Dr. Ahmed Fathalla Ibrahim LOWER RESPIRATORY ORGANS LARYNX TRACHEA BRONCHI LUNGS PHARYNGEAL ARCHES RESPIRATORY PRIMORDIUM 6 middle of 4 th weekBegins to form in the middle of 4 th week Laryngotracheal groove:Laryngotracheal groove: a median outgrowth from the caudal end of the floor (ventral wall) of primordial pharynx, caudal to 4 th pharyngeal pouch Respiratory diverticulum:Respiratory diverticulum: produced by deepening (evagination) of the groove, located ventral to caudal part of foregut RESPIRATORY PRIMORDIUM Tracheal bud:Tracheal bud: it is the enlarged distal end of diverticulum tracheoesophageal folds tracheoesophageal septumLongitudinal tracheoesophageal folds develop in diverticulum, approach each other & fuse to form tracheoesophageal septum dividing cranial part of foregut into: 1.Ventral part: laryngotracheal tube: 1.Ventral part: laryngotracheal tube: primordium of larynx, trachea, bronchi & lungs 2.Dorsal part: pharynx, esophagus primordial laryngeal inletBoth parts maintain communication through primordial laryngeal inlet EMBRYOLOGICAL ORIGIN Endoderm lining laryngotracheal tube:Endoderm lining laryngotracheal tube: epithelium & glands of all respiratory organs Fourth & sixth pharyngeal arch cartilages: epiglottisFourth & sixth pharyngeal arch cartilages: all laryngeal cartilages EXCEPT epiglottis Sixth pharyngeal arch muscles:Sixth pharyngeal arch muscles: intrinsic muscles of larynx Mesenchyme of hypopharyngeal eminence:Mesenchyme of hypopharyngeal eminence: epiglottis Splanchnic mesoderm surrounding foregut:Splanchnic mesoderm surrounding foregut: connective tissue, cartilage, smooth muscles, blood & lymphatic vessels of trachea, bronchi & lungs DEVELOPMENT OF BRONCHI & LUNGS During 5 th week, 2 primary bronchial budsDuring 5 th week, tracheal bud divides into 2 primary bronchial buds Main bronchus: divided into: secondary (lobar), tertiary (segmental) & intrasegmental branchesMain bronchus: formed by primary bronchial bud together with its surrounding splanchnic mesoderm, divided into: secondary (lobar), tertiary (segmental) & intrasegmental branches By 24 weeks, 17 orders of branches respiratory bronchiolesBy 24 weeks, 17 orders of branches have formed & respiratory bronchioles have developed After birth, 24 orders of branches are presentAfter birth, 24 orders of branches are present DEVELOPMENT OF BRONCHI & LUNGS As the lungs expand: visceral layer of pleura (derived from splanchnic mesoderm) 1.They invaginate pleura & acquire a visceral layer of pleura (derived from splanchnic mesoderm) a parietal layer of pleura(derived from somatic mesoderm) 2.They grow caudally into body wall & lie close to heart. The thoracic body wall becomes lined by a parietal layer of pleura (derived from somatic mesoderm) MATURATION OF LUNGS 1.Pseudoglandular period 2.Canalicular period 3.Terminal saccular period 4.Alveolar period MATURATION OF LUNGS PSEUDOGLANDULAR PERIOD (6-12 WEEKS) Lung resembles an exocrine gland bronchial & primordial terminal bronchiolesOnly bronchial & primordial terminal bronchioles are formed Respiration is not possible CANALICULAR PERIOD (16-26 WEEKS) Lumina of bronchi & terminal bronchioles enlarge Respiratory bronchioles & alveolar ductsRespiratory bronchioles & alveolar ducts develop Lung tissue highly vascularLung tissue becomes highly vascular Respiration is possible Fetus born at this period often dies because of immaturity of systems TERMINAL SACCULAR PERIOD (26 BIRTH) terminal saccules (primordial alveoli)Alveolar ducts give rise to terminal saccules (primordial alveoli) Terminal saccules: squamousTerminal saccules: cuboidal epithelium begins to change into squamous Capillariesbulge into alveoliCapillaries begin to bulge into alveoli to establish blood-air barrier that permits adequate gas exchange Terminal saccules are lined with: Type I pneumocytes main type of cells 1. Type I pneumocytes (across which gas exchange occurs): main type of cells 2.Type II pneumocytes 2.Type II pneumocytes (secrete surfactant) TERMINAL SACCULAR PERIOD (26 BIRTH) Surfactant: Role:Role: counteracts surface tension forces & facilitates expansion of alveoli Production: 20 weeks last 2 weeks of pregnancyProduction: begins by 20 weeks & increases during last 2 weeks of pregnancy By weeks:By weeks: surfactant is produced in sufficient amount for survival of prematurely born fetus ALVEOLAR PERIOD (32 WEEKS 8 YEARS) Number of respiratory bronchioles and terminal saccules (primordial alveoli) increasesNumber of respiratory bronchioles and terminal saccules (primordial alveoli) increases Type I pneumocytes of terminal saccules becomes extremely thin squamous so that adjacent capillaries bulge into it Alveolocapillary membraneAlveolocapillary membrane is sufficiently thin to allow gas exchange ALVEOLAR PERIOD (32 WEEKS 8 YEARS) after birthLungs begin to work after birth Transition from dependence on placenta for gas exchange to lungs requires: 1.Production of sufficient amount of surfactant 2.Development of alveolocapillary membrane that is sufficiently thin ALVEOLAR PERIOD (32 WEEKS 8 YEARS) after birthMaturation of alveoli (about 95%) occurs after birth From birth to third year:From birth to third year: Increase in size of lungs is mostly due to increase in number of respiratory bronchioles & primordial alveoli Immature alveoli have the potential for forming additional primordial alveoli Immature alveoli increases in size & become mature TRACHEOESOPHAGEAL FISTULA Most common anomalyMost common anomaly esophageal atresiaAssociated with esophageal atresia in more than 85% of cases polyhydramniosOften associated with polyhydramnios Cause: defective tracheoesophageal septumCause: incomplete fusion of treacheoesophageal folds resulting in a defective tracheoesophageal septum & faulty partitioning of foregut into trachea & esophagus TRACHEOESOPHAGEAL FISTULA Clinical manifesttaions: Infants cough & choke when swallowing (due to accumulation of saliva in upper respiratory tract) Infants regurgitate when swallowing milk (milk fills esophageal pouch & is regurgitated) Pneumonitis RESPIRATORY DISTRESS SYNDROME hyaline membrane diseaseAlso known as hyaline membrane disease prematureAffects about 2% of newborn (especially premature newborn) Cause:Cause: deficiency of surfactant Manifestation:Manifestation: lungs are underinflated, alveoli contain a fluid with a high protein content that resembles a hyaline membrane RESPIRATORY DISTRESS SYNDROME Predisposing factors:Predisposing factors: 1.Prolonged intrauterine asphyxia 2.Deficiency of thyroxine Treatment:Treatment: glucocorticoid OTHER ANOMALIES Stenosis (narrowing) or atresia (obstruction) of trachea:Stenosis (narrowing) or atresia (obstruction) of trachea: due to unequal partitioning of foregut into esophagus & trachea Congenital lung cysts: disturbance in bronchial development leads to dilation of terminal bronchi & formation of cysts filled with fluid or air Agenesis of lung:Agenesis of lung: due to failure of bronchial bud to develop