Chapter 4 Study Guide

1. Face Development Overview

• Begins predominantly in week 4 and is largely complete by week 8.

• Formed by the fusion of five prominences surrounding the stomodeum (primitive mouth):

  • One frontonasal prominence (superior)

  • Two maxillary prominences (lateral)

  • Two mandibular prominences (inferior)

2. Nasal Cavity and Structures

• Nasal placodes: Ectodermal thickenings appearing on the frontonasal prominence around week 4.

  • Invaginate to form nasal pits, which deepen into nasal sacs.

• Medial nasal processes: Develop on either side of the nasal pits, contribute to the middle part of the nose and the intermaxillary segment.

• Lateral nasal processes: Develop lateral to the nasal pits, form the alae (sides) of the nose.

3. Intermaxillary Segment and Upper Lip

• Intermaxillary segment: Formed by the fusion of the two medial nasal processes in the midline.

  • Gives rise to:

    • Philtrum of the upper lip

    • Primary palate (anterior part of the hard palate)

    • Four maxillary incisor teeth

• Upper lip: Formed by the fusion of the maxillary prominences with the medial nasal processes.

  • Philtrum: The vertical groove in the upper lip, derived from the intermaxillary segment.

  • Commissure: The corners of the mouth, formed by the fusion of maxillary and mandibular prominences.

  • Cleft lip: A common congenital anomaly resulting from failed fusion of the maxillary prominence with the medial nasal process, often occurring in week 6.

4. Branchial (Pharyngeal) Arches and Neck Development

• Time frame: Branchial arches appear during weeks 4-5.

• Embryonic layers involvement: Each arch consists of a core of mesenchyme (derived from paraxial and lateral plate mesoderm, and neural crest cells) covered externally by ectoderm and internally by endoderm (pharyngeal pouches).

  • Foregut origin: The pharynx, from which the pharyngeal arches develop, is derived from the foregut endoderm.

4.1. Mandibular Arch (Branchial Arch I)

• Cartilage: Meckel's cartilage

  • Derivatives: Malleus, incus, sphenomandibular ligament.

• Muscles: Muscles of mastication (masseter, temporalis, medial/lateral pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini.

• Nerve: Trigeminal nerve ($CN V$).

4.2. Branchial Arch II (Hyoid Arch)

• Cartilage: Reichert's cartilage

  • Derivatives: Stapes, styloid process, stylohyoid ligament, lesser horn and upper part of the body of the hyoid bone.

• Muscles: Muscles of facial expression (e.g., orbicularis oculi, orbicularis oris, buccinator, platysma), posterior belly of digastric, stylohyoid, stapedius.

• Nerve: Facial nerve ($CN VII$).

5. Clinical Relevance

• Branchial cleft cyst: Results from the incomplete obliteration of the cervical sinus (a space formed by the overgrowth of arch II over arches $III$ and $IV$).

  • Typically presents as a palpable mass or sinus along the anterior border of the sternocleidomastoid muscle, lateral to the neck.

6. Key Time Frames

• Face Development: Weeks 4-8.

• Nasal Placodes Appearance: Week 4.

• Primary Palate Fusion: Weeks 5-6.

• Secondary Palate Fusion: Weeks 7-10.

• Branchial Arches Appearance: Weeks 4-5.

• Neck Development: Primarily weeks 4-8, involving the transformation of branchial arches and pouches.