Dr. Ahmed Fathalla Ibrahim

LOWER RESPIRATORY ORGANS LARYNX TRACHEA BRONCHI LUNGS

PHARYNGEAL ARCHES

RESPIRATORY PRIMORDIUM

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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