Cranial Development Flashcards
Formation of Facial Structures
Nose Formation (Week 5-6):
The frontonasal prominence differentiates into medial and lateral nasal processes.
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The medial nasal processes merge in the midline to form the bridge of the nose.
The lateral nasal processes form the ala (sides) of the nose.
1st pharyngeal arches
differentiate into maxillary
and mandibular processes
Upper Lip Formation (Week 6):
The medial nasal processes merge with the maxillary processes to form the philtrum and upper lip.
If this fusion fails, it results in a cleft lip.
Lower Lip Formation (Week 7):
The mandibular processes (from the 1st pharyngeal arch) fuse to form the lower lip and mandible.
Formation and Malformations of the Upper Lip & Palate
Upper lip: Fusion of medial nasal processes with the maxillary
processes
Palate Formation (Week 7):
- Medial nasal processes fuse → Forms the front of the palate.
- Maxillary processes fuse → Forms most of the palate.
Cleft Lip and Cleft Palate:
Cleft lip: Failure of medial nasal process and maxillary process fusion.
Cleft palate: Failure of medial nasal processes and maxillary processes to properly join.
Common (1:700 births), more frequent in females.
Formation of the Ear
Outer Ear:
1st and 2nd pharyngeal arches form external ear structures.
1st cleft → External acoustic meatus.
1st pouch → Middle ear & Eustachian tube.
Inner Ear (Cochlea Nerve):
Cochlear nerve develops → responsible for hearing.
Malformations (Treacher Collins Syndrome):
Disorder affecting 1st and 2nd pharyngeal arches.
Hearing loss, craniofacial defects, cleft palate.
Formation of Salivary Glands
Epithelial buds enlarge and branch, forming tree-like glandular structures (arborization).
Salivary glands undergo branching morphogenesis to reach their final structure.
basic mechanisms of tissue fusion during embryonic development
Epithelial to Mesenchymal Transition (EMT)
Epithelial cells lose polarity and cell-cell adhesion.
They gain migratory and invasive properties, transforming into mesenchymal cells.
This process is essential for:
Neural crest cell formation
Palate and lip fusion (important in preventing cleft lip and cleft palate).
Extrusion
As cells migrate toward each other, some crowding occurs.
Extra cells are removed by extrusion, meaning they are displaced out of the fusion site.
Extrusion = physically displacing cells away from the fusion site.
This helps maintain a smooth fusion process without excessive cell accumulation.
Apoptosis (Lethal EMT)
Cells at the fusion line undergo programmed cell death (apoptosis).
[When two tissues meet (e.g., in palate fusion or lip fusion), the cells at the junction need to be cleared.Apoptosis removes these excess cells so that the tissues merge properly.]
This ensures that the two merging tissue layers fully integrate without gaps or excessive cells.
If apoptosis fails, abnormal tissue growth or cleft defects can occur.
Formation of the Maxilla (Upper Jaw)
Occurs in Week 6 of development.
Origin: The maxilla is derived from the 1st pharyngeal arch, specifically from the maxillary processes.
Development:
- The maxilla grows postnatally, including the palate, until 14-15 years of age.
- The palate forms as part of maxillary development, ensuring the separation of the oral and nasal cavities.
Formation of the Lower Lip and Mandible
Occurs in Week 7 of development.
Lower lip:
Forms by the fusion of the two mandibular processes.
Failure of proper fusion results in a lower lip cleft.
Mandible (Lower Jaw):
Forms from mandibular processes.
Continues to develop until 20 years of age.
The mandible is the largest and strongest bone in the face.
Cochlear Nerve + placodes
Placodes are specialized regions of ectoderm that form structures like the lens, otic (ear), and olfactory (smell) systems.
They can invaginate (fold inward) or delaminate (cells migrate away) to contribute to sensory organs and nerves.
The otic placode (highlighted) forms the inner ear, including the cochlea and vestibular system for hearing and balance.
The cochlear nerve is essential for hearing. It develops from the otic placode, which invaginates and forms the otic vesicle.
=> This vesicle then differentiates into the cochlea and vestibular system.
During development, neural crest cells and placodal cells contribute to forming the cochlear nerve.
== Any disruption in these processes (e.g., genetic mutations or environmental factors) can lead to hearing impairments.
Development of Salivary Glands
Begins with epithelial buds that grow and branch into solid structures.
These structures arborize (branch out like a tree) to form ductal and glandular systems.
Mesenchymal signaling regulates this development.
