DEVELOPMENT OF THE FACE, NASAL CAVITY, PALATE, SALIVARY GLANDS, TEETH AND HYPOPHYSISAn appreciation of the development of the structures of the head and neck needs to include knowledge of the pharyngeal (branchial) arches. These arches form on either side of the foregut and correspond to the primitive vertebral gill bars or branchial arches. The pharyngeal arch consists of a core of mesenchyme covered externally by ectoderm and internally by endoderm. These arches are separated from each other externally by a pharyngeal cleft and internally by a pharyngeal pouch.
The ectoderm which contributes to the formation of the face are well around the stomodeum by the fourth week of embryonic life. The oral plate of the embryo is located at a level just in front of the palatine tonsil. It is evident that the ectodermal structures bounding the stomodeum participate not only in the formation of the face, but also in the formation of the nasal and oral cavities.
The mesenchyme that fills the pharyngeal arches is derived from three sources. The first source is paraxial mesoderm. The second is lateral plate mesoderm. Both contribute to the musculature that develops in each particular arch. This musculature is innervated by one nerve that innervates all the muscles of that particular arch. In addition, the third source of mesenchymal cells is neural crest. Neural crest cells contribute to the skeletal portions of each arch. Finally each arch has its own artery.
Development of the Face
By week six, a median prominence overhangs the cephalic end of the oral cavity. This is called the frontonasal prominence (or process). Nasal (olfactory) pits are located on either side of the frontonasall prominence and are surrounded by horseshoe-shaped eminences. The medial portion of these eminences is called the medial nasal process. The lateral portion of which is called the lateral nasal process. The lateral nasal process is separated from the maxillary process (the more rostral portion of the first branchial arch) by a furrow which reaches the medial aspect of the developing eye. This furrow is called the nasolacrimal groove (naso-optic furrow). The oral cavity is bounded inferiorly by the mandible which has formed by the fusion of the right and left mandibular processes of the first branchial arch. The maxillary processes grow and as they do they crowd the medial nasal processes toward the midline where they unite with one another. The medial nasal processes also crowd the frontal prominence upward. At the same time, the medial nasal processes fuse laterally with the maxillary processes. Later, the lateral nasal processes fuse to the maxillary processes, obliterating the nasolacrimal groove.
Development of the Nasal Cavities and Palate
Thickening of the surface ectoderm on either side of the frontal prominence just above the stomodeum is the first indication of the nasal cavity. These are called the nasal (olfactory) placodes. These placodes begin to invaginate by the 5th week and are called the nasal pits. The nasal pits invaginate by 1) forward growth of the medial nasal and lateral nasal processes, and, 2) posteroinferior growth of the pits themselves, the placode tissue comes to line the roof of each pit. The pits grow and approach the primitive oral cavity. A thin oronasal membrane is located between the pits and the oral cavity. This membrane then ruptures and forms the primitive choanae. The placode tissue differentiates into the olfactory epithelium while the general epithelium of the nasal cavity differentiates from the remainder of the surface ectoderm lining the nasal pits.
At the end of the second month, as the maxilla develops, a partition between the primitive nasal chambers and the oral cavity begins to form. The anterior aspect of this partition is derived from the area of the upper jaw formed by the medial nasal processes (intermaxillary segment) and is called the primary palate (median palatine process). Most of the palatine partition, however, is derived from the medial growth of shelf-like processes originating from the maxillary process called the palatine shelves (lateral palatine processes). This segment of the palate is called the secondary palate. As the secondary palate is formed, the nasal septum grows inferiorly toward it. The nasal septum and the two palatine shelves unite to form separate right and left nasal chambers, an oral cavity, and the definitive choanae.
Summary of Development of Major Salivary Glands and Teeth
The salivary glands develop by the evagination of the buccal ectoderm during the 6th and 7th week of development. A band of buccal ectoderm (dental ledge) pushes into the underlying mesenchymal tissue around the entire arc of both the upper and lower jaws. This ectoderm forms the enamel organ, whose ameloblasts give rise to the enamel of each tooth. The cementum and dentin of each tooth develop from mesenchyme (ectomesenchyme) lying internal to the enamel organ. Odontoblasts lining the tooth pulp form the dentin.
Development of the Hypophysis
In 3 1/2 week embryos, the ectoderm found in front of the oral plate forms a diverticulum. This diverticulum develops into Rathke's pouch. It makes contact with a diverticulum from the floor of the forebrain called the infundibulum which is destined to be the infundibular (neural) stalk and the pars nervosa of the adult hypophysis. After the pouch breaks away from the stomodeum, the portion of Rathke's pouch contacting the infundibulum becomes the pars intermedia of the adult pituitary gland. The more rostral portion of Rathke's Pouch thickens and becomes pars distalis. The original lumen of Rathke's pouch remains for a time, but in humans usually obliterates. A pair of lateral buds arise from Rathke's pouch and form a collar about the infundibular stalk. This collar of tissue becomes the pars tuberalis.
1. Cleft (hare) lip and cleft jaw: the medial nasal processes do not fuse with the maxillary processes.
2. Median cleft of upper lip: The 2 medial nasal processes fail to fuse. This is often accompanied by mental retardation.
3. Median cleft of lower lip: Failure of fusion of 2 mandibular arches.
4. Cleft palate and/or uvula: the palatine shelves fail to fuse with each other and/or with the primary palate.
5. Oblique facial cleft: The nasolacrimal duct remains exposed due to the lateral nasal process failing to fuse with the maxillary process It can occur along with a cleft lip.
Other Web Resources
The Pediatric Pathology Index: Look specifically at the pictures of cleft lip & palate.
American Cleft Palate-Craniofacial Association (ACPA)
WIDE SMILES: Cleft Lip and Palate Resource
Cleft lip/palate picture from the University of Utah