Embryology Week 1 & 4 Overview

Early Embryogenesis: Week 1

• The inner cell mass congregates on one side, forming the embryonic disk and three embryonic layers.
• Stem cells migrate throughout the embryo, giving rise to cranial and sensory ganglia, peripheral nerves, and connective tissues in the head, face, and oral cavity.
• Ectomesenchyme forms specialized connective tissues in the head, face, and oral cavity, such as dentine, pulp, and cementum, but NOT enamel.

Early Embryogenesis: Week 4

• The embryo starts to take shape through folds, forming the stomodeum (primitive mouth) and the bucco-pharyngeal membrane.
• The bucco-pharyngeal membrane separates the primitive mouth from the primitive gut.
• The bucco-pharyngeal membrane ruptures, allowing communication between the stomodeum and the gut.
• Bulges called pharyngeal arches (branchial arches) develop, forming the building blocks for face and oral cavity structures.
• Each pharyngeal arch has its own nerve supply, blood supply, and cartilage.

Pharyngeal Arches and Derivatives

• Each arch contributes to specific structures:
  • 1st Arch (Mandibular): Trigeminal nerve, 1st aortic arch blood vessel, muscles of mastication, Merkel’s cartilage, Mandible, Maxilla, Zygomatic bones, Squamous portion of temporal bone, Malleus and Incus of ear.
  • 2nd Arch (Hyoid): Facial nerve, 2nd aortic arch blood vessel, Muscles of facial expression, Reichert’s cartilage, Part of the hyoid bone, Body of hyoid, Styloid process, Stapes.
  • 3rd Arch: Glossopharyngeal nerve, 3rd aortic arch blood vessel, Stylopharyngeal muscle, Connective tissue of the thymus, Inferior parathyroid.
  • 4th Arch: Superior laryngeal branch of the Vagus nerve, 4th aortic arch blood vessel, Laryngeal muscles, Cartilage of the larynx (Thyroid, corniculate, and cuneiform).

Development of the Face: Week 4-6

• The face continues to develop in detail with the fusion of prominences:
  • 1x Frontonasal prominence
  • 2x Maxillary prominences
  • 2x Mandibular prominences

Development of the Primary Palate: Week 6-7

• The primary palate forms from the fusion of the frontonasal prominence and the medial nasal prominences.
• The developing tongue occupies the space where the secondary palate will form, hence the primary palate forms first due to limited space.

Development of the Secondary Palate: Week 7-8

• The palatine shelves develop and grow around the developing tongue.
• The palatine shelves fuse together along the midline with the primary palate and the nasal septum as the tongue retracts.
• The incisive foramen forms at the midline between the secondary and primary palates.

Development of the Tongue: Week 4-7

• The tongue develops from the 1st, 2nd, and 4th pharyngeal arches.
  • Anterior 2/3: 2x lateral lingual swellings (1st arch), Tuberculum impar (2nd arch)
  • Posterior 1/3: Copula/hypobranchial eminence.
• Cellular apoptosis allows the tongue to separate from the floor of the mouth, leaving a frenulum.

Development of the Alveolar Bone: Week 7

• The maxilla and mandible jaws undergo ossification, starting in the respective prominences.

Embryonic Origins

• Neural crest cells
• Pharyngeal arches

Clinical Significance

• Developmental anomalies can occur due to disruptions in the developmental processes.
• Example: Cleft lip and palate.