Development of Orofacial Structures – Chapter 5 Study Notes week 3

Let's break down how our face and mouth develop before we're born, making it easier to understand.

What We'll Learn

• Understand the order of events as the face and mouth grow.

• Connect what happens during development to the parts of the mouth we see today.

• Learn how the mouth opening (stomodeum) and early mouth space form.

• Be able to describe and label:

  • How the roof of the mouth (palate) develops (the first and second parts).

  • How the nose and the wall separating the nostrils (nasal septum) form.

  • How the tongue develops, becomes complete, and its important landmarks.

  • The parts of the branchial apparatus (gill arches) and what they turn into.

• Figure out the first (primary) versus second (secondary) parts of the palate.

• Link problems during development (like clefts) to what we might see later on.

Important Timelines (Key Weeks)

• Palate (roof of mouth): Starts forming around the 5^{th} week and finishes by the 12^{th} week.

• Nasal cavity & septum (nose and middle wall): Develops at the same time, from 5^{th} to 12^{th} weeks. The septum starts joining up at the 9^{th} week.

• Tongue: Begins forming from the 4^{th} to 8^{th} weeks, with its final pieces joining by the end of the 8^{th} week. The back of the tongue moves down to its final position around 4 years after birth.

How the Palate Develops

Basic Ideas

• The palate forms from two main parts:

  1. The primary palate (which comes from the intermaxillary segment).

  2. The secondary palate (which is made from shelves growing out from the jaw area).

• These parts join from the front to the back. They need to come into contact from different angles (top, sides, bottom) of the embryo to fuse properly.

Primary Palate (First Stage)

• Where it comes from: The intermaxillary segment, which is formed when the medial nasal processes (parts that form the middle of the nose) fuse together.

• When it appears: Around the 5^{th} week.

• What it becomes:

  • The front part of the upper jaw (maxilla) that holds the four top incisor teeth.

  • It starts to partly separate the nose and mouth cavities early on.

Secondary Palate (Second Stage)

• When it starts: The 6^{th} week.

• How it forms:

  • Two palatal shelves (like ledges) grow from each side of the upper jaw area.

  • These shelves first grow downwards, then they lift up, get longer, and swing inwards over the top of the tongue.

  • Finally, these shelves join together to create a solid secondary palate.

• What it becomes:

  • The back two-thirds (\frac{2}{3}) of the hard roof of the mouth – this part supports the upper canine teeth and all back teeth.

  • The entire soft palate (the soft, flexible part at the back of the roof of the mouth) and the uvula (the small hanging ball at the back of the throat).

Finishing the Palate (Third Stage)

• The process:

  • The back edge of the primary palate meets the front edge of the secondary palate.

  • Fusion continues from the front to the back until it's completely joined (by the 12^{th} week).

• The result: This creates a clear separation between the nose and mouth cavities, which is essential for speaking clearly, swallowing food, and breathing properly.

How the Nasal Cavity & Septum Develop

• This happens at the same time as the palate is forming, from 5^{th} to 12^{th} weeks.

• The vertical nasal septum (the wall dividing your nostrils) grows downwards from the fused medial nasal processes.

• When the septum fuses:

  • Starts at the 9^{th} week.

  • Is fully joined by the 12^{th} week when it touches the horizontally formed final palate.

• What it does: This completely separates the nasal (nose) and oral (mouth) spaces, allowing air and food to travel through different paths.

Problems During Development: Clefts

• If these shelves or the primary/secondary palate parts don't join together properly, it results in cleft lip and/or palate.

• Consequences: This can lead to problems with feeding, speech difficulties, a higher risk of ear infections, and social/emotional challenges.

• This shows how important the precise timing and exact positioning of these developing tissues are.

How the Tongue Develops

Where the Tongue Comes From (Branchial Arches)

• Body (front two-thirds, or anterior rac{2}{3}): Forms from the 1^{st} branchial arch.

• Base (back one-third, or posterior rac{1}{3}): Forms from parts of the 2^{nd}, 3^{rd}, and 4^{th} arches.

Steps in Forming the Tongue (Swellings)

1. Tuberculum impar

   • A small, triangular bump in the middle of the floor of the early throat (appears around the 4^{th} week).

2. Lateral lingual swellings (a pair of them)

   • These grow on each side of the tuberculum impar; they quickly get bigger and grow over the tuberculum impar.

   • The line where these fuse together is seen in adults as the median lingual sulcus (the groove down the middle of your tongue).

3. Copula

   • A bump in the middle, located behind the lateral lingual swellings; it mostly comes from the 3^{rd} arch (and grows over the 2^{nd} arch).

   • It helps form the base of the tongue.

4. Epiglottic swelling

   • Comes from the 4^{th} arch; it forms the epiglottis (the flap that covers your windpipe when you swallow) and the very back of the tongue.

Important Fusion Marker

• Sulcus terminalis: This is a V-shaped groove on the tongue that shows where the body (from arch 1) and the base (from arches 2–4) joined together. At the point of the 'V' is the foramen cecum—this is where the thyroglossal duct started, which is the path the thyroid gland took as it moved down to its final position.

Final Touches & Changes After Birth

• All these swellings have joined together by the end of the 8^{th} week; the tongue then moves up into its proper place in the mouth.

• The base (back one-third) of the tongue doesn't fully move down into its adult position in the throat until about 4 years of age.

• Growth: The tongue roughly doubles in length, width, and thickness from birth until puberty.

What are Branchial Arches?

• Even though we focused on the tongue, remember that each branchial (or pharyngeal) arch provides:

  • Cartilage/bone (skeletal parts).

  • Muscles.

  • Blood vessels.

  • Nerves that are specific to its section.

• Proper development of these arches ensures that the tongue, voice box (larynx), face, and neck structures form correctly and work together.

How This Connects to Real Life (Clinical & Functional)

• If the palate and septum don't join correctly, it can lead to cleft palate/lip, openings between the nose and mouth, and problems with feeding and speech.

• Tongue problems (like macroglossia, which is an abnormally large tongue, or ankyloglossia, which is 'tongue-tie') happen when swellings are abnormal or don't move correctly.

• Knowing about this development helps surgeons decide when to perform operations (e.g., cleft repair around 9–18 months) and anticipate potential breathing or feeding difficulties.

Why This Information Matters

• Understanding how these embryonic parts join helps doctors predict where cleft lines will be and plan surgeries (like customizing surgical flaps).

• Knowing the origin of the sulcus terminalis and foramen cecum helps diagnose thyroglossal cysts, which can form along the path where the thyroid gland moved.

• Oral histologists (people who study mouth tissues) link the stages of palate and tongue development with how tissues become specialized (e.g., bone formation in the hard palate, the way muscle fibers are arranged in the tongue).