Overview
This lecture covers high-yield embryology of the head and neck, focusing on germ layers, pharyngeal arches, and related developmental processes relevant for dental and medical board exams.
Early Embryonic Development
- Germinal stage: fertilization to week 2; highest risk for miscarriage.
- Fertilization occurs in the fallopian tube; forms the zygote.
- Zygote undergoes cleavage to morula, then blastula (blastocyst in mammals).
- Implantation: blastula embeds in the uterine wall.
- Blastocyst consists of the inner cell mass (embryoblast) and trophoblast (forms placenta).
Formation of Germ Layers
- Week 2: embryoblast forms the bilaminar disk—epiblast (upper) and hypoblast (lower).
- Amniotic cavity (above) and yolk sac cavity (below) arise.
- Week 3: gastrulation forms three germ layers—ectoderm, mesoderm, endoderm.
- Embryonic period (weeks 3-8): organ systems develop; sensitive to teratogens.
- Ectoderm: forms skin, nervous system, teeth enamel; mesoderm: muscles, skeleton, connective tissue; endoderm: linings of hollow organs.
Neurulation and Body Segmentation
- Week 4: neurulation—neural plate forms neural tube (future CNS).
- Neural crest cells migrate and contribute to facial skeleton, nerves.
- Somites (body segments) and pharyngeal arches (head/neck segments) arise.
Pharyngeal Arch Derivatives
- Five key arches: 1 (mandibular), 2 (hyoid), 3, 4, 6; each has artery, nerve, muscle, cartilage.
- Arch 1: mandibular structures, muscles of mastication, innervated by CN V.
- Arch 2: facial expression muscles, stapes, facial nerve (CN VII).
- Arch 3: part of hyoid, stylopharyngeus, glossopharyngeal nerve (CN IX).
- Arches 4 & 6: laryngeal structures, vagus nerve (CN X).
- Arch 5 is transient and regresses.
Clefts, Pouches, and Placodes
- Ectodermal clefts and endodermal pouches separate arches; placodes develop sensory ganglia.
- Cleft 1: external acoustic meatus; clefts 2-4 form cervical sinus (can form branchial cysts if persistent).
- Pouch 1: middle ear, auditory tube; pouch 2: palatine tonsil; pouch 3: thymus, inferior parathyroids; pouch 4: superior parathyroids, thyroid C cells.
Development of Pituitary, Tongue, Thyroid, and Ear
- Pituitary gland: anterior from oral ectoderm, posterior from neural ectoderm.
- Tongue: sensation/taste/motor divided by CN V, VII, IX, X, and XII (from occipital somites).
- Thyroid: develops from the foramen cecum at tongue base; migrates down via thyroglossal duct.
- External ear: forms from six auricular hillocks (arches 1 and 2).
Cleft Lip and Palate Formation
- Cleft lip: failure of fusion between maxillary and medial nasal prominences (weeks 4-6).
- Cleft palate: failure of palatal shelf fusion (weeks 6-8); primary palate from medial nasal, secondary palate from palatal shelves.
Clinical Correlates
- DiGeorge syndrome: defects in pouches 3/4 lead to thymic aplasia, hypocalcemia, congenital heart defects (CATCH-22 mnemonic).
- Teratogens like retinoic acid can disrupt first/second arch development (e.g., microtia, micrognathia).
Key Terms & Definitions
- Gastrulation — formation of three germ layers from the epiblast.
- Neurulation — process by which the neural tube forms from the neural plate.
- Pharyngeal arches — embryonic structures that form head and neck components.
- Cleft (branchial, palatal, lip) — failure of normal tissue fusion during development.
- DiGeorge syndrome — disorder due to abnormal development of 3rd/4th pharyngeal pouches.
Action Items / Next Steps
- Memorize key pharyngeal arch derivatives (nerves, muscles, bones).
- Review diagrams of germ layers, neural tube, and pharyngeal arches.
- Study cranial nerve associations with arches.
- Practice drawing developmental pathways for head and neck structures.