Development Of The Respiratory System PDF

DEVELOPMENT OF THE RESPIRATORY SYSTEM Assoc.Prof. Özlem Tuğçe Çilingir-Kaya Dept of Histology and Embryology Marmara Uni. School of Medicine DIGESTIVE & RESPIRATORY Initially, the digestive and respiratory systems form in a common body cavity. To separate the lung bud from the gut tube 2 longitudina...

DEVELOPMENT OF THE RESPIRATORY SYSTEM Assoc.Prof. Özlem Tuğçe Çilingir-Kaya Dept of Histology and Embryology Marmara Uni. School of Medicine DIGESTIVE & RESPIRATORY Initially, the digestive and respiratory systems form in a common body cavity. To separate the lung bud from the gut tube 2 longitudinal folds, creating the tracheoesophageal septum. This division splits the dorsal foregut (esophagus) from the ventral lung bud (larynx, trachea and lung). These structures remain in communication superiorly through the laryngeal orifice. Common body cavity 1. Thoracic 2. Abdominal On day 22, the foregut produces a ventral evagination called the respiratory diverticulum or lung bud  The primordium of the lungs. As the lung bud grows, it remains ensheathed in a covering of splanchnopleuric mesoderm, which will give rise to the lung vasculature and the CT, cartilage, and muscle within the bronchi. As the laryngotracheal or respiratory diverticulum elongates, its distal end enlarges to form a globular respiratory bud. This diverticulum soon separates from the primordial pharynx, but it maintains communication with it through the primordial laryngeal inlet. Longitudinal tracheoesophageal folds develop in the laryngotracheal diverticulum, fuse to form a partition  Tracheoesophageal septum This septum divides the cranial part of the foregut into 1. Ventral part (the laryngotracheal tube) 2. Dorsal part (primordium of the oropharynx and esophagus) The opening of the laryngotracheal tube into the pharynx becomes the primordial laryngeal inlet. DEVELOPMENT OF THE LARYNX 4th-5th weeks Epithelium and glands  Endoderm of laryngotracheal tube Cartilages and muscles  4th and 6th pharyngeal arches Laryngeal cartilages  Mesenchyme that is derived from neural crest cells. laryngotracheal tube) Mesenchyme of LTT proliferate  Arytenoid swellings  Grow toward tongue  Convert primordial glottis into Tshaped laryngeal inlet Laryngeal epithelium proliferates  Temporary occlusion of the lumen Recanalization by 10th week THE EPIGLOTTIS Develops from the caudal part of the hypopharyngeal eminence A prominence produced by the proliferation of the mesenchyme in the ventral ends of the 3rd and 4th pharyngeal arches. The rostral part of this eminence forms the posterior 3rd or pharyngeal part of the tongue. Growth of the larynx & epiglottis is rapid during the first 3 years after birth. By this time it has reached its adult form. Laryngeal Atresia Rare birth defect Obstruction of upper fetal airway (congenital high airway obstruction syndrome) Airways dilated distal to atresia The lungs are hyperplastic (causing compression of the heart and great vessels), the diaphragm is either flattened or inverted, and fetal hydrops (accumulation of fluid in two or more compartments) and/or ascites (abdominal fluid) is present. Prenatal USG Laryngeal atresia is a complete blockage of the opening of the larynx, preventing air from reaching the lungs. This blockage is present from birth and is considered a form of congenital high airway obstruction syndrome (CHAOS). Consequences: Due to the blockage, air cannot flow past the larynx to the lungs. This leads to underdeveloped lungs (hyplastic lungs). The underdeveloped lungs take up less space in the chest cavity, causing them to compress the heart and major blood vessels. As the baby tries to breathe against the blockage, the diaphragm (the muscle that separates the chest from the abdomen) becomes flattened or inverted. In some cases, this struggle to breathe can lead to fetal hydrops, which is a buildup of fluid around the body and organs, including the abdomen (ascites). Prenatal Laryngeal WEB Incomplete recanalization of larynx during 10th week Membranous web at the level of vocal folds mostly anteriorly. Partial obstruction of the airway Difficult to breathe normally Development of the esophagus, stomach, trachea, and lungs from the foregut region are tightly linked. THE TRACHEA Endoderm of LTT distal to larynx  Epithelium and glands of trachea & pulmonary epithelium Splanchnic mesenchyme around LTT  Cartilage, CT & muscles A, 4 weeks. B, 10 weeks. C, 11 weeks (drawing of micrograph in D). Note that the endoderm of the tube gives rise to the epithelium and the glands of the trachea and that the mesenchyme surrounding the tube forms the connective tissue, muscle, and cartilage (drawing of the micrograph shown in D). D, Photomicrograph of a transverse section of the developing trachea at 12 weeks. Tracheoesophageal Fistula (TEF) Most common anomaly 1:3000-4500 live births Mostly males >85%: Esophageal atresia Mostlyuperior part A, Esophageal atresia is associated with TEF in more than 85% of cases. B, Fistula between the trachea and esophagus; this type of birth defect accounts for approximately 4% of cases. Incomplete division of foregut into respiratory and esophageal parts during 4th w Incomplete fusion of tracheoesophageal folds results in defective tracheoesophageal septum & abnormal communication between trachea & esophagus Tracheal Stenosis and Atresia Uncommon narrowing (stenosis) & obstruction (atresia) Usually associated with TEF Unequal partitioning of foregut into esophagus and trachea Sometimes a web of tissue obstructing airflow (incomplete atresia) C, Atresia of the proximal esophagus ending in a tracheoesophageal fistula with the distal esophagus having a blind pouch. Air cannot enter the distal esophagus and stomach. D, Atresia of the proximal segment of the esophagus with fistulas between the trachea and both the proximal and distal segments of the esophagus. Air can enter the distal esophagus and stomach. Tracheal Diverticulum Extremely rare Blind, bronchus-like projection from trachea Outgrowth may terminate in normal-appearing lung tissue, forming a tracheal lobe of the lung THE BRONCHI & LUNGS The lung is a composite of endodermal and mesodermal tissues. Lung bud  Bronchial buds  Grow into Pericardioperitoneal canals  Bronchi & their ramification in lungs Early in 5th week: Primordium of the main bronchus Embryonic R main bronchus larger & more vertical Foreign body The first round of branching is highly stereotypical and yields three secondary bronchial buds on the R and two on the L. The secondary bronchial buds give rise to the lung lobes: 3 in the R and 2 in the L lung. During the 6th week, yields 10 tertiary bronchial buds on both sides  Bronchopulmonary segments of the mature lung Main bronchi  Secondary or stem bronchi Tertiary or segmental bronchi  10 in R lung & 8-9 in L, 7th week Surrounding mesenchyme also divides 10 day-old mouse embryo Approx. human age: 6 weeks Transverse cut Each segmental bronchus with its mesenchyme: Primordium of bronchopulmonary segment 24 weeks: 17 orders of branches, respiratory bronchioles Additional 7 orders of airways develop after birth Surrounding mesenchyme  Cartilaginous plates, bronchial smooth musculature & CT and pulmonary CT & capillaries Splanchnic mesoderm  Visceral pleura Somatic mesoderm  Parietal pleura 5th Week 6th Week THE LUNGS 1. 2. 3. 4. 5. Embryonic Pseudoglandular Canalicular Saccular Alveolar Lung development is not completely synchronous. More rapid in cranial regions than in caudal areas. The 5 developmental stages can be identified by histology, and each stage of lung development is marked by a major developmental milestone. Take a Break! Embryonic Stage Respiratory diverticulum arises as a ventral outpouching of foregut endoderm and undergoes three initial rounds of branching, producing the primordia successively of the two lungs, the lung lobes, and the bronchopulmonary segments. The stem of the diverticulum forms the trachea and larynx. Embryonic stage: normal lung at 7 weeks of gestation. Mesenchyme shows some condensation around endodermal tubules (H&E, 20×) Pseudoglandular Period (5-17ws) The respiratory tree undergoes fourteen more generations of branching, resulting in the formation of terminal bronchioles Like exocrine gland All major elements have formed by the 17th week, except those involved with gas exchange Respiration is not possible; unable to survive Canalicular Period (16-25 ws) Lumina of bronchi & terminal bronchioles become larger 24 weeks: Each terminal bronchiole  2 or more respiratory bronchioles Respiratory bronchiole  3-6 alveolar ducts Respiratory vasculature begins to develop. Blood vessels come into close apposition with the lung epithelium. The lung epithelium also begins to differentiate into specialized cell types (ciliated, secretory, and neuroendocrine cells proximally and precursors of the alveolar type II and I cells distally) Respiration is possible toward the end Terminal Sac Period (24-36 ws) Respiratory bronchioles  Terminal sacs (primitive alveoli) Many more terminal sacs with very thin epithelium Development of lymphatic capillaries Capillaries begin to bulge into developing alveoli Contact between epithelial & endothelial cells: Air Blood Barrier 24 wks: Terminal sacs lined mainly by squamous epithelial cells (type I) Rounded secretory type II cells Terminal sacs continue to be produced until well into childhood. SURFACTANT Production begins in 20 wks, and increases during the last 2 wks 26-28 wks: Sufficient terminal sacs and surfactant for survival