Pre-natal Facial Growth and Development Flashcards
What determines malocclusion?
Malocclusion is determined by the size, shape, and position of the underlying jaws. These factors are influenced by both genetic and environmental factors during development.
What are the two main phases of life in utero?
The two phases are:
Embryonic phase (1-8 weeks) - when all limbs, organs, and facial structures form
Fetal phase (8 weeks to term) - period of growth and refinement of structures
Why is the embryonic phase (1-8 weeks) particularly critical for facial development?
The embryonic phase is critical because all limbs and organs, including the face, form during this period. The embryo develops a characteristic human form, and external environmental influences during this time can cause craniofacial abnormalities.
What are neural crest cells and why are they important in facial development?
Neural crest cells are specialized cells that:
Develop from ectoderm along the edge of the neural groove
Undergo extensive migration within the developing embryo
Differentiate into many cell types including spinal and autonomic ganglia, Schwann cells, adrenal medulla, and meninges of the brain
Their migration is crucial for facial development as they contribute to pharyngeal arch cartilage, bone, connective tissue, and dental tissues (pulp, dentine, cementum, and periodontal ligament)
Describe the migration pattern of cranial neural crest cells.
- Form in neuroectoderm at the tips (crest) of the neural folds
- Migrate ventrally into the craniofacial region
- Contribute to many structures in the head and neck region, including facial bones, skin dermis, connective tissue, and cranial nerves V, VII, IX, and X sensory ganglia
- By week 4, these migrating cells form the frontonasal process and laterally the pharyngeal arches
What are pharyngeal arches and when do they appear?
Pharyngeal arches are:
Paired structures of the developing embryo
5 well-developed arches, numbered I, II, III, IV, VI (IV and VI are very close)
Appear during the 4th and 5th week of embryonic development
Each has a mesenchymal core containing one artery (aortic arch), one nerve, and one cartilage
Describe the structure of a pharyngeal arch.
Each pharyngeal arch has:
A mesenchymal core consisting of neural crest cells and paraxial or lateral plate mesoderm
External covering of ectoderm
Internal lining of endoderm
External indentations between arches called pharyngeal clefts
Internal indentations between arches called pharyngeal pouches
What happens to the pharyngeal clefts during development?
Only the 1st pharyngeal cleft makes a contribution, forming the external auditory meatus and the ectodermal covering of the tympanic membrane
The remaining pharyngeal clefts are obliterated by caudal growth of the 2nd pharyngeal arch that merges with another proliferation of tissue called the epicardial ridge (forming the cervical sinus)
Sometimes this process leaves remnants of these clefts as a cyst or fistula
What are cervical cysts and fistulas, and where are they located?
Cervical cysts and fistulas:
Are developmental anomalies resulting from incomplete obliteration of pharyngeal clefts
Located laterally in the neck
Always positioned anterior to the sternocleidomastoid muscle
What is the timeline for facial formation, and why is this period critical?
Formation of the face occurs during the first eight weeks after fertilization
Weeks 5-7 see the extension and fusion of the facial processes
This early period is critical because environmental factors may lead to significant malformations
Most of the face forms from migrating neural crest cells
Interference with neural crest cell migration can lead to severe facial deformities
What structures are apparent in week 4 of facial development?
In week 4, the following structures are apparent:
The frontonasal prominence
Two maxillary prominences
Mandibular arch
The nasal placodes (two thickenings of ectoderm) begin to form on the lateral surfaces of the frontonasal prominence
Describe how the nasal processes form.
Ectoderm from the center of nasal placodes proliferates and invaginates to form nasal pits (primitive nares)
This, together with proliferation of surrounding mesenchyme, forms the nasal processes (medial and lateral) on each side
The maxillary prominences continue to enlarge and grow anteriorly
The mandibular prominences merge across the midline to form the mandible
Explain the fusion of facial prominences to form the nose and upper lip.
The two medial nasal prominences (yellow in diagram) merge to form the tip of the nose, philtrum region of the upper lip, and primary palate
The maxillary prominences fuse with the medial nasal prominences to complete formation of the upper lip and alveolar ridge
The lateral nasal prominences form the sides (alae) of the nose
The frontonasal prominence forms the bridge of the nose and forehead
What is the primary palate and how is it formed?
The primary palate forms from the medial nasal prominences that create the intermaxillary segment
The intermaxillary segment is composed of:
- A labial component forming the philtrum
- An upper jaw component including the 4 incisor teeth
- A palatal component forming the primary palate
Describe the formation of the secondary palate.
The secondary palate forms through the following process:
- The maxillary prominences extend into the oral cavity
- Palatal shelves from the maxillary prominences extend into the oral cavity and initially assume a vertical position on either side of the tongue
- The palatal shelves “flip up” to assume a horizontal position dorsal to the tongue
- For this movement to occur, the tongue must “drop” out of the way
- The mandible must grow forward to create the necessary space for the tongue to move ventrally
- Once the palatal shelves have elevated, they grow and fuse across the midline forming the secondary palate, soft palate, and uvula
- Fusion occurs by mechanisms similar to those in neural tube closure
How do clefts of the palate occur?
The fusion process of the palatal shelves fails
The palatal shelves fail to elevate
This can happen due to genetic factors, environmental factors, or a combination of both
How is the skull divided anatomically and developmentally?
The neurocranium - forms a protective case around the brain. Vault of the skull: flat bones that develop through intramembranous ossification
Base of the skull: forms through endochondral ossification
The viscerocranium - forms the skeleton of the face
What is intramembranous bone formation?
Intramembranous bone formation:
- Bone is deposited directly into primitive mesenchymal tissue without a cartilage model
- Forms the vault of the skull, the maxilla, and most of the mandible
- Needle-like bone spicules form, which progressively radiate from primary ossification centers to the periphery
- Progressive bone formation results in the fusion of adjacent bony centers
What is endochondral bone formation?
which zones does it occur in?
Endochondral bone formation:
Bones are preceded by a hyaline cartilage “model”
Forms the base of the skull
Has several centers of ossification which eventually fuse
Occurs in 5 zones:
- Resting cartilage
- Proliferating cartilage
- Hypertrophic cartilage
- Degeneration and mineralization
- Woven bone
Describe the development of the base of the skull.
Base of the skull development:
A series of cartilages forms the base of skull
These undergo endochondral ossification from multiple centers, starting with the basi-occiput at 10-12 weeks
At birth, cartilaginous growth centers remain between the sphenoid and occipital bones (synchondroses) and in the nasal septum
Explain the development of the vault of the skull.
Vault of the skull development:
Intramembranous ossification commences in the third month in several centers
Fusion is incomplete at birth, leaving widenings known as fontanelles to allow flexibility during birth
The anterior fontanelle closes at about 2 years of age, the posterior at about 1 year
Growth occurs at fibrous sutures in response to intracranial pressure
Growth of the skull continues until the seventh year, but some sutures remain open until adulthood
What are the embryonic facial cartilages that precede the maxilla and mandible?
Although the maxilla and mandible form intramembranously, they develop adjacent to pre-existing cartilaginous skeletons:
- The nasal capsule (precedes the maxilla)
- Meckel’s cartilage (precedes the mandible)
These cartilages are present by 6 weeks of embryonic development
Describe the development of the mandible.
The mandible develops intramembranously as several units, all responding to different growth stimuli:
- A condylar unit - forms the articulation and contains the largest secondary cartilage formation
- An angular unit - forms in response to the lateral pterygoid and masseter muscles
- A coronoid unit - responds to temporalis muscle development
- The body of the mandible - forms in response to the inferior dental nerve
- An alveolar unit - forms only if teeth are developing
What are the secondary cartilages of the mandible and why are they important?
The three main sites of secondary cartilage formation in the mandible appear between 12 and 14 weeks:
- Coronoid cartilage - disappears long before birth
- Symphyseal cartilage in midline of mandible - at birth the mandible is in two halves; the midline symphysis fuses a few months after birth
- Condylar cartilage - very important for postnatal mandibular growth, as growth continues until about 20 years of age
What is a syndrome?
A syndrome is a group of symptoms and physical findings that occur together, for which a direct cause is not necessarily understood.
Distinguish between primary and secondary abnormalities.
Primary abnormality: A defect in the structure of an organ or part of an organ that can be traced back to an anomaly in its development (e.g., spina bifida, cleft lip, congenital heart defects)
Secondary abnormality: An interruption of the normal development of an organ that can be traced back to other influences such as:
- Teratogenic agents: infection (rubella virus), chemicals (thalidomide, lithium)
- Trauma: amniotic bands
- A congenital abnormality is not necessarily inherited
Define deformation, agenesia, and sequence in the context of craniofacial abnormalities.
Deformation: Anomalies that occur due to outer mechanical effects on existing structures
Agenesia: Absence of an organ due to failed development during the embryonic period
Sequence: A single factor that results in numerous secondary effects (e.g., Pierre-Robin sequence: micrognathia → tongue positioned high → palatal cleft)
What are facial syndromes that can be associated with clefts?
Apert’s Syndrome (acrosyndactyly)
Crouzon’s Syndrome (craniofacial dysostosis)
Oral-Facial Digital Syndrome
Cleidocranial dysostosis
Treacher Collins Syndrome (mandibulofacial dysostosis)
Pierre-Robin sequence
Stickler’s Syndrome
Van der Woude Syndrome
Hemifacial Microsomia
What are facial syndromes arising from early problems with facial development (1-8 weeks)?
environmental, genetic/multifactorial
Environmental: Fetal Alcohol Syndrome
Genetic/Multifactorial:
- Hemifacial microsomia
- Treacher Collins syndrome (mandibulofacial dysostosis)
- Clefts of lip and palate
Describe Fetal Alcohol Syndrome and its characteristics.
anatomical
Environmental cause: high maternal intake of alcohol
Can occur from very early in development (as early as day 17)
Features include:
Microcephaly (small head)
Typical facies with short palpebral fissures
Short nose
Long upper lip with deficient philtrum
Small midface
Small mandible
Mild mental retardation
What is Hemifacial Microsomia and what is it characterised by?
Etiology: Multifactorial, possibly related to neural crest cell migration issues (day 19-28)
Characterized by:
Progressive facial asymmetry in 3D
Unilateral mandibular hypoplasia
Zygomatic arch hypoplasia
High arched palate
Malformed pinna
Clinical spectrum ranges from ear tags only to complete lack of condyle, coronoid, and ramus
Normal intellect, but may have deafness, cardiac and renal problems
Describe Treacher Collins Syndrome (Mandibulofacial Dysostosis) and it’s features
Inheritance: Autosomal dominant
Etiology: Deformity of 1st and 2nd branchial arches (day 19-28)
Features include:
Anti-mongoloid slant of palpebral fissures
Coloboma of lower lid outer 1/3
Hypoplastic or missing zygomatic arches
Hypoplastic mandible with antigonial notch
Deformed pinna, conductive deafness
What is the incidence and distribution of cleft lip and/or palate?
70% are sporadic cases
Sex distribution:
Cleft Lip with or without Palate: males > females
Cleft Lip: males:females 3:1 (Scotland)
Cleft Palate: females > males 3:2 (EU)
Regional variations:
CLP > CP in England and Wales
CP = CLP in Scotland
What is the timeline for the development of cleft lip and palate?
- Cleft Lip with or without Palate: develops around day 28-38 (during primary palate formation)
- Cleft Palate: develops around day 42-55 (during secondary palate formation)
What factors contribute to the etiology of clefts?
genetic, environmental
Genetic factors:
Higher concordance in monozygotic twins
Association with syndromes
Familial patterns
Epidemiological factors (sex ratio, laterality with 66% on left side, ethnic distribution)
Environmental factors:
Social deprivation
Smoking (increases risk 2.5x)
Alcohol consumption
Anti-epileptic medications
Multivitamins (can reduce risk by 25%)
What dental features are associated with clefts?
Impacted teeth
Crowding
Hypodontia (missing teeth)
Supernumerary teeth
Hypoplastic teeth (with defective enamel formation)
Increased caries risk
Who makes up the cleft team for patient management?
Specialist cleft nurse
Speech therapist
Orthodontist
Pediatric dentist
Cleft surgeon (OMFS/Plastics)
ENT specialist
Geneticist
Psychologist
Describe Achondroplasia and its features.
Inheritance: Autosomal dominant
Etiology: Problem with endochondral ossification
Features:
Defects in long bones, short limbs leading to dwarfism (70% of cases)
Defects in base of the skull
Retrusive middle third of the face
Frontal bossing
Depressed nasal bridge
What is Crouzon’s Syndrome (Craniofacial Dysostosis)?
features and management
Inheritance: Autosomal dominant
Main feature: Premature closure of cranial sutures (especially coronal and lambdoid)
Clinical manifestations:
Proptosis (shallow orbits)
Orbital dystopia
Mild hypertelorism
Retrusion and vertical shortening of midface
Prominent nose
Class III malocclusion
Narrow spaced teeth
Management: Requires surgical intervention (e.g., distraction osteogenesis) for early closure of sutures (cranial synostosis)
What is Apert’s Syndrome (Acrosyndactyly)?
Inheritance: Autosomal dominant
Features:
Premature closure of almost all cranial sutures
Exophthalmos
Hypertelorism
Maxillary hypoplasia
Class III occlusion, anterior open bite (AOB)
Narrow spaced teeth
Parrot’s beak nose
Syndactyly of fingers and toes
Narrow high arched palate (30% have cleft palate)
Conductive deafness
