Development of Respiratory System PDF

Development of Respiratory System The lower respiratory organs (larynx, trachea, bronchi, and lungs) begin to form during the fourth week of development. The primordium of the tracheobronchial tree develops caudal...

Development of Respiratory System The lower respiratory organs (larynx, trachea, bronchi, and lungs) begin to form during the fourth week of development. The primordium of the tracheobronchial tree develops caudal to the fourth pair of pharyngeal pouches. The endoderm lining the laryngotracheal groove gives rise to the pulmonary epithelium and glands of the larynx, trachea, and bronchi. The connective tissue, cartilage, and smooth muscle in these structures develop from the splanchnic mesoderm surrounding the foregut. By the end of the fourth week, the laryngotracheal groove has evaginated (protruded) to form a pouch-like laryngotracheal diverticulum (lung bud), which is located ventral to the caudal part of the foregut. During its separation from the foregut, the laryngotracheal diverticulum forms the trachea and two lateral outpouchings, the primary bronchial buds. The endodermal lining of the laryngotracheal tube distal to the larynx differentiates into the epithelium and glands of the trachea and the pulmonary epithelium. The cartilage, connective tissue, and muscles of the trachea are derived from the splanchnic mesenchyme surrounding the laryngotracheal tube  Development of the trachea Endoderm of the tube gives rise to the epithelium and glands of the trachea and that mesenchyme surrounding the tube forms the connective tissue, muscle, and cartilage.  Development of the larynx The epithelium lining of the larynx is of endodermal origin. The cartilages and muscles of the larynx arise from mesenchyme in the fourth and sixth pairs of pharyngeal arches.  Development of bronchi and lungs The respiratory bud develops at the caudal end of the laryngotracheal diverticulum during the fourth week. The bud soon divides into two outpouchings—the primary bronchial buds. These buds grow laterally into the pericardioperitoneal canals, the primordia of the pleural cavities. Secondary and tertiary bronchial buds soon develop. As the lungs develop, they acquire a layer of visceral pleura from the splanchnic mesenchyme. The thoracic body wall becomes lined by a layer of parietal pleura, derived from the somatic mesoderm. During the fourth week of gestation, the respiratory passages first appear as an endodermal bud arising from the caudal end of the laryngotracheal groove. As the lung bud grows, it becomes covered by mesenchyme that later differentiates into blood vessels and the cartilage, smooth muscle, and connective tissues that support the walls. The endoderm provides the lining epithelium for the entire respiratory tract. The lung bud grows caudally and undergoes successive divisions, the first branches representing the main bronchi. Subsequent divisions provide finer and finer branches of a complex respiratory tree, ultimately ending in small expansions called infundibula. These minute pouches eventually become the definitive alveoli. Successive stages in the development of the bronchial buds, bronchi, and lungs During their development, the lungs undergo considerable change in appearance, and histologically, three phases can be identified. The first or glandular phase extends from the fifth to the seventeenth week and consists mainly of development of the branching air-way passages. The repeated divisions of the endodermal tubes within a bed of mesenchyme give the lung the appearance of a gland. The tubes are lined by undifferentiated columnar cells that have few organelles but are rich in glycogen and contain fat. Nuclei are variable in size and show one or two nucleoli. Mitoses are common. The cells are united at their apices by tight junctions and desmosomes. Even in the earliest stages, the epithelial cells are separated from the mesenchyme by a delicate basement membrane. The cells later become cuboidal, and by the thirteenth week, cilia and goblet cells appear, first in the trachea and then progressively at more distal levels. By sixteen weeks Clara cells are differentiating and endocrine cells can be found. Maturation of the lungs is divided into four stages: pseudoglandular, canalicular, terminal sac, and alveolar. The canalicular phase extends from the seventeenth to the twenty- sixth week and is marked by rapid growth of the tubular elements to provide the finer twigs of the respiratory tree, and by delineation of the respiratory parts. The terminal buds (infundibula) become less spherical and are intimately related to capillary networks. Transformation of tubules to canaliculi is accompanied by flattening of the epithelium and considerable cytodifferentiation. Osmiophilic lamellar bodies associated with production of surfactant appear in some of the epithelial cells, which now have fairly well developed endoplasmic reticulum and plentiful ribosomes. These cells identifiable as type II pneumocytes and surfactant first appear at the 28th week of gestation. During this phase, the cells reduce their height, and their intercellular junctions lie at the lower half of the cell rather than at the apex as in the glandular phase. These low cells are the precursors of type I pneumocytes of saccules and alveoli. The alveolar (terminal sac) phase extends from the twenty- sixth week until term. By this time respiratory bronchioles can be recognized as tubes lined on one side by flattened epithelium and on the other by a ciliated cuboidal epithelium. Branches that had been buds lined by cuboidal epithelium now are clusters of five to six generations of saccules lined by flattened epithelial cells closely associated with capillaries. The septa between capillaries are thick, but gradually thin. At birth, alveoli are small and shallow, and only a few solitary alveoli are present in respiratory bronchioles. Inflation of alveoli occurs after birth with the onset of respiration, first in the hilar region, then in the central portion of the lungs. Only after several hours of breathing are the distal alveoli inflated. New alveoli continue to be formed after birth, and it is not until about the eighth postnatal year that the adult number of alveoli is attained. The transition from dependence on the placenta for gas exchange to autonomous gas exchange requires the following adaptive changes in the lungs: Production of surfactant in the alveolar sacs Transformation of the lungs from secretory into gas exchanging organs Establishment of parallel pulmonary and systemic circulations Pseudoglandular stage, 8 weeks. The “glandular” appearance of the lung. Canalicular stage, 16 weeks. The lumina of the bronchi and terminal bronchioles are enlarging. Canalicular stage, 18 weeks. Terminal sac stage, 24 weeks. The thin-walled terminal sacs (primordial alveoli) that have developed at the ends of the respiratory bronchioles. The number of capillaries have increased and some of them are closely associated with the developing alveoli. Once the pericardioperitoneal canals separate from the pericardial and peritoneal cavities, respectively, the lungs expand in the pleural cavities. Note the visceral and parietal pleura and definitive pleural cavity. The visceral pleura extends between the lobes of the lungs. Histological and functional development of the lung. A. The canalicular period lasts from the 16th to the 26th week. Note the cuboidal cells lining the respiratory bronchioli. B. The terminal sac period begins at the end of the sixth and beginning of the seventh prenatal month. Cuboidal cells become very thin and intimately associated with the endothelium of blood and lymph capillaries or form terminal sacs [primitive alveoli]. Lung tissue in a newborn. Note the thin squamous epithelial cells [also known as alveolar epithelial cells, type I] and surrounding capillaries protruding into mature alveoli. Maturation of the Lungs