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normal lung
Tuesday 31 January 2012
normal lungs
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Embryology
The lung begins as a pouch or groove originating from the primitive foregut in week 3 of embryologic development, when the embryo is 3 mm long.
As the groove enlarges caudally, a tubular lung bud is formed; the upper portion devel-ops into the epithelium of the larynx, and the caudal portion into the epithelium of the tracheobronchial tree.
The embryonic period of lung development begins in week 4 of gestation as the single lung bud from the foregut divides into two primary bronchial buds, the forerunners of the right and left lungs.
During week 5 of gestation, the primary bronchi divide. Each forms three lobar buds that, by the end of week 6 of gestation, divide again to form 10 segmental bronchi on the right and eight to nine on the left.
These potential airways consist of a central core of epithelial cells surrounded by loose primitive mesenchyme that contains widely separated capillaries.
The primitive pulmonary arteries begin to form from the sixth aortic arch, near the end of the embryonic period.
The pulmonary veins begin as evaginations of the left atrium during week 4 of gestation and coalesce with the mesenchymal capillary plexus early in week 5.
The pseudoglandular period (weeks 6 to 16 of gestation) begins with the completion of the proximal airways and encompasses the development of the conducting airway sys-tem to the level of the terminal bronchioles.
The pseudostratified columnar epithelium of the proximal airway displays cilia at week 10 of gestation.
The appearance of cilia extends to the epithelial cells of the peripheral airways by week 13.
Goblet cells appear in the bronchial epithelium at weeks 13 to 14 of gestation, and submucosal glands begin as solid buds originating from the basal layers of the epithelium by weeks 15 to 16.
Smooth muscle cells develop around airways by the end of gestational week 7 and organize to form an identifi able wall to the larger bronchi by week 12.
Lymphatics appear fi rst in the hilar region of the lung in gestational week 8 and in the lung itself by week 10.
Cartilage is fi rst seen in week 4 of gestation and formsdistinct rings along the trachea and main bronchi by the end of week 10.
The acinar or canalicular period extends from weeks 17 to 28 of gestation and is characterized by the development of the basic structure of the gas-exchanging portion of the lung.
Smooth-walled respiratory bronchi-oles, lined by cuboidal epithelium, subdivide into multiple, irregular alveolar ducts.
By week 20 of gestation, the cells lining the ducts develop into type II alveolar lining cells with lamellar and multivesicular bodies associated with surfactant synthesis.
Type I alveolar lining cells then differentiate from type II cells to form the thin air-blood interface required for gas exchange.
As the interstitium thins in the latter portion of the acinar period, the capillaries of the interstitium proliferate and come to lie beneath the type I cells.
Submucosal glands in the trachea and bronchi progress from tubules to mucus-containing acini. By week 24, the cartilage has extended to the most distal bronchi.
The saccular period begins at week 28 of gestation with the development of secondary crests, which are formed as distal airspaces divide into smaller units.
With an accompanying marked decrease in the interstitial tissue and
further increase in the capillary bed, a complex, interwoven capillary network develops in the wall of the saccules.
This provides for effective gas exchange as alveoli begin to develop at the end of the period (32 to 36 weeks of gestation).
The fi nal period of development, the alveolar period, begins in utero at 32 to 36 weeks of gestation and extends until 18 to 24 months after birth.
Alveoli develop as fl ask-shaped structures with thin walls whose double capillary network meshes to appear as a single capillary bed.
At term, type I alveolar cells are extremely thin, resulting in an air-blood barrier of only 0.2 mm including the type I cell, the underlying basement membrane, and the cytoplasm of the capillary endothelial cell.
Lymphatic channels are distrib-uted around pulmonary arteries, bronchi, and bronchiolesand extend along interlobular septa to anastomose with a plexus beneath the pleura. Lymphatic spaces do not exist between alveoli.
The vascular supply of the lung changes appreciably in late gestation and infancy.
The bronchial arterial circula-tion, originating from the aortic arch, supplies the bronchi, bronchioles, and interlobular septa in older childrenand adults; however, the bronchial artery contributes substantially to the circulation of the alveolar ducts and alveoli in the central portions of the lungs through bron-chopulmonary artery anastomoses in utero and in early infancy.
At birth, the surface area of the lung is about 4 m2, with the number of alveoli ranging from 10 to 150 million (mean of 53 million).
Alveoli increase in number after birth, reaching the adult range of 300 to 600 million alveoli by 2 years of age. Thereafter, lung growth occurs in terms of volume and alveolar size, with no further increase in alveolar numbers.
See also
Lungs (lungs)