Embryology of the Respiratory System PDF

Embryology L9 RESPIRATORY SYSTEM  FORMATION OF THE LUNGS BUDS When the embryo is approximately 4 weeks old, the respiratory diverticulum (lung bud) appears as an outgrowth from the ventral wall of the foregut. The appearance and location of the lung bud are dependent upon an increa...

Embryology L9 RESPIRATORY SYSTEM  FORMATION OF THE LUNGS BUDS When the embryo is approximately 4 weeks old, the respiratory diverticulum (lung bud) appears as an outgrowth from the ventral wall of the foregut. The appearance and location of the lung bud are dependent upon an increase in retinoic acid (RA) produced by adjacent mesoderm. Epithelium of the internal lining of the larynx, trachea, and bronchi, as well as that of the lungs, is entirely of endodermal origin. The cartilaginous, muscular, and connective tissue components of the trachea and lungs are derived from splanchnic mesoderm surrounding the foregut. Initially, the lung bud is in open communication with the foregut. When the diverticulum expands caudally, however, two longitudinal ridges, the tracheoesophageal ridges, separate it from the foregut. Subsequently, when these ridges fuse to form the tracheoesophageal septum, the foregut is divided into a dorsal portion, the esophagus, and a ventral portion, the trachea and lung buds. The respiratory primordium maintain its communication with the pharynx through the laryngeal orifice.  LARYNX The internal lining of the larynx originates from endoderm, but the cartilages and muscles originate from mesenchyme of the fourth and sixth pharyngeal arches. As a result of rapid proliferation of this mesenchyme, the laryngeal orifice changes in appearance from a sagittal slit to a T-shaped opening. Subsequently, when mesenchyme of the two arches transforms into the thyroid, cricoid, and arytenoid cartilages. At about the time that the cartilages are formed, the laryngeal epithelium also proliferates rapidly, resulting in a temporary occlusion of the lumen. Musculature of the larynx is derived from mesenchyme of the fourth and sixth pharyngeal arches, all laryngeal muscles are innervated branches of the tenth cranial nerve, ( vagus nerve).  TRACHEA, BRONCHI, AND LUNGS During its separation from the foregut, the lung bud forms the trachea and two bronchial buds. At the beginning of the fifth weeks, each of these buds enlarges to form right and left main bronchi. The right then forms three secondary bronchi, and the left, two, thus foreshadowing the three lobes on the right side and two on the left. With subsequent growth in caudal and lateral direction, the lung buds expand into the body cavity. The spaces for the lungs, the pericardioperitoneal canals, are narrow. They lie on each side of the foregut and are gradually filled by the expanding lung buds. The pleuroperitoneal and pleuropericardial folds separate the pericardioperitoneal canals from the peritoneal and pericardial cavities, and the remaining spaces form the primitive pleural cavities. The mesoderm, which covers the outside of the lung, develops into the visceral pleura. The somatic mesoderm layer, covering the body wall from the inside, becomes the parietal pleura. The space between the parietal and visceral pleura is the pleural cavity.  MATURATION OF THE LUNGS Up to the seventh prenatal month, the bronchioles divide continuously into more and smaller canals. Respiration becomes possible when some of the cells of the cuboidal respiratory bronchioles change into thin, flat cells. These cells are intimately associated with numerous blood and lymph capillaries, and the surrounding spaces are now known as terminal sacs or primitive alveoli. During the seventh month, sufficient numbers of capillaries are present to guarantee adequate gas exchange. During the last 2 months of prenatal life and for several years thereafter, the number of terminal sacs increases steadily. In addition, cells lining the sacs, known as type I alveolar epithelial cells, become thinner, so that surrounding capillaries protrude into the alveolar sacs. This intimate contact between epithelial and endothelial cells makes up the blood-air barrier. Mature alveoli are not present before birth. In addition to endothelial cells and flat alveolar epithelial cells, another cell type develops at the end of the sixth month. These cells, type II alveolar epithelial cells, produce surfactant, a phospholipidrich fluid capable of lowering surface tension at the airalveolar interface.  MATURATION OF THE LUNGS Before birth. The lungs are full of fluid that contains a high chloride concentration, little protein, some mucus from the bronchial gland, and surfactant from the alveolar epithelial cells (type II). The amount of surfactant in the fluid increases, particularly during the last 2 weeks before birth. As concentration of surfactant increase during the 34 th week of gestation, some of this phospholipid enters the amniotic fluid and acts on macrophages in the amniotic cavity. Fetal breathing movements begin before birth and cause aspiration of amniotic fluid. These movements are important for stimulating lung development and conditioning respiratory muscles. When respiration begins at birth, most of the lung fluid is rapidly resorbed by the blood and lymph capillaries, and a small amount is probably expelled via the trachea and bronchi during delivery. When the fluid is resorbed from alveolar sacs, surfactant remains deposited as a thin phospholipid coat on alveolar cell membranes. With air entering alveoli during the first breath, the surfactant coat prevents development of an air-water interface with high surface tension. Without the fatty surfactant layer, the alveoli would collapse during expiration (atelectasis). SUMMARY THANK YOU！