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Questions and Answers
Which prominence contributes to the formation of the upper lip?
At what week does the nasal placode begin to flatten and depress?
Which of the following structures is derived from the lateral nasal prominence?
What separates the nasal and oral cavities during development?
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Which cranial nerve is primarily responsible for the special sensory innervation of the tongue's anterior two-thirds?
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What structure do nasal pits develop into by week 5?
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Which structures are involved in the formation of the posterior tongue?
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What is the role of the tensor veli palatini?
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What structures develop from the first pharyngeal arch?
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Which cranial nerve is associated with the second pharyngeal arch?
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What is the evolutionary significance of the pharyngeal arches?
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Which structure does the third pharyngeal arch contribute to?
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Which pairs are associated with the internal pouches of the pharyngeal apparatus?
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From which embryonic lineages do the structures of the pharyngeal arches derive?
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What evolutionary function did the pharyngeal arches evolve to support in tetrapods?
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Which structures are derived from the fourth and sixth pharyngeal arches?
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What structure begins the descent of the thyroid gland?
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At which week does the thyroid gland typically finish its descent?
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Which part of the palate merges to form the premaxillary part of the maxilla?
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What is formed during the cap stage of tooth development?
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Which week marks the beginning of primary dentition?
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What role do the palatine shelves play in the development of the secondary palate?
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Which cells contribute to the formation of the dental papilla?
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At what stage do cell differentiation of ameloblasts and odontoblasts occur?
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Study Notes
Pharyngeal Apparatus Review
- Pharyngeal arches are essential for development of head and neck structures.
- Arches 1-6, each with unique components, including muscles, nerves, and blood vessels.
- Arch 1: maxillary and mandibular structures, malleus and incus, CN V, muscles of mastication.
- Arch 2: muscles of facial expression, stapes, styloid, part of hyoid, ECA branches, CN VII.
- Arch 3: part of hyoid, CN IX, stylopharyngeus, parts of ICA and CCA.
- Arches 4 and 6: laryngeal cartilages, CN X, pharyngeal and laryngeal muscles.
- Pharyngeal spaces include clefts (external ectoderm) and pouches (internal endoderm).
- Clefts: four pairs between arches.
- Pouches: four pairs between arches; each contributes to specific structures like ear cavity, pharyngotympanic tube, tonsils, parathyroid glands, and thyroid gland.
Evolutionary History of Head and Neck
- Pharyngeal arches initially functioned for filter feeding.
- Anterior arches evolved into jaws.
- Neural crest cells and mesoderm contributed to skeletal structures and muscles.
- Development of a predatory lifestyle influenced arch functions.
- Respiratory role of arches led to a reduction in arch number.
Evolutionary History of Head and Neck (Continued)
- Transition to land (tetrapods) resulted in a closing of the posterior pharynx.
- Posterior arches were reduced, leading to the evolution of the parathyroid gland (calcium regulation).
- Development of an air chamber in the ear allowed for sound conduction.
Evolution of Mammalian Ear: Ossicles
- Tetrapod jaw joint initially involved quadrate and articular bones.
- In mammals, the jaw joint utilizes the dentary and squamosal bones.
- The articular bone evolved into the malleus.
- The quadrate bone evolved into the incus.
Viscerocranium
- Pharyngeal arch 1 is responsible for the mandibular process, forming the mandible, malleus, and incus.
First Pharyngeal Arch: Muscles
- Muscles derived from arch 1 include the tensor veli palatini and tensor tympani.
Face Development
- Facial primordia begin to form by week 4.
- The stomodeum is the precursor to the mouth.
- Five prominences contribute to facial development:
- Frontonasal prominence
- Two maxillary prominences
- Two mandibular prominences
Placodes
- Placodes are thickened ectodermal regions that give rise to sensory structures.
- Lens placode: forms the eyes.
- Otic placode: forms the ears.
- Nasal placode: forms the nose.
Nasal Development
- The nasal placode flattens and then depresses.
- Forms a horseshoe shape:
- Nasal pit: precursor to the nasal cavity.
- Lateral nasal prominence: contributes to the ala, nasolacrimal duct.
- Medial nasal prominence: contributes to the nose, philtrum, upper lip, and part of the palate.
Nasal Cavity
- The nasal pits develop into the nasal sac.
- The oronasal membrane separates the nasal and oral cavities.
- Paranasal sinuses form late in fetal development or postnatally.
Forehead and Nose
- The frontonasal prominence contributes to the forehead.
- The medial nasal prominence contributes to the nose.
- The lateral nasal prominence contributes to the ala of the nose and nasolacrimal duct.
Lips
- The upper lip forms from the fusion of the medial nasal prominences and maxillary prominences.
- The lower lip forms from the fusion of the mandibular prominences.
Finalizing Facial Development
- The development of paranasal sinuses and the accommodation of teeth contribute to finalizing facial features.
- Brain enlargement also influences facial development.
Tongue Development
- The tongue starts developing around week 4.
- Contributions from various pharyngeal arches:
- Arch 1: forms the lateral swellings and tuberculum impar.
- Arch 2: forms the hypobranchial eminence (copula).
- Arch 4: forms the epiglottal swelling.
Tongue Swellings
- The lateral swellings overgrow the tuberculum impar, forming the anterior two-thirds of the tongue.
- The posterior tongue develops from the third arch tissue overgrowing the second arch.
- The terminal sulcus marks the boundary between the anterior and posterior thirds of the tongue.
Tongue Innervation
- Somatic sensory:
- Body: CN V
- Root: CN IX (with slight contribution from CN X)
- Special sensory:
- Body: CN VII
- Root: CN IX
- Motor: hypoglossal nerve (CN XII)
Thyroid Gland
- The thyroid gland is the first endocrine gland to develop.
- It originates as a thickening in the developing tongue between the tuberculum impar and copula.
- The foramen cecum marks the point of origin.
Thyroid Gland Descent
- The thyroid gland descends from the tongue via the thyroglossal duct.
- It fuses with the ultimopharyngeal body (from the 4th pharyngeal pouch).
- Descent is complete by week 7.
Palate Development
- The palate forms the roof of the mouth, separating the oral and nasal cavities.
- Primary palate: anterior portion, including the incisive foramina, derived from the medial nasal prominence.
- Secondary palate: anterior two-thirds of the bony palate and the soft palate, derived from the maxillary prominences.
Primary Palate
- Formed by the merging of the two medial nasal prominences.
- This merging forms the intermaxillary segment.
- The premaxillary part of the maxilla develops, containing the four incisor teeth.
- The primary palate separates the nasal and oral cavities early in development.
- The incisive foramina allow for the passage of blood vessels and nerves.
Secondary Palate
- The palatine shelves, derived from the maxillary prominences, project below the tongue around week 6.
- The palatine shelves rise and fuse, resulting in the secondary palate.
- The tongue is pulled into the mouth during shelf fusion.
- The shelves become horizontal around week 7-8.
Tooth Development
- Tooth development begins during the bud stage (week 6).
- Primary dentition (deciduous teeth): 10 buds per jaw.
- The dental lamina originates from the oral ectoderm.
- Neural crest cells contribute to the development of the teeth.
Tooth Development: Cap Stage
- The bud invaginates, forming the inner and outer dental epithelia.
- The enamel knot forms.
- The enamel organ develops.
- The dental papilla, composed of neural crest cells, forms dentin and pulp.
- The dental sac contributes to the development of the periodontium.
- The beginnings of permanent teeth develop during this stage.
Tooth Development: Bell Stage
- Cell differentiation occurs, leading to the formation of ameloblasts (enamel), odontoblasts (dentin), and the crown.
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