Development of human lung
The development of human lung arises from the laryngotracheal groove.
The larynx, trachea, bronchi and lungs begin to form during the fourth week of embryonic development. At this time, the respiratory diverticulum (lung bud) appears ventrally to the caudal portion of the foregut. The location of the diverticulum along the gut tube is directed by various signals from the surrounding mesenchyme, including fibroblast growth factors. As the lung bud grows, its distal end enlarges to form the tracheal bud. At the same time the future trachea separates from the foregut through the formation of tracheoesophageal ridges, which fuse to form the tracheoesophageal septum.
The tracheal bud divides into two primary bronchial buds. During the fifth week of development, the bronchial buds enlarge to form right and left main bronchi. These continue to develop into secondary and tertiary bronchi.
The maturation of the lungs occurs in several phases:
|Pseudoglandular period (also known as "glandular period")||weeks 6 to 16||The developing lung resembles an endocrine gland at this time. By the end of this period, all of the major lung elements, except those required for gas exchange (e.g. alveoli), have appeared. Respiration is not possible during this phase, and fetuses born during this period are unable to survive.|
|Canalicular period||weeks 16 to 26||The lumina of the bronchi enlarge and lung tissue becomes highly vascularized during the canalicular period. By week 24, respiratory bronchioles and alveolar ducts have developed from the terminal bronchioles. Respiration is possible towards the end of this period, but few fetuses born during this time will survive.|
|Terminal saccular period||week 26 to birth||The important blood-air barrier is established during the terminal saccular period. Specialized cells of the respiratory epithelium appear at this time, including type I alveolar cells across which gas exchange occurs, and type II alveolar cells which secrete pulmonary surfactant. This surfactant is important in reducing the surface tension at the air-alveolar surface, allowing expansion of the terminal saccules. During this time, the lungs are rock-like and will sink if placed in water but will expand after the first breath, a trait which is used to determine if babies were born alive.|
|Alveolar period||birth to 2 years of age||During this stage the terminal saccules, alveolar ducts, and alveoli increase in number. True alveoli appear as indentations in the saccular wall and septae form to produce divisions in the wall.|
At birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. When the newborn is expelled from the birth canal, its central nervous system reacts to the sudden change in temperature and environment. This triggers it to take the first breath, within about 10 seconds after delivery. The newborn lung is far from being a miniaturized version of the adult lung. It has only about 20,000,000 to 50,000,000 alveoli or 6 to 15 percent of the full adult compliment. Although it was previously thought that alveolar formation could continue to the age of eight years and beyond, it is now accepted that the bulk of alveolar formation is concluded much earlier, probably before that age of two years. The newly formed inter alveolar septa still contain a double capillary network instead of the single one of the adult lungs. This means that the pulmonary capillary bed must be completely reorganized during and after alveolar formation, it has to mature. Only after full microvascular maturation, which is terminated sometime between that ages of two and five years, is the lung development completed and the lung can enter a phase of normal growth.
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