Development of the Tongue - Complete Notes

Overview

Developmental Swellings - Step by Step

Step 1: Tuberculum Impar (Median Lingual Swelling)

• Appears at the end of the 4th week of IUL
• A small, median, triangular elevation in the floor of the primitive pharynx
• Located just cranial (rostral) to the foramen cecum
• Derived from 1st pharyngeal arch mesenchyme
• Also called the median tongue bud - it is the first indication of tongue development
• Fate: It is ultimately overgrown by the lateral lingual swellings and does not form a recognizable part of the adult tongue

Step 2: Lateral Lingual Swellings (Distal Tongue Buds)

• Appear soon after the tuberculum impar
• Two oval swellings, one on each side of the tuberculum impar
• Placed slightly distal to the tuberculum impar - hence called distal tongue buds
• Derived from proliferation of mesenchyme in the ventromedial parts of the 1st pharyngeal arch
• They rapidly increase in size, merge with each other, and overgrow the tuberculum impar

• They merge and form the anterior 2/3 (oral part / body) of the tongue
• The midline groove on the tongue surface + the internal fibrous lingual septum mark the fusion site of the two lateral swellings

Step 3: Hypobranchial Eminence / Copula of His

• Develops caudal to the tuberculum impar
• A large, second median swelling in the floor of the primitive pharynx
• Derived from mesoderm of the 2nd, 3rd, and part of the 4th pharyngeal arches
• It soon subdivides into two parts:
  • Large cranial part = the true hypobranchial eminence (from 3rd and 4th arch mesenchyme)
  • Small caudal part = the copula (from 2nd arch fusion) - this is eventually overgrown and disappears

The floor of the primitive pharynx showing pharyngeal arches 1-4, with the lateral lingual swellings (blue, arch 1), median lingual swelling/tuberculum impar, foramen cecum, copula/hypobranchial eminence (arches 2-4), and the laryngotracheal groove caudally.

Step 4: Epiglottal Swelling

• A third median swelling formed by the posterior part of the 4th arch
• Marks development of the epiglottis
• The laryngeal orifice lies immediately behind this swelling, flanked by the arytenoid swellings

Summary Diagram of Derivatives

Left panel (early): tuberculum impar, lateral lingual swellings, copula (hypobranchial eminence), epiglottal swelling. Right panel (later): body of tongue, terminal sulcus, foramen cecum, root of tongue, palatine tonsil, epiglottis.

Adult Tongue: What Each Swelling Becomes

Nerve Supply - Explained by Development

Muscle Development

• Most tongue muscles are derived from myoblasts migrating from occipital somites (2nd to 5th occipital myotomes) - NOT from pharyngeal arch mesenchyme
• The hypoglossal nerve (CN XII) accompanies these myoblasts during migration and innervates all intrinsic + most extrinsic tongue muscles
• Palatoglossus is the exception - innervated by CN X (vagus) via the pharyngeal plexus, reflecting its palatal origin
• Some muscles may also differentiate in situ within the tongue
• Cranial neural crest cells migrate into the developing tongue and give rise to connective tissue and vasculature

Boundary Between Anterior and Posterior Tongue

• The line of fusion of the two embryological parts is marked by the terminal sulcus - a V-shaped groove on the dorsum of the tongue
• At the apex of the V lies the foramen cecum - the site where the thyroid diverticulum originated and descended to become the thyroid gland
• If the thyroglossal duct fails to obliterate (normally by 6 weeks), a thyroglossal duct cyst results - it runs from foramen cecum down to the thyroid, passing through or near the hyoid bone

The spatial relationship between tongue swellings and the thyroid diverticulum at 4 weeks.

Papillae and Taste Buds - Timeline

• Taste buds form by inductive interaction between tongue epithelium and invading gustatory nerve fibers
• Fetal taste responses to bitter substances are detectable at 26-28 weeks

Molecular Signals

• Myogenic regulatory factors - guide muscle development
• Wnt/Notch signaling pathway - pattern formation
• PAX3 and PAX7 (paired box genes) - specification of myogenic precursors

Clinical Correlation: Ankyloglossia (Tongue-Tie)

Key Points to Remember

1. Tongue development begins at the end of week 4
2. Tuberculum impar = first swelling to appear; median, triangular, from 1st arch
3. Lateral lingual swellings = distal tongue buds; overgrow tuberculum impar → form anterior 2/3
4. Hypobranchial eminence (copula of His) = from 2nd, 3rd, 4th arches; cranial part → posterior 1/3 of tongue
5. 2nd arch (copula) is overgrown by 3rd arch tissue - that's why posterior tongue is CN IX, not CN VII
6. Foramen cecum = remnant of thyroid descent; sits at the apex of terminal sulcus
7. Tongue muscles = from occipital myotomes; innervated by CN XII
8. Terminal sulcus = boundary between anterior 2/3 and posterior 1/3

Development of the Tongue - Simple Notes

When Does It Start?

The 4 Swellings (in order of appearance)

1. Tuberculum Impar

• What: Small, median (midline), triangular swelling
• Where: Just in front of (cranial to) the foramen cecum
• From: 1st pharyngeal arch
• Fate: Gets covered over by lateral swellings - forms no recognizable part of the adult tongue

2. Lateral Lingual Swellings (x2)

• What: Two oval swellings, one on each side of tuberculum impar
• Also called: Distal tongue buds (because they are placed slightly distal to tuberculum impar)
• From: 1st pharyngeal arch
• Fate: They enlarge, merge together, and overgrow the tuberculum impar → become the anterior 2/3 of the tongue

3. Hypobranchial Eminence (Copula of His)

• What: Large median swelling, caudal to tuberculum impar
• From: 2nd, 3rd, and 4th pharyngeal arches
• Divides into:
  • Large cranial part → becomes posterior 1/3 of the tongue
  • Small caudal part → becomes the epiglottis
• The 2nd arch portion is overgrown by 3rd arch tissue (important - see nerve supply below)

What Each Part Becomes (Adult Tongue)

Nerve Supply (follows arch of origin)

Why CN IX for posterior tongue and not CN VII? Because the 3rd arch tissue overgrows the 2nd arch tissue. The 2nd arch (CN VII) contribution disappears entirely.

Muscles of the Tongue

• Come from occipital somites (not pharyngeal arches)
• Innervated by CN XII (hypoglossal)
• Exception: Palatoglossus → innervated by CN X

Two Important Landmarks

• Foramen cecum - pit at the apex of the V-groove; marks where the thyroid gland originated and migrated downward
• Terminal sulcus - V-shaped groove on the tongue dorsum; separates anterior 2/3 from posterior 1/3

Quick Memory Table

1. Tuberculum impar
2. Lateral lingual swellings
3. Hypobranchial eminence and its subdivision into cranial and caudal parts

• Adult derivatives table
• Nerve supply (CN V, VII, IX, X)
• Tongue muscles and CN XII
• Foramen cecum / terminal sulcus explanation
• Memory table
• The epiglottis mention

Development of the Tongue – Notes (Strictly from your text)

Where & When

• Develops in the floor of the developing mouth
• From 1st, 2nd, 3rd, and 4th pharyngeal arches
• Begins at the end of the 4th week of IUL

Swelling 1 – Tuberculum Impar

• Small, median, triangular swelling
• Appears in the floor of the primitive pharynx
• Located just cranial to the foramen cecum

Swelling 2 – Lateral Lingual Swellings

• Two oval swellings
• Appear soon after tuberculum impar
• One on each side of tuberculum impar
• Placed slightly distal to tuberculum impar
• Hence also called distal tongue buds

Swelling 3 – Hypobranchial Eminence (Copula of His)

• Develops caudal to tuberculum impar
• A second large median swelling
• Located in the floor of the primitive pharynx
• Related to 2nd, 3rd, and 4th pharyngeal arches
• Soon subdivides into:
  • Large cranial part
  • Small caudal part