Development of human lung

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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.^[1] 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.

[edit] Phases

The maturation of the lungs occurs in several phases:^[2]

Period	Time	Description
Pseudoglandular period (also known as "glandular period"^[3])	weeks 6 to 16	The developing lung resembles an exocrine 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 lumens 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.^[4]
Alveolar period	birth to 8 years of age	During this stage the terminal saccules, alveolar ducts, and alveoli increase in number.

[edit] References

^ Moore KL, Persaud TVN (2002). The Developing Human: Clinically Oriented Embryology, 7th ed., Saunders. ISBN 0-7216-9412-8.
^ Sadler T (2003). Langman's Medical Embryology, 9th ed., Lippincott Williams & Wilkins. ISBN 0-7817-4310-9.
^ Kyung Won, PhD. Chung (2005). Gross Anatomy (Board Review). Hagerstwon, MD: Lippincott Williams & Wilkins, 156. ISBN 0-7817-5309-0.
^ U.S. V. Nelson. Retrieved on 2007-11-22.