Development of the Respiratory System

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)

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.

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.

• 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.

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

• 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).

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

• 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).

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.

• Laryngeal orifice and surrounding swellings at successive stages of development:

• A. 6 weeks.• B. 12 weeks

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.

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

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 .

• 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.

• 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.

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

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.

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

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

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

THE END