Development of the Face and Palate

The face and palate are forming on a similar timeline early in prenatal development.
These areas are among the most susceptible to malformations during human development.

Following this chapter, students will be able to:

Describe facial development during the 4th to 7th weeks of gestation.
Explain palatal development during the 7th to 9th weeks.
Discuss the development of the tongue and thyroid gland.
Understand the implications of facial and palatal clefts and other facial defects.
Familiarize with key terms relevant to facial and palatal development.

Palatal Process Formation:
- Medial and lateral palatal processes develop during this period.
Cellular Dynamics:
- Rapid cell division occurs during embryonic development, which heightens susceptibility to disruptions.
Facial Structure Development:
- Frontal Area (Pink): Develops into the forehead and nose.
- Mandibular Arch (Blue): First pharyngeal arch forming the lower jaw.
- Maxillary Processes (Green): Left and right sides lateral to the oral pit (stomodeum) develop into cheeks and palate.
- Medial Nasal Process (Orange): Contributes to the median palatal segment, separating nasal and oral cavities.
- Lateral Nasal Process (Yellow): Closes anteriorly to pharynx.
The tongue is developing simultaneously, influencing the closing of the palate processes.

Pharyngeal arches appear below the oral pit; the frontal processes of the brain begin to bulge.
The mandibular arch shows constriction at the midline. The heart is positioned below the face and starts pumping blood.
Maxillary Processes develop into cheek structures and are wedge-shaped tissues.

Bilateral Nasal Placodes: Appear at the upper lip border; these will grow into nostrils.
The frontal area is designated as the frontonasal process.
Internasal Area: The distance between nostrils indicates facial width; eyes become prominent.

Facial Expansion: The lateral parts of the face broaden; upper lip fuses to create the philtrum between the medial nasal process and lateral maxillary segments.
The upper lip's formation is critical—cleft potential is evaluated now based on fusion of nasal and maxillary processes.
Auricular Hillocks: Six hillocks forming the external ear develop from both the mandibular and hyoid arches.
The nasal fin is formed where processes fuse around the oral pit.

The face shows a more human-like appearance; the upper lip has completed fusion, forming the philtrum and the ear hillocks have combined into auricles.
Cleft Lip Risks: Cleft lips are evaluated, but cleft palate fusion occurs later (week 8).

Structures Involved:
- The primary palate develops from an anterior wedge-shaped medial part, and the secondary palate arises from two lateral palatine processes, which grow from maxillary tissues.
As palatine shelves grow medially, they must make contact with the upwardly migrated tongue, positioned within the nasal cavity.
Palatal Shelf Elevation:
- In week 8, the shelves elevate and push forward and downwards, facilitating contact and closure while covering the tongue.
Palatal Fusion: Involves the merge of the palatine processes and the removal of the epithelial barrier through apoptosis, after which connective tissue allows for continued fusion.
They also fuse with the nasal septum, completing the division of oral and nasal cavities back to the nasopharynx.

The tongue originates from occipital myotomes and merges anteriorly with muscles from the first and second arches, forming its body and base.
The regions are separated by the terminal sulcus; the surface features include the circumvallate papilla which serves as an anatomical landmark.
Waldeyer Tonsillar Ring: The base forms the lingual tonsil, part of the ring including palatine and pharyngeal tonsils. Clinically significant as tonsils can be sites for oral cancer.

The thyroid gland derives from an epithelial proliferation at the foramen cecum, migrating ventrally to position in front of the trachea by the 7th week.
Thyroglossal Duct: Links the thyroid to the tongue during migration; issues like cysts along this route can occur (e.g., thyroglossal cysts and fistulas).
Functional thyroid development completes by the 3rd prenatal month.

Malformations are associated with genetic and environmental factors; cleft lip is notably common—rates vary by ethnicity:
- Caucasians: 1 in 700 births
- African Americans: 1 in 2000 births
- Asians: 3 in 2000 births
Facial clefts are classified by position (unilateral/bilateral) and extent (complete/incomplete).
Incomplete Cleft Lip: Does not involve nasal opening; complete forms can significantly affect function and require surgical intervention.
Other rare malformations include cleft mandible and conditions like harelip, each varying in degree and developmental implications.
Bifid Uvula/Cleft Uvula: The mildest form of cleft palate, potentially indicating more serious underlying structural issues.