6. Development of the face, formation of the nasal and oral cavities. Malformations

Page created on July 31, 2020. Last updated on August 2, 2020 at 15:58

Learning objectives

• What are facial prominences?
• In which week do the facial prominences appear?
• What forms from the frontonasal prominence?
• What forms from the nasal prominences?
• What forms from the maxillary prominences?
• What forms from the mandibular prominences?
• How is the nasolacrimal duct formed?
• Describe the development of the palate
• What does the nasal septum develop from?
• Describe the development of the paranasal sinuses
• Describe the development of the tongue
• Describe the development of the epiglottis
• What are the three stages of tooth development?
• Which embryonic tissues give rise to odontoblasts, ameloblasts, cementoblasts, and the dental pulp, respectively?
• When do the milk teeth erupt?
• When do the permanent teeth start erupting?
• What is cleft lip, and why does it occur?
• What is cleft palate, and why does it occur?
• What is ankyloglossia?

Development of the face

The face is built up from so-called facial prominences. These prominences are swellings of neural crest-derived mesenchyme which lifts the surface ectoderm. They appear in the fourth week and originate from the first and second pharyngeal arches.

Recall from topic 2 the stomodeum, the embryonic precursor of the oral cavity. It is important in the development of the face.

The frontonasal prominence

The frontonasal prominence is the only facial prominence which is unpaired. It lies superiorly to the stomodeum. This prominence will form the forehead and the bridge of the nose.

On each side of the frontonasal prominence a nasal placode forms. These structures will give rise to the olfactory epithelium. Placodes are further discussed in topic 19.

The nasal prominences

The nasal placodes eventually invaginate, forming nasal pits. Around these pits, nasal prominences form. The prominences on the outer edge of the pits are called the lateral nasal prominences, while those on the inner edge are called the medial nasal prominences.

The maxillary prominences

The maxillary prominences are paired structures which lie laterally to the stomodeum. These prominences give rise to cheeks and the lateral portion of the upper lip. They also give rise to the maxilla, zygomatic bone, and part of the temporal bone.

The mandibular prominences

The mandibular prominences are paired structures which lie caudally to the stomodeum. These prominences give rise to the lower lip. They also gives rise to the mandible, as well as the Meckel cartilage, which gives rise to the incus and malleus.

Nose

The adult nose is formed from five facial prominences. The frontonasal prominence forms the bridge of the nose, the merged nasal prominences form the nasal crest and tip, while the lateral nasal prominences form the alae of the nose.

Lips

The philtrum of the upper lip is formed by the medial nasal prominence, while the lateral part is formed by the maxillary prominences.

The lower lip is formed by the mandibular prominences.

Development of the nasal cavity

Nasolacrimal duct

The maxillary prominence and the lateral nasal prominences are separated by a groove, the nasolacrimal groove. This groove eventually closes to form the nasolacrimal duct.

The palate

The two medial nasal prominences fuse not only on the surface, but beneath the surface as well. This forms the intermaxillary segment. This segment has multiple components, one of which is the primary palate, a triangular plate which is a small part of the definitive palate. The other parts of the intermaxillary segments form the four upper incisor teeth and the philtrum.

The rest of the definitive palate develops from two shelf-like outgrowths called the palatine shelves from the maxillary prominence, which grown medially and fuse in the midline. When fusing these outgrows form the secondary palate. The secondary palate fuses with the primary palate anteriorly. The incisive foramen forms at the point in the midline where the primary and secondary palates fuse.

Nasal cavity proper

During the sixth week, the nasal pits deepen, forming the nasal cavity. They deepen to the point where they’re only separated from the primitive oral cavity by the oronasal membrane. This membrane soon breaks down, allowing the nasal and oral cavities to communicate by the primary choanae.

These choanae lie directly behind the primary palate. After the formation of the secondary palate the secondary choanae are formed, which lie behind the secondary palate, equal to how it is in adults.

Nasal septum

The nasal septum is formed by the frontonasal prominence. It grows in the caudal direction from the cephalic side of the embryo and eventually fuses with the secondary palate. The two nasal cavities are now separated.

Paranasal sinuses

The paranasal sinuses develop as the lateral wall of the nasal cavity extends into the maxilla, ethmoid, frontal, and sphenoid bones. These sinuses reach their maximum size during puberty and contribute to the definitive shape of the face.

Development of the oral cavity

Oral cavity proper

After the oropharyngeal membrane is broken down, the gut tube is opened to the amniotic cavity through the stomodeum, the “mouth” of the embryo. The oral cavity proper is established once the nasal and oral cavities are separated by the fusion of the primary and secondary palate.

Tongue

The anterior two thirds of the tongue, which form the body of the tongue, are derived from the first pharyngeal arch. They are formed from two lateral swellings, the lateral lingual swellings, and one medial swelling, the tuberculum impar. These three swellings fuse, forming the anterior two thirds.

The posterior third of the tongue, which forms the root of the tongue, is derived from the second, third, and fourth pharyngeal arches. From these three pharyngeal arches another median swelling develops, the copula.

The epiglottis is derived from the epiglottic swelling, which is formed from the fourth pharyngeal arch. The laryngeal orifice develops immediately behind the epiglottic swelling.

The foramen coecum, the origin of the thyroglossal duct, is formed in the midline at the border between the body and root of the tongue.

Teeth

The teeth arise from an interaction between the oral epithelium and neural crest-derived mesenchyme. 20 so-called dental buds form, 10 on the upper jaw and 10 on the lower. The formation of dental buds signifies the “bud stage” of tooth development.

The lower part of the dental buds invaginate, forming a “cap” around the mesenchyme beneath it. The mesenchyme becomes the dental papilla. This signifies the “cap stage” of tooth development.

The cap has three layers. The innermost layer is the inner dental epithelium, and the outermost layer is the outer dental epithelium. Between these layers is the stellate reticulum.

The cap continues to invaginate, eventually forming a “bell” around the dental papilla rather than a cap. This signifies the “bell stage” of tooth development. Mesenchyme cells of the dental papilla which lie adjacent to the inner dental epithelium differentiate into odontoblasts, which will produce dentin. The cells of the inner dental epithelium differentiate into ameloblast, which will produce enamel. Mesenchymal cells on the outside of the tooth differentiate into cementoblasts. The remaining cells of the dental papilla form the dental pulp of the tooth.

The eruption of the milk teeth occurs 6 – 24 months after birth. However, the permanent teeth have by then already been in development for months. During month 3 of gestation the permanent tooth bud is formed. They lie dormant until the sixth year of life, at which point they will grow and push out the milk teeth.

Malformations

Facial clefts

During facial development, many embryonical structures fuse together. Abnormal fusion of these structures paves the way for developmental malformations. Cleft lip and cleft palate are two such common malformations. Some cases of facial clefts occur together with other developmental malformations, or as part of chromosomal abnormalities. All facial clefts are repaired surgically.

There are two forms of cleft lip, oblique and midline. Oblique cleft lip is the more common and occurs due to failure of the lateral nasal prominence to merge with the maxillary prominence. This causes the nasolacrimal duct to be exposed to the surface.

Midline cleft lip is less common and is due to failure of the two medial nasal prominences to fuse.

Cleft palate occurs due to failure of the two palatine shelves to fuse.

Ankyloglossia

Normally the tongue is attached to the floor of the oral cavity only by the frenulum. Ankyloglossia is a condition where the tongue is not as mobile as normal, due to it being abnormally attached to the floor of the oral cavity. The most common form of ankyloglossia is when the frenulum is attached to the tip of the tongue.

Summary

• What are facial prominences?
  • They are swellings of neural crest-derived mesenchyme which lifts the surface ectoderm
• In which week do the facial prominences appear?
  • Week 4
• What forms from the frontonasal prominence?
  • The forehead and the bridge of the nose
• What forms from the nasal prominences?
  • The nasal pits, which will form the nasal cavity
  • The lateral nasal prominences, which will form the alae of the nose
  • The medial nasal prominences, which will fuse and form the nasal crest and tip, and the philtrum
• What forms from the maxillary prominences?
  • The cheeks and the lateral portion of the upper lip
  • The maxilla, zygomatic bone, and part of the temporal bone
• What forms from the mandibular prominences?
  • The lower lip
  • The mandible
  • The Meckel cartilage -> incus, malleus
• How is the nasolacrimal duct formed?
  • The nasolacrimal duct is formed from the nasolacrimal groove between the maxillary and lateral nasal prominences
• Describe the development of the palate
  • When the two medial nasal prominences fuse the intermaxillary segment is formed, containing the primary palate
  • Palatine shelves grow out of the lateral nasal wall and fuse in the midline and with the primary palate anteriorly, forming the secondary palate
• What does the nasal septum develop from?
  • From the frontonasal prominence
• Describe the development of the paranasal sinuses
  • The paranasal sinuses are formed as the lateral wall of the nasal cavity extends into the maxilla, ethmoid, frontal, and sphenoid bones
• Describe the development of the tongue
  • The body of the tongue is derived from the lateral lingual swellings and the tuberculum impar, all from the first pharyngeal arch
  • The root of the tongue is derived from the second, third, and fourth pharyngeal arches
• Describe the development of the epiglottis
  • The epiglottis is formed from the epiglottic swelling, which derives from the fourth pharyngeal arch
• What are the three stages of tooth development?
  • The bud stage, the cap stage, and the bell stage
• Which embryonic tissues give rise to odontoblasts, ameloblasts, cementoblasts, and the dental pulp, respectively?
  • Mesenchyme cells of the dental papilla which lie adjacent to the inner dental epithelium -> odontoblasts
  • Cells of the inner dental epithelium -> ameloblast
  • Mesenchymal cells on the outside of the tooth -> cementoblasts
  • The remaining cells of the dental papilla form the dental pulp of the tooth
• When do the milk teeth erupt?
  • 6 – 24 months after birth
• When do the permanent teeth start erupting?
  • After year 6
• What is cleft lip, and why does it occur?
  • Cleft lip is a defect in the face which occurs due to failure of fusion of the lateral nasal prominence and the maxillary prominence, or the fusion of the two medial nasal prominences
• What is cleft palate, and why does it occur?
  • Cleft palate is a defect in the face which occurs due to failure of fusion of the two palatine shelves
• What is ankyloglossia?
  • Ankyloglossia is a condition of decreased tongue mobility due to the tongue being abnormally attached to the floor of the oral cavity