EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
54. The Face
Review of MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D.
- The face is built up from "facial swellings" as a result of mesodermal masses lifting the surface ectoderm. At the end of week 4, the center of the developing facial structures is formed by an ectodermal depression, the stomodeum, or primitive mouth surrounded by the first pair of pharyngeal arches. The 5 facial primordia, formed by proliferation of mesenchyme, appear around the stomodeum early in week 4
- THE UNPAIRED MEDIAN FRONTONASAL (FRONTAL) PROMINENCE (SWELLING) constitutes the upper boundary of the stomodeum resulting from proliferation ventral to the brain
- By weeks 3 to 4, the anterior neuropore can be seen in the center of this swelling
- THE PAIRED MAXILLARY PROMINENCES (SWELLINGS) of branchial arch I form the lateral boundaries or sides of the stomodeum
- THE PAIRED MANDIBULAR PROMINENCES (SWELLINGS) of arch I make up the lower stomodeal boundary or floor
- The nasal placodes: bilateral oval-shaped thickenings of surface ectoderm that develop on each side of the lower part of the frontonasa1 prominence and just above the stomodeum by the end of week 4
- THE HORSESHOE-SHAPED MEDIAL AND LATERAL NASAL PROMINENCES (RIDGES) are produced by mesenchymal proliferation at the placode margins during week 5, and the nasal placodes now lie in depressions called nasal pits
- The lateral swellings form the alae of the nose; the medial swellings form the middle part of the nose, the middle upper lip, the middle part of the maxilla, and the entire primary palat By weeks 6 to 7, the nasal cavities are well formed
- The maxillary prominences approach each other and the medial nasal prominences
- THE NASOLACRIMAL GROOVE marks the cleft or furrow separating the lateral nasal and maxillary prominences
- The nasolacrimal ducts form from a linear ectodermal thickening that forms in the floor of the nasolacrimal groov This epithelial cord sinks into the mesenchyme, canalizes and forms the duct, while its upper part expands to form the lacrimal sa The duct drains into the inferior meatus of the nose
- DURING WEEKS 6 TO 7, the medial nasal and maxillary prominences merge
- Merging of the former forms the intermaxillary segment of the upper jaw, giving rise to middle part of philtrum of upper lip, middle part of upper jaw and associated gingiva (gums), and primary palate
- The maxillary prominences form the lateral parts of the upper lip, the upper jaw, and the secondary palat They merge laterally with the mandibular
- THE PRIMITIVE LIPS AND CHEEKS are invaded by branchial arch II (nerve VII) mesenchyme, giving rise to the facial muscles (innervated by cranial nerve VII)
- The frontonasal prominence forms the forehead and dorsum and apex of the nose
- The mandibular prominences merge in week 4, and the groove between them vanishes by the end of week 5. They give rise to the lower jaw, lower lip, and lower part of face
- Primitive jaws until end of week 6 are solid tissue masses
- LIPS AND GINGIVAE (GUMS) develop when the ectodermal linear thickening, the labiogingival lamina, grows into the mesenchyme
- It gradually degenerates leaving a labiogingival groove or lip sulcus between the gingivae and lips. Only the frenulum remains between the lips and gingiva
- Slowness of facial development is result of proportional and position change of components. Face smallness at birth is due to: unerupted teeth, small maxillary air sinuses and nasal cavities, and rudimentary upper and lower jaws. The fetal face is almost definitively formed by about weeks 9 to 10
