Embryology of Respiratory System

1. Development of the
Respiratory System

2. FORMATION OF THE LUNG BUDS
• Embryo is 4 weeks old
• respiratory diverticulum (lung bud) appears as an
outgrowth from the ventral wall of the foregut .
• Dependent upon: retinoic acid (RA) produced by
adjacent mesoderm.
• Epithelium of the internal lining of the larynx,
trachea; bronchi; lungs, is Endoderm
• The cartilaginous; muscular,; connective tissue
components of the trachea; lungs are splanchnic
mesoderm ( that surrounds the foregut)

3. A.Embryo of approximately 25 days' gestation showing
the relation of the respiratory diverticulum to the
heart, stomach, and liver.
B. Sagittal section through the cephalic end of a 5-week
embryo showing the openings of the pharyngeal
pouches and the laryngotracheal orifice.

4. FORMATION OF THE LUNG BUDS -2
• The diverticulum expands caudally, and two
longitudinal ridges, the tracheoesophageal
ridges, separate it from the foregut.
• These ridges fuse to form the tracheoesophageal
septum
• The respiratory primordium communicates with
the pharynx through the laryngeal orifice
• Foregut Divides into:
1) Dorsal portion: Oesophagus.
2) Ventral portion: Trachea & Lung Buds.

5. • A-C. Successive stages in development of the
respiratory diverticulum showing the
tracheoesophageal ridges and formation of the septum,
splitting the foregut into esophagus and trachea with
lung buds.
• D. The ventral portion of the pharynx seen from above
showing the laryngeal orifice and surrounding swelling.

6. TRACHEOESOPHAGEAL FISTULAS
(TEFS)
• Abnormalities in partitioning of the esophagus
and trachea by the tracheoesophageal septum
ensuing Esophageal Atresia with or w/o TEFs.
• Approx. 1/3,000 births
• Blind pouch and the lower segment forming a
fistula with the trachea (90% of cases)
• Isolated Oesophageal Atresia
• H-type TEF without oesophageal Atresia

7. • A. most frequent
abnormality (90% of
cases) occurs with
the upper
oesophagus ending
in a blind pouch and
the lower segment
forming a fistula
with the trachea.
• B. Isolated
oesophageal atresia
(4% of cases).
• C. H-type
tracheoesophageal
fistula (4% of cases).
• D,E. Other
variations (each 1%
of cases).

8. BUT THESE ABNORMALITIES ARE
ASSOCIATED WITH OTHER BIRTH DEFECTS
• Including cardiac abnormalities (33% of cases)
• TEFs are a component of the VACTERL Group:
• Vertebral anomalies
• Anal Atresia
• Cardiac Defects
• Tracheoesophageal Fistula
• Esophageal Atresia,
• Renal Anomalies
• Limb Defects

9. • A, Tracheoesophageal fistula (TEF) in a 17-week male fetus. The upper
esophageal segment ends blindly (pointer).
• B, Contrast radiograph of a newborn infant with TEF. Note the
communication (arrow) between the esophagus (E) and trachea (T).

10. LARYNX
• Internal lining: originates from endoderm.
• Cartilages; muscles originate from mesenchyme
of the 4th & 6th pharyngeal arches.
• Laryngeal orifice changes from a sagittal slit to a
T-shaped opening.
• Caracteristic adult shape of the laryngeal orifice
can be recognized when mesenchyme of the two
arches transforms into the thyroid; cricoid;
arytenoid cartilages.

11. • Laryngeal orifice and surrounding swellings at
successive stages of development:
• A. 6 weeks.
• B. 12 weeks

12. TRACHEA & BRONCHI & LUNGS
• The bronchial buds forms
• 5th week, each of these buds enlarges to form
right and left main bronchi.
• The right forms three secondary bronchi.
• The left forms two.

13. • Stages in development of the trachea and lungs:
• A. 5 weeks.
• B. 6 weeks.
• C. 8 weeks

14. Further Development
• Subsequent growth in caudal and lateral
directions, the lung buds expand into the body
cavity .
• The spaces for the lungs, which are the
pericardioperitoneal canals, are narrow.
• The mesoderm[covers outside of the lung]
develops into the visceral pleura.
• The somatic mesoderm layer[covering the body
wall from the inside] becomes the parietal pleura
• The space between the parietal and visceral
pleura is the pleural cavity .

15. • Expansion of the lung buds into the pericardioperitoneal
canals. At this stage, the canals are in communication with
the peritoneal and pericardial cavities.
• A. Ventral view of lung buds.
• B. Transverse section through the lung buds showing the
pleuropericardial folds that will divide the thoracic portion of
the body cavity into the pleural and pericardial cavities.

16. • pericardioperitoneal
canals separate from
the pericardial and
peritoneal cavities,
• the lungs expand in
the pleural cavities.
• Note the visceral and
parietal pleura and
definitive pleural
cavity. The visceral
pleura extends
between the lobes of
the lungs.

17. Further development
• Secondary bronchi divide repeatedly in a dichotomous
fashion forming tertiary (segmental) bronchi.
• Right lung: forming ten.
• Left: forming eight.
• Creating the bronchopulmonary segments of the adult
lung.
• By the end of the sixth month, approximately 17
generations of subdivisions have formed. Before the
bronchial tree reaches its final shape.
• But, an additional six divisions form during postnatal life.
• For Maturation of the lungs, check your book pg: 205

18. Surfactant
• Important for survival of the premature infant.
• When insufficient, the air-water (blood)
surface membrane tension becomes high,
bringing great risk that alveoli will collapse
during expiration.
• Resulting in respiratory distress syndrome
(RDS)
• Common cause of death in the premature
infant.

19. SUMMARY
• The respiratory system is an outgrowth of the ventral wall of the foregut
• Epithelium of the larynx; trachea; bronchi; alveoli originates is endoderm.
• The cartilaginous; muscular; connective tissue components arise from
mesoderm.
• In the fourth week of development, the tracheoesophageal septum
separates the trachea from the foregut
• Dividing the foregut into the lung bud anteriorly & esophagus posteriorly.
• Contact between the two is maintained through the larynx, which is
formed by tissue of the fourth and sixth pharyngeal arches.
• The lung bud develops into two main bronchi:
• the right forms three secondary bronchi and three lobes;
• the left forms two secondary bronchi and two lobes.
• Faulty partitioning of the foregut by the tracheoesophageal septum
causes oesophageal atresias and tracheoesophageal fistulas

20. Quiz - 1
• Q: A prenatal ultrasound revealed polyhydramnios, and at
birth, the baby had excessive fluids in its mouth. What type of
birth defect might be present, and what is its embryological
origin? Would you examine the child carefully for other birth
defects? Why?
• A: This infant most likely has some type of tracheoesophageal atresia with
or without a tracheoesophageal fistula.
• The baby cannot swallow, and this condition results in polyhydramnios.
• The defect is caused by abnormal partitioning of the trachea and
oesophagus by the tracheoesophageal septum.
• These defects are often associated with other malformations, including a
constellation of vertebral anomalies, anal atresia, cardiac defects, renal
anomalies, and limb defects known as the VACTERL association

21. Quiz - 2
• A baby born at 6 months' gestation is having
trouble breathing. Why?
• Babies born before 7 months of gestation do not produce
sufficient amounts of surfactant to reduce surface tension in
the alveoli to permit normal lung function.
• Consequently, alveoli collapse, resulting in respiratory
distress syndrome. Recent improvements in artificial
surfactants have improved the prognosis for these infants

22. THE END