EMBRYOLOGY OF THE RESPIRATORY SYSTEM,
DIAPHRAGM & THORACIC CAVITY
High-Yield Study Guide, Clinical Correlation & Review Summary
Based on: Langman's Medical Embryology (Sadler), 15th ed. — Chapter 14 Ideal for: Exam preparation, review, and clinical correlation
1. GENERAL INTRODUCTION
💡 KEY CONCEPT
The respiratory system begins developing at week 4 of embryonic life, long before the baby needs to breathe. It all starts from
a small "bud" of the foregut called the respiratory diverticulum or lung bud.
Although the respiratory apparatus only functions after birth, most of its anatomical structures are already established before
the third trimester. Its formation depends on continuous interaction between two germ layers:
Figure 1. Embryological derivatives of the Endoderm and Splanchnic Mesoderm.
🧠 MNEMONIC
"ENDO = EPITHELIUM"
Everything that is internal lining (epithelia and glands) comes from endoderm. Everything that is support (cartilage, muscle,
vessels, pleura) comes from splanchnic mesoderm.
2. REGULATORY MOLECULES
These molecular signals "command" how the respiratory system grows and branches:
Molecule Main Function If It Fails (Pathology)
Retinoic Acid Initial induction of the lung bud; proximal-distal specification. Pulmonary agenesis, severe malformations.
FGF-10 Stimulates bronchial growth and branching. Absence of lung buds.
BMP-4 Regulates branching pattern and epithelial differentiation. Altered branching pattern.
SHH (Sonic Hedgehog) Controls anteroposterior patterning and proliferation. Holoprosencephaly-associated defects.
🧠 MNEMONIC
"FABS make lungs grow"
FGF-10 · Acid retinoic · BMP-4 · SHH — All these molecules work together so the lung forms and "fabulously" grows.
Langman's Medical Embryology - 15th Ed. Ch. 14 Page 1 of 5
, 3. LUNG BUD FORMATION & TRACHEOESOPHAGEAL SEPARATION
📍 Week 4 — The Beginning: A small ventral bud appears on the wall of the foregut. It is known as the respiratory
diverticulum, laryngotracheal groove, or lung bud. Growth direction is caudal (downward).
The Separation Process (Step by Step)
Step 1: Initially there is a single common tube. The foregut is not yet divided. The ventral portion will become the
respiratory system; the dorsal portion will become the esophagus. Step 2: Tracheoesophageal folds appear — two
longitudinal elevations that grow toward each other from the sides of the primitive tube. Step 3: They fuse to form the
tracheoesophageal septum, which definitively separates the digestive tube from the respiratory tube.
Ventral Portion (Respiratory) Dorsal Portion (Digestive)
Larynx, Trachea, Bronchi, Lungs Esophagus
🏥 CLINICAL CORRELATION: TRACHEOESOPHAGEAL FISTULA (TEF)
• Cause: Alteration in the formation/fusion of the tracheoesophageal septum.
• Most common variant: Proximal esophageal atresia + distal fistula (85% of cases).
• Manifestations: Coughing and choking during feeding, cyanosis (due to aspiration), recurrent aspiration pneumonia,
abdominal distension (air passes from respiratory to digestive tract).
• 💡 Exam tip: If a newborn has cyanosis while feeding and recurrent cough → think TEF!
4. DEVELOPMENT OF THE LARYNX
📍 Origin: The entrance of the respiratory diverticulum forms the laryngeal primordium.
• Epithelium: Endoderm.
• Cartilages and muscles: 4th and 6th pharyngeal arches (mesoderm, innervated by vagus nerve - CN X).
Recanalization Process
During development, the laryngeal epithelium proliferates so much that the lumen temporarily disappears (obstruction). Later,
recanalization occurs via vacuolization and cell death to re-establish the airway.
Recanalization Defect Anatomical Description Severity
Laryngeal Webs Residual tissue that was not reabsorbed. Mild to moderate
Laryngeal Stenosis Narrow lumen; partial recanalization. Moderate to severe
Laryngeal Atresia Complete absence of lumen; total failed recanalization. Life-threatening emergency at birth
5. DEVELOPMENT OF THE BRONCHIAL TREE
📍 Initial Division: The lung bud divides into:
• Right main bronchus: 3 lobar bronchi → 10 segmental bronchi.
• Left main bronchus: 2 lobar bronchi → 8-10 segmental bronchi.
🧠 MNEMONIC
"3-2 = RIGHT-LEFT"
The right lung has 3 lobes; the left has 2. This is defined from embryology: 3 right lobar bronchi vs 2 left lobar bronchi.
Dichotomous Branching: Each bronchus divides into two daughter branches. This process repeats throughout gestation. At
birth: ~17 bronchial generations; postnatal: 6 additional generations.
Langman's Medical Embryology - 15th Ed. Ch. 14 Page 2 of 5
DIAPHRAGM & THORACIC CAVITY
High-Yield Study Guide, Clinical Correlation & Review Summary
Based on: Langman's Medical Embryology (Sadler), 15th ed. — Chapter 14 Ideal for: Exam preparation, review, and clinical correlation
1. GENERAL INTRODUCTION
💡 KEY CONCEPT
The respiratory system begins developing at week 4 of embryonic life, long before the baby needs to breathe. It all starts from
a small "bud" of the foregut called the respiratory diverticulum or lung bud.
Although the respiratory apparatus only functions after birth, most of its anatomical structures are already established before
the third trimester. Its formation depends on continuous interaction between two germ layers:
Figure 1. Embryological derivatives of the Endoderm and Splanchnic Mesoderm.
🧠 MNEMONIC
"ENDO = EPITHELIUM"
Everything that is internal lining (epithelia and glands) comes from endoderm. Everything that is support (cartilage, muscle,
vessels, pleura) comes from splanchnic mesoderm.
2. REGULATORY MOLECULES
These molecular signals "command" how the respiratory system grows and branches:
Molecule Main Function If It Fails (Pathology)
Retinoic Acid Initial induction of the lung bud; proximal-distal specification. Pulmonary agenesis, severe malformations.
FGF-10 Stimulates bronchial growth and branching. Absence of lung buds.
BMP-4 Regulates branching pattern and epithelial differentiation. Altered branching pattern.
SHH (Sonic Hedgehog) Controls anteroposterior patterning and proliferation. Holoprosencephaly-associated defects.
🧠 MNEMONIC
"FABS make lungs grow"
FGF-10 · Acid retinoic · BMP-4 · SHH — All these molecules work together so the lung forms and "fabulously" grows.
Langman's Medical Embryology - 15th Ed. Ch. 14 Page 1 of 5
, 3. LUNG BUD FORMATION & TRACHEOESOPHAGEAL SEPARATION
📍 Week 4 — The Beginning: A small ventral bud appears on the wall of the foregut. It is known as the respiratory
diverticulum, laryngotracheal groove, or lung bud. Growth direction is caudal (downward).
The Separation Process (Step by Step)
Step 1: Initially there is a single common tube. The foregut is not yet divided. The ventral portion will become the
respiratory system; the dorsal portion will become the esophagus. Step 2: Tracheoesophageal folds appear — two
longitudinal elevations that grow toward each other from the sides of the primitive tube. Step 3: They fuse to form the
tracheoesophageal septum, which definitively separates the digestive tube from the respiratory tube.
Ventral Portion (Respiratory) Dorsal Portion (Digestive)
Larynx, Trachea, Bronchi, Lungs Esophagus
🏥 CLINICAL CORRELATION: TRACHEOESOPHAGEAL FISTULA (TEF)
• Cause: Alteration in the formation/fusion of the tracheoesophageal septum.
• Most common variant: Proximal esophageal atresia + distal fistula (85% of cases).
• Manifestations: Coughing and choking during feeding, cyanosis (due to aspiration), recurrent aspiration pneumonia,
abdominal distension (air passes from respiratory to digestive tract).
• 💡 Exam tip: If a newborn has cyanosis while feeding and recurrent cough → think TEF!
4. DEVELOPMENT OF THE LARYNX
📍 Origin: The entrance of the respiratory diverticulum forms the laryngeal primordium.
• Epithelium: Endoderm.
• Cartilages and muscles: 4th and 6th pharyngeal arches (mesoderm, innervated by vagus nerve - CN X).
Recanalization Process
During development, the laryngeal epithelium proliferates so much that the lumen temporarily disappears (obstruction). Later,
recanalization occurs via vacuolization and cell death to re-establish the airway.
Recanalization Defect Anatomical Description Severity
Laryngeal Webs Residual tissue that was not reabsorbed. Mild to moderate
Laryngeal Stenosis Narrow lumen; partial recanalization. Moderate to severe
Laryngeal Atresia Complete absence of lumen; total failed recanalization. Life-threatening emergency at birth
5. DEVELOPMENT OF THE BRONCHIAL TREE
📍 Initial Division: The lung bud divides into:
• Right main bronchus: 3 lobar bronchi → 10 segmental bronchi.
• Left main bronchus: 2 lobar bronchi → 8-10 segmental bronchi.
🧠 MNEMONIC
"3-2 = RIGHT-LEFT"
The right lung has 3 lobes; the left has 2. This is defined from embryology: 3 right lobar bronchi vs 2 left lobar bronchi.
Dichotomous Branching: Each bronchus divides into two daughter branches. This process repeats throughout gestation. At
birth: ~17 bronchial generations; postnatal: 6 additional generations.
Langman's Medical Embryology - 15th Ed. Ch. 14 Page 2 of 5
