Describe the early development of the lower respiratory tract, illustrating the formation of the laryngotracheal tube and its derivatives
Week 4 of Development – lung bud forms (laryngotracheal diverticulum). The lung bud appears on day 22 and grows in a ventrocaudal direction through the mesoderm. It is initially connected to the gut tube foregut. Tracheoesophageal ridges separate the lung bud from the foregut. The lung bud now begins to form two bronchial buds.
Tracheoesophageal fistulas – an abnormal communication between the esophagus and trachea. 90% of TEFs are associated with esophageal atresia (congenital absence of a normal lumen). Results from incomplete division of the foregut. Clinical features include episodic gagging and cyanosis after feeding, gastric reflux into the lungs, and accumulation of saliva and mucus in the mouth.
Components of TEF. VATER is less common than VACTERL. The causes are unknown and still debated. Questionable defect in growth and migration of mesoderm during the third week.
Describe development of the larynx and trachea
Development of the larynx – derived from endoderm from cartilages from the 4th and 6th pharyngeal arch – specifically from NCC that surround the mesoderm in the 4th and 6th arch. Mesoderm gives rise to muscles of the larynx. Proliferation of the mesoderm changes the shape of the laryngeal arch and makes it T-shaped.
Development of the trachea – derived from respiratory diverticulum. In Week 4 of Development the trachea is a circular tube of endoderm surrounded by mesoderm. The surrounding mesoderm differentiates into cartilage (C-plates), muscle (Trachealis), and connective tissue by the third month. In addition, respiratory epithelium and glands from endodermal origin are visible.
Describe development of the bronchi
Week 5 of Development – further growth and differentiation results in the formation of main bronchi and secondary bronchi with corresponding to the lobes of the lungs. The right bronchus forms more vertically and has a larger lumen due to the primitive heart formation.
Week 6 of Development – tertiary bronchi formation occurs (10 in the right lung and 8 in the left). Each tertiary bronchi with surrounding mesenchyme is the primordium of a bronchopulmonary segment. The bronchi branch up to 24 times. The branching is regulated by the surrounding splanchnic mesoderm.
List the histological stages of lung development, and discuss the main events that occur during each period
Pseudoglandular (Week 5-16) – ling tissue appears as compound exocrine and no alveoli have developed. Respiration is not possible and premature infant would die.
Canalicular(Week 16-25) – matures craniocaudal. Terminal bronchioles give rise to respiratory bronchioles which give rise to alveolar ducts. Mesodermal tissue becomes highly vascularized. Respiration is possible toward the end of the period, but low chance of fetal survival.
Terminal sac (Week 24-term) – Terminal sacs further develop. Epithelium thins and capillaries come into close proximity with respiratory epithelium. Blood-air barrier formation. Epithelium differentiation of Type I and II pneumocytes. Surfactant production begins at Week 20.
Alveolar (birth to 8 years of age) – growth of the lungs is an increase in the number of respiratory bronchioles. 90% of mature alveoli do not develop until after birth. Approximately 300 million alveoli are present by age 8.
Write short notes on fetal breathing
Fetus practices breathing movements before birth. This trains the respiratory muscles. These movements cause aspiration of amniotic fluid into the lungs. Amniotic fluid is necessary for normal lung development and insufficient (oligohydramnios) can retard lung development and cause pulmonary hypoplasia. At birth the fetus has amniotic fluid in the lungs. The fluid is cleared either by vaginal canal pressure on the fetal thorax, into the pulmonary capillaries, into the lymphatics and pulmonary arteries.
Identify three methods of intra-alveolar fluid clearance
Know the cause and consequences of the selected congenital abnormalities
Unilateral pulmonary agenesis – bronchial bud fails to split. Infant develops respiratory distress within the first year of life. 60% have other congenital abnormalities. Right lung abnormalities occur more than left. Bi-lateral pulmonary agenesis – is fatal.
Pulmonary hypoplasia – incomplete growth of the lung; inadequate size. Often found with bilateral renal agenesis. The severity determines the respiratory compromise.
Congenital bronchial cysts – abnormal dilation of the bronchi. May be filled with air or fluid. Multiple cysts are visible on radiograph.
Write brief notes on the surfactant and respiratory distress syndrome
Fetuses born prematurely (24 Weeks) can survive given intensive care but may suffer from Respiratory Distress Syndrome. Infants born via C-section can initially suffer respiratory distress due to the lack of compression on the thorax by the vagina. Respiratory distress syndrome is labored breathing and lack of surfactant causing possible detachment of alveolar lining. It is treated with glucocorticoids and artificial surfactant therapy.