Face, Nose and Palate - Overview
External features of the face develop from five primordia.
- Frontonasal process (prominence) - forms upper part of face; forehead, nose, orbit, lacrimal apparatus and nasal cavity
- Paired Maxillary Processes - from 1st Pharyngeal Arch - forms upper jaw and palate
- Paired Mandibular Processes - from 1st Pharyngeal Arch - form lower jaw
The skull and hyoid bone are the major skeletal components of the head region.
The anterior part of the skull, forming the facial skeleton is made of bones that arise from neural crest cells (blue) in the five primordia that form the face. The hyoid is from neural crest of the 2nd and 3rd pharyngeal arches.
The posterior part of the skull. such as the parietal bones and upper part of the occipital bone, are from somitic mesoderm (pink).
Development of the Skull
The skull is divided into two main parts:
- Viscerocranium - bones that make up the facial part of the skull and the hyoid bone.
- Neurocranium - or the cranial vault which covers the brain and lined by dura mater.
The viscerocranium consists of bones that form the face, lower jaw and along with the hyoid are derived from neural crest mesenchyme of the first three pharyngeal arches.
The maxillary process of the first pharyngeal arch gives rise to the maxilla, zygomatic and the squamous part of the temporal bones.
The mandibular process of the first pharyngeal arch forms the malleus and incus and parts of the middle ear cavity by endochondral ossification.
The mandible forms by intramembranous ossification.
The second and third pharyngeal arches form the stapes and styloid process along with the hyoid bone.
The neurocranium surrounds and protects the brain with bones that form by both intramembranous and endochondral ossification and which are derived from mesenchyme of both neural crest and somitic mesoderm origin.
The chondrocranium, which is the base of the skull and floor of the cranial cavity, develops by endochondral ossification in mesenchyme of both neural crest and somitic mesoderm origin.
The bones of neural crest origin (blue) include:
- ethmoid bones
- greater and lesser wings of the sphenoid bone
- anterior part of the sella turcica
The bones of somitic mesoderm origin (pink) include:
- posterior part of the sella turcica
- body of the sphenoid
- base of the occipital bone
- petrous part of the temporal bone.
The membranous neurocranium (dermatocranium) consists of flat bones in the roof of the skull that form by intramembranous ossification. It is also called dermatocranium because the flat bones resemble dermal plates found in the heads of lower vertebrates.
Those derived from neural crest (blue) include:
- frontal bone
- squamous part of temporal bone
- parietal bones
- upper part of the occipital bone
Face
The frontonasal prominence forms midline structures. Around week 5 of development, nasal placodes develop on the lower, lateral part of the frontonasal prominence.
These become surrounded by the medial and lateral nasal swellings to form the nasal pits.
The nasolacrimal groove is between the lateral nasal swelling and the maxillary process and will contribute to formation of the nasolacrimal duct.
Medial nasal swellings (folds) are pushed toward the midline by the medial growth of the maxillary processes.
Eventually, they fuse forming the intermaxillary segment of the face, which gives rise to a midline portion of the lip, nose, upper jaw and palate.
The fused medial nasal swellings, or intermaxillary segment of the face and the maxillary processes form the upper lip, jaw and palate.
The intermaxillary segment of the face has several components:
- Upper Lip Component - midline of upper lip with philtrum
- Upper Jaw Component - midline of maxillary bone with incisor teeth
- Palatal Component - primary palate
- Nasal Component - forms midline parts of the nose, crest and tip of nose
The lateral parts of the upper jaw are from the maxillary processes of the 1st pharyngeal arch.
The secondary palate forms by fusion of the palatine processes of the maxillary processes, with each other and with the primary palate anteriorly. The incisive foramen on the hard palate marks the boundary between primary palate and secondary palate.
At 8 weeks of development, the face has a distinctively human appearance. Note that the fused medial nasal swellings, in addition to contributing to the midline of the upper lip, jaw and palate also contribute to the nose.
Observe that the auricles of the ear begin development in the cervical region, The elastic cartilage forming the ears is from neural crest derived mesenchyme of the 1st and 2nd pharyngeal arches.
Nose
The nose is a prominent feature of the human face. It is composed mainly of cartilage derived from five primordia. The bridge of the nose is bony and is formed by the frontonasal prominence itself. The crest and the tip of the nose are formed by the fused medial nasal swellings and are largely cartilage. The sides of the nose and alae are largely cartilaginous and formed from the lateral nasal swellings.
The medial and lateral nasal swellings (folds) surround the external nares, openings into
the nasal cavity.
Nasolacrimal Apparatus
At five weeks of development, the beginnings of the lacrimal apparatus are evident as a thickened cord of ectodermal cells located in the nasolacrimal groove, which extends from the anterior aspect of the eye between the lateral nasal swellings and the maxillary process of each side of the face. The ectodermal cord of cells in the nasolacrimal groove will sink beneath the surface ectoderm and will become canalized to form the lacrimal canaliculi and the nasolacrimal duct.
The entire lacrimal apparatus consists of the lacrimal gland and the ducts derived from the nasolacrimal groove.
The lacrimal gland itself is formed from ectoderm of the frontonasal prominence that invaginated into the superior lateral roof of the orbit.
Its function is to produce tears to keep the anterior surface of the eye clean and moist at all times. Tears collect in the medial angle of the eye and are discretely siphoned into the inferior meatus of the nasal cavity by the lacrimal canaliculi and nasolacrimal duct.
There they are mixed with other nasal secretions and for the most part swallowed. Crying may result in the production of copious amounts of tears that flood the system, spilling onto the cheeks and causing a runny nose.
Palate
Anatomically the palate consists of the anterior hard palate & posterior soft palate.
Developmentally it forms from two primordia:
Primary Palate
- Forms from medial nasal swellings
- Anterior part of maxillary bones
- Forms from palatine processes of the maxillary processes
- Lateral and posterior parts of maxillary bones
- Palatine bones
- Soft palate
The incisive foramen forms the boundary between the primary and secondary palates.
Clinically is it the boundary between anterior (purple line) and posterior (green line) cleft defects.
Around 6.5 weeks, the medial nasal swellings are fused with each other but not yet with the maxillary processes. In this superior view of the palate, we can see up into nasal cavity to observe the nasal septum growing downward toward the palatine processes. The palatine processes are growing toward each other in the midline. The green line marks the relative location of the coronal cut below.
In the coronal section to the right, note that the palatine processes are directed downward and that the tongue protrudes into the nasal cavity.
The primary palate is fusing with secondary palate. The palatine processes of the maxillary processes continue to grow toward midline where they will eventually fuse with each other and the nasal septum.
The nasal septum continues its downward growth toward the palate.
In the coronal section above the palatine processes can be see to have returned to a more horizontal position and the tongue is now confined to the oral cavity.
Around 10 weeks of development, the primary and secondary palates have fused and the secondary palate is completed by fusion of the palatine processes of the maxillary processes.
In the coronal section above, we see that the nasal septum has also fused with the completed palate, dividing the nasal cavity into right and left sides.
Clinical Considerations
Treacher Collins Syndrome
Cleft Lip & Cleft Palate
Unilateral Cleft Lip
Bilateral Cleft Lip
Anterior cleft deformities may involve the lip, upper jaw and/or primary palate. These are located anterior to the incisive foramen, the boundary between the primary and secondary palates.
They may be unilateral as in the image above on the left, or bilateral as in the image above on the right.
They may also coexist with cleft of the secondary palate.
Due to a deficiency in mesenchyme in the intermaxillary segment
- Mainly occur as a result of genetic defects but other causes are identified
- 1 in 700 births
- Prevalent in males
- Highest incidence among Asians and Native Americans
- Lowest incidence in African Americans
Unilateral Cleft Lip at and Cleft Palate
Bilateral Cleft Lip and Cleft Palate
Posterior cleft deformities are located posterior to the incisive foramen and involve failure of fusion of the palatine processes of the maxillary processes.
They may be unilateral or bilateral and can occur in conjunction with cleft lip and primary palate as seen in the images above.
Due to a failure of the palatine processes to fuse
- Associated with environmental factors
- Anticonvulsant drugs associated with a high incidence
- 1 in 1500 births
- Prevalent in females, perhaps because the palatal processes fuse about 1 week later in females.