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RESPIRATORY PRIMORDIUM It is formed in the (4th ) week as median outgrowth at the caudal end of the Pharynx (Foregut). • • LARYNGOTRACHEAL GROOVE The respiratory • primordium is defined at (26-27) days as a Laryngotracheal groove caudal to the 4th pair of Pharyngeal Pouches. LARYNGOTRACHEAL DIVERTICULUM By the end of the (4th week) the laryngotracheal groove has Evaginated to form the pouch like LaryngoTracheal Diverticulum. • LARYNGOTRACHEAL DIVERTICULUM The Endoderm of the • Foregut will form: The Epithelial lining and • glands of the larynx, trachea, bronchi and lungs. The surrounding • Splanchnic mesoderm will form: The connective tissue, • cartilage and smooth muscles of these structures. LUNG BUD It is the expanded • distal end of the laryngotracheal diverticulum. Initially, it is in open • communication with the foregut. TRACHEOESOPHAGEAL SEPTUM Two longitudinal • Tracheoesophageal folds (ridges) develop in the diverticulum. They approach each • other and fuse to form the tracheoesophageal septum. TRACHEOESOPHAGEAL SEPTUM The foregut will be divided • into: Dorsal portion • (Esophagus). Ventral portion (trachea • and lung bud). The respiratory primordium • keeps its communication with the pharynx through the Laryngeal Inlet. LARYNX CARTILAGES : • Derived from • proliferation of mesenchyme in the 4th and 6th pharyngeal arches. This mesenchyme is • derived from Neural Crest cells. CARTILAGES The mesenchyme of the two arches transforms into Arytenoid, Thyroid and Cricoid cartilages. • CARTILAGES This proliferation • changes the appearance of the laryngeal orifice (primordial Glottis) from a narrow slit into a T -shaped laryngeal inlet. EPIGLOTTIS It is developed from • the caudal part of the Hypobranchial Eminence. It reaches its adult • form during the first three years after birth. EPITHELIAL LINING It is derived from the Endoderm of the cranial part of the laryngotracheal tube. Its rapid proliferation causes temporary occlusion of the laryngeal inlet. Recanalization • occurs during the 10th week. • • EPITHELIAL LINING The ventricles of the larynx form during this recanalization. They are bounded by two folds of mucous membrane : Vocal Folds (Cords) and Vestibular Folds. • • • MUSCLES They are derived from myoblasts in the 4th and 6th pharyngeal arches. They have nerve • supply from the laryngeal branches of the Vagus nerves that supply these arches. • TRACHEA The Endoderm of the • laryngotracheal tube distal to the larynx gives rise to : Epithelial lining and • Glands. Splanchnic Mesoderm gives rise to: Connective tissue, • Cartilages and Muscles. • BRONCHI Bronchial Buds • develop as two outpouchings of the lung bud. They grow laterally into the pericardio peritoneal canals (primordia of the pleural cavities). • PRIMARY (MAIN) BRONCHI It is the enlarged • connection between the trachea and the bronchial buds. Right Primary • Bronchus: Larger and more vertical • than the left(this embryonic picture is maintained till adult life). SECONDARY (STEM) BRONCHI They develop from • subdivision of the Primary (main ) Bronchi. They supply the lobes of • the lung. On the Right : • They are : Superior, and • Inferior (subdivide into Middle and lower). On the Left : • They are: Superior and • Inferior. TERTIARY (SEGMENTAL) BRONCHI In the Right Lung : (10) in number. In the Left Lung : • (8) or (9) in number. • BronchoPulmonary • segment : It is the segmental • bronchus with its surrounding mass of mesenchyme. • TERTIARY (SEGMENTAL) BRONCHI At (24) weeks : • (1)17 order of • branches are formed. After birth (7) • orders of branches are formed. (2) Respiratory • Bronchioles have developed . PLEURA VISCERAL : • Is derived from the • Splanchnic mesoderm. PARIETAL : • Is derived from the • Somatic mesoderm. It lines the thoracic body wall. DEVELOPMENT OF THE LUNGS It depends on : • (1) Adequate thoracic • space. (2) Fetal breathing • movements before birth. It stimulates lung development by creating pressure gradient between the lungs and the amniotic fluid. MATURATION OF THE LUNGS (3) Adequate volume • of amniotic fluid. Maturation of the • lung passes through four periods. (1)PSEUDO GLANDULAR PERIOD (5-17) WEEKS: • 1. Glandular • appearance of the lung (as an exocrine gland). 2. All major elements • of the lung are formed. 3. No Respiratory • Bronchioles or Alveoli. PSEUDO GLANDULAR PERIOD 4. Respiration is Not possible. 5. Fetuses born • during this period are Unable to survive. • (2) CANALICULAR PERIOD (16-25) WEEKS: • 1. The lumina of • Bronchi and Terminal Bronchioles are larger. 2. Respiratory • Bronchioles : Each terminal bronchiole divides into two or more respiratory bronchioles. CANALICULAR PERIOD 3. Alveolar ducts : The • respiratory bronchioles divide into (3-6) tubular passages. 4. Many Blood vessels • are developing in the mesenchyme surrounding the bronchi and terminal bronchioles. CANALICULAR PERIOD 5. (Some) Terminal sacs : appear at the ends of the respiratory bronchioles. 6. Lung tissue is well vascularised. • • • CANALICULAR PERIOD 7. Respiration is • Possible toward the end of this period. 8. Fetuses born May • survive if given intensive care. They often die • because of immaturity of the respiratory and other systems. (3) TERMINAL SAC PERIOD (24 WEEKS- • BIRTH) 1. Many terminal • sacs develop. 2. Their epithelium • becomes very thin. 3. Capillaries begin • to bulge into these developing alveoli. TERMINAL SAC PERIOD 4. This establishes • intimate contact between the Epithelial and Endothelial cells. 5. The Cuboidal • Endodermal epithelium becomes thin Squamous (Type 1 Alveolar epithelial cells) . TERMINAL SAC PERIOD Type 1 Alveolar cells • (Pneumocytes) permit adequate gas exchange Blood –Air Barrier (for the survival of premature fetuses). (6) PULMONARY SURFACTANT Production: • From Type (11) Alveolar cells, these are rounded secretory epithelial cells scattered among the squamous cells. STRUCTURE : • A complex mixture of • phospholipids. • PULMONARY SURFACTANT TIME of production: • It begins by (20) weeks. • It increases during the • last (2) weeks before birth. FUNCTIONS : • It forms a monomolecular • coat over the internal walls of the terminal sacs. PULMONARY SURFACTANT (a) It lowers the surface • tension at the air- alveolar interface by counteracting the tension forces. (b) It facilitates the expansion of the terminal sacs. Deficiency : causes • Respiratory Distress because of collapse of the primitive alveoli. • (4) ALVEOLAR PERIOD Late Fetal period- • Childhood 1. Each respiratory • bronchiole terminates in a cluster of thin walled terminal sacs (future alveolar ducts). 2. The epithelial lining of the terminal sacs becomes Extremely Thin squamous. • ALVEOLAR PERIOD 3. Loose connective tissue • with large pulmonary lymphatic vessels separates the terminal sacs. 4. The AlveolCapillary • Membrane (Respiratory Membrane) is sufficiently thin to allow gas exchange. AUTONOMOUS GAS EXCHANGE It requires : • (1) Adequate amount of Surfactant. (2) The lung is changed from secretory to gas exchanging organ. (3) Existence of parallel pulmonary and systemic circulations. • • • ALVEOLI 1. A full term newborn • has (50) million alveoli (1/6) of the adult number. 2. On chest radiographs, • the lungs of newborn infants are denser than adult lungs. 3. 95% of alveoli develop • after birth( first 8 years after birth) . ALVEOLI 4. Immature alveoli • have the potential for forming additional alveoli. 5. The primordial alveoli • become mature by increasing in size. MATURATION OF THE LUNGS 1. Cranial segments • mature faster than caudal ones. 2. Growth of the lungs after birth is primarily due to an increase in the Number of respiratory bronchioles and alveoli and not to an increase in the Size of the alveoli. • TRACHEOESOPHAGEAL FISTULA Etiology • Incomplete division of • the cranial part of the foregut into respiratory and esophageal parts. Defective • tracheoesophageal septum causes abnormal communication between the trachea and esophagus. TRACHEOESOPHAGEAL FISTULA It is the most • common anomaly of the lower respiratory tract especially in Male infants. It is usually • associated with other congenital anomalies. TRACHEOESOPHAGEAL FISTULA Most common • variety: Blind ending of the • superior part of the esophagus (esophageal atresia) and joining of the inferior part to the trachea near its bifurcation. TRACHEOESOPHAGEAL FISTULA Manifestations : • Cough and choke during • swallowing because of accumulation of excessive saliva in the mouth and upper respiratory tract. Gastric contents may • reflux from the stomach through the fistula into the trachea and lungs.