches. The first pharyngeal arch splits into maxillary and mandibular processes.
Facial development begins in week 4 with the proliferation of ectoderm on the frontal prominence, forming nasal placodes. By week 5, mesodermal cells around these placodes form horseshoe-shaped swellings: the medial nasal process and lateral nasal process. This region is called the frontonasal process. As mesoderm proliferates, nasal placodes sink to form nasal pits, lined by the oronasal membrane.
Maxillary processes proliferate towards the center, separated from nasal processes by grooves. By the end of week 6, maxillary processes fuse with the medial nasal processes, forming the upper lip. The two medial nasal processes merge to form the inter-maxillary segment, which develops into the bridge of the nose, philtrum of the upper lip, part of the maxillary bone, incisor teeth, and the primary palate. The naso-optic groove becomes the nasal lacrimal duct.
During weeks 6 and 7, nasal pits deepen to form nasal sacs behind the inter-maxillary segment. By week 8, the oronasal membrane disintegrates, forming a primitive choana connecting the nasal and oral cavities. A primary palate grows from the inter-maxillary segment.
Maxillary processes develop palatine shelves that initially grow vertically. As the mandibular process grows and the tongue descends, the palatine shelves elevate to a horizontal position and fuse with each other and the primary palate. Concurrently, the nasal septum forms and fuses with the secondary palate. Secondary palate development is typically complete by week 12. The critical period for face and palate development is between weeks 6 and 12.
Facial development starts in week 4 from three main structures: the frontonasal process, maxillary process, and mandibular process. The frontonasal process gives rise to medial and lateral nasal processes. The medial nasal processes merge to form the intermaxillary segment, which forms the philtrum. Fusion of maxillary and intermaxillary segments completes the upper lip. The mandibular process forms the lower jaw and lip.