Chapter 4 – Development of the Face and Neck week 3

Timeline Overview

• Facial and neck development happens early, from about week 4 to week 12 of pregnancy (when the baby is an embryo turning into an early fetus).

• This process uses cells from all three main layers that form the embryo (ectoderm, mesoderm, and endoderm).

• The rapidly growing front part of the brain pushes out over the developing mouth area (the oropharyngeal membrane) and heart, giving the face and head their early shape.

• Important steps during this time:

  • Week 4: Facial parts start to form, the primitive mouth (stomodeum) appears, and the branchial (gill-like) arches become visible.

  • Week 7: All branchial grooves (folds on the outside) except the first one disappear.

  • Week 12: The main shape of the face and neck is mostly finished.

  • This period aligns with Carnegie stages 13 to 23, which is roughly 28 to 56 days after ovulation.

Embryonic Folding (Week 4)

• The trilaminar disc (a flat, three-layered sheet of cells) begins to fold, especially at the head end (cephalic folding). This transforms it from a flat sheet into a tube-like embryo.

• This folding correctly places the different cell layers so they can form organs later.

• It also creates the front wall of the body and clearly defines the primitive foregut (the upper part of the digestive tube) and the stomodeum (which will become the mouth).

Primary Facial Processes (Start in Week 4)

• Five main swellings (or bumps) develop around what will be the mouth:

  • One frontonasal process (single, in the middle, at the top).

  • Two maxillary processes (one on each side).

  • Two mandibular processes (one on each side, at the bottom).

• The growth, joining, and reshaping of these five bumps create the final shape of the face.

Facial vs. Palatal Fusion

• Facial fusion: This is when a groove or crease between two swellings on the same surface disappears, making them smooth (e.g., the two lower jaw processes joining in the middle).

• Palatal fusion: This is when structures from opposite sides join together (e.g., the shelves that form the roof of the mouth). If this fails, it leads to a cleft palate.

Stomodeum & Oral-Cavity Formation

• The stomodeum begins as a shallow dip in the outer skin layer (ectoderm) before week 4.

• At first, it's separated from the inner lining of the primitive pharynx (early throat) by a thin wall called the oropharyngeal membrane.

• The FIRST major facial event is when this membrane breaks down in week 4. This makes the stomodeum deeper and connects the early mouth cavity to the primitive gut.

Mandibular Processes & Lower-Face Formation

• These are the bottom two processes, filled with special cells (neural-crest mesenchyme) and covered by outer skin (ectoderm) and inner lining (endoderm).

• They join in the middle to form the mandibular arch (also called the first branchial arch).

• The point where they fuse can be seen after birth as a slight line on the chin, called the mandibular symphysis.

• What they form:

  • Bones: The lower jaw bone (mandible), and Meckel’s cartilage, which acts as a guide for some inner ear bones (malleus) and a ligament (sphenomandibular ligament).

  • Teeth: All lower teeth and the tissues that hold them in place (like periodontal ligament and bone).

  • Muscles: Muscles for chewing, parts of the roof of the mouth, and some muscles under the chin (all supplied by the trigeminal nerve, CV V).

  • Lining: The lower lip and the front part of the tongue.

Frontonasal Process & Upper-Face Primordium

• This is the bump at the top, forming the upper border of the primitive mouth (stomodeum).

• It develops into:

  • The forehead, the bridge and root of the nose.

  • The primary palate (the very front part of the roof of the mouth).

  • The nasal septum (the wall separating the nostrils).

  • The nasal placodes (thickened areas that become sense organs) and later the nasal pits.

  • The medial and lateral nasal processes.

  • The intermaxillary segment (a key part that grows between the maxillary processes).

Placodes (Special-Sense Ectoderm)

• These are thickened areas of the outer skin (ectoderm) that show where future sensory organs will develop.

  • Lens placodes: Paired, on the frontonasal process, lead to the eyes and their structures.

  • Otic placodes: Located more to the side and back, become the inner ear. They start low and move up as the head grows.

  • Nasal placodes: Button-like, at the front-middle, become the lining inside the nose responsible for smell.

Nasal Pits, Sacs & Nose Development

• By week 4, the tissue around the nasal placodes grows, causing the placodes to sink inwards, forming nasal pits.

• These pits continue to deepen, forming nasal sacs that grow towards the brain.

• A membrane called the oronasal membrane initially separates these sacs from the stomodeum. When it breaks down, it connects the early nasal and oral cavities (which will become the choanae, or the back openings of the nasal passages into the throat).

Medial Nasal Processes

• These are two crescent-shaped swellings located between the nasal pits; they join in the middle (both externally and internally):

  • External fusion: Forms the bridge to the tip of the nose, the philtrum (the groove below your nose and above your upper lip), and the central part of the upper lip.

  • Internal fusion: Forms the intermaxillary segment.

    • This segment contains special cells (ectomesenchyme) that help form the upper front teeth (maxillary incisors) and their supporting tissues, the primary palate, and part of the nasal septum.

Lateral Nasal Processes

• These border the nasal pits on the sides and form the alae (the flaring sides or wings) of the nose.

• They are separated from the maxillary processes by a groove that later becomes the nasolacrimal (tear) duct.

Maxillary Processes & Mid-Face Formation

• These processes grow outwards from the upper part of the mandibular arch.

• What they form:

  • Bones: Most of the upper jaw bone (maxilla, except for where the incisors are), the cheekbones (zygomatic bones), and parts of the temporal bones.

  • Teeth: Enamel organs (which form enamel) for canine, premolar, and molar teeth, along with their supporting tissues.

  • Soft tissue: The cheeks, the secondary palate (the main part of the roof of the mouth), the side parts of the upper lip, and parts of the tear duct system.

Upper-Lip Morphogenesis

• How the upper lip forms: Each maxillary process joins with the nearby medial nasal process.

  • The medial nasal part contributes to the philtrum and the central red part of the lip.

  • The maxillary part contributes to the side segments of the lip.

• The corners of the mouth (commissures) are formed where the maxillary process blends with the side part of the mandibular arch.

Developmental Disturbance – Commissural & Congenital Lip Pits

• Commissural pits: These are small, blind-ended tubes or depressions at the corners of the mouth (labial commissures); their depth can vary.

• Midline congenital lip pits: Can sometimes appear in the middle of the upper lip.

• These are usually harmless and don't require treatment unless they cause cosmetic concerns or get infected.

Neck Development Overview

• The neck develops around the primitive pharynx (which will become the upper throat) and the branchial apparatus.

• The branchial apparatus consists of: arches, grooves (or clefts), membranes, and pharyngeal pouches.

Branchial (Pharyngeal) Arches

• These appear as stacked, paired swellings below the mouth area in week 4.

• Each arch contains its own cartilage, artery, nerve, and muscle-forming cells (derived from neural crest cells).

Specific Arches

1. Mandibular arch: (See above, forms lower face and jaw-related structures).

2. Hyoid arch: Its cartilage is called Reichert’s cartilage.

   • Forms parts of the inner ear (stapes bone), the styloid process (a slender bone near the ear), the stylohyoid ligament, and the upper part/lesser horns of the hyoid bone (a U-shaped bone in the neck); also forms muscles used for facial expressions (supplied by the facial nerve, CN VII).

3. Third arch: Its cartilage is unnamed, but it forms the larger horns and lower part of the hyoid bone; also the stylopharyngeus muscle (supplied by the glossopharyngeal nerve, CN IX).

4 & 6. These two arches fuse. Their cartilages form most of the voice box (larynx) framework; their muscles form parts of the larynx and pharynx (supplied by branches of the vagus nerve, CN X).

Branchial Grooves (Clefts)

• These are outer dips in the skin (ectoderm) found between the branchial arches.

• Only the first groove remains in adults, forming the external auditory meatus (the ear canal).

• Grooves 2 through 4 disappear by week 7, which helps to smooth out the contour of the neck.

Pharyngeal Pouches (Endodermal)

• These are balloon-like outgrowths from the inside lining (endoderm) of the primitive pharynx.

1. First pouch: Forms the Eustachian tubes (connecting the middle ear to the back of the throat) and the middle-ear cavity.

2. Second pouch: Forms the inner lining (crypt epithelium) of the palatine tonsils.

3. Third pouch: Forms the inferior parathyroid glands (involved in calcium regulation) and the thymus gland (involved in immune system development).

4. Fourth pouch: Forms the superior parathyroid glands and special cells (parafollicular C-cells) in the thyroid gland that produce calcitonin (a hormone for calcium regulation).

Embryo Growth Metrics (Visualized in Multi-Dimensional Imaging)

• This section provides measurements of embryo size (crown-rump length, or CRL) at different post-ovulatory ages (days after egg release):

  • 28 days: 4–6\,\text{mm} (Carnegie stage 13)

  • 32 days: 5–7\,\text{mm}

  • 33 days: 7–9\,\text{mm}

  • 37 days: 11–14\,\text{mm}

  • 44 days: 13–17\,\text{mm}

  • 47–50 days (Stages 19–20): Approximately 15\,\text{mm}

  • 56 days (Stage 23, end of embryonic period): Approximately 25\,\text{mm}

Clinical & Educational Significance for Dental Professionals

• Knowing how these parts develop and when helps explain:

  • Why and how often cleft lip/palate occurs (due to failed fusions between weeks 4 and 8).

  • Where to find cysts or fistulas (abnormal tracts) along the paths where branchial clefts or tear ducts formed.

  • The connection between the trigeminal and facial nerves and the muscles/structures that form from the first and second branchial arches.

  • Why certain issues might occur together (e.g., ear problems with jaw defects, because they share a common origin from the same arch).

• Early diagnosis allows different specialists (surgeons, orthodontists, speech therapists) to work together for the best care.

Key Take-Home Points

• Face development is a very precise process involving FIVE facial processes starting around the 4^{th} week of pregnancy.

• The ways in which parts fuse are different for the outer facial surfaces compared to the roof of the mouth; problems during these fusions cause different birth defects.

• Placodes are key areas that mark where special sense organs (like eyes and nose) will form; their inward folding creates cavities and ducts.

• The branchial apparatus forms the bones, muscles, and glands of the neck; only the first groove remains as the ear canal in adults.

• Continuous growth from head to tail and from side to side combines the face, mouth, and throat parts into a working unit by week 12.