Lung development begins on day 26 of gestation and continues through the toddler years. There are five phases in development. The embryonic period extends from day 26 through 52 of gestation. During this time, the primitive lung bud arises from the foregut, elongates caudally and branches to form the major airways. The pseudoglandular phase encompasses the time from day 52 through the end of 16
weeks gestation. During this phase of development, the process of branching continues and the smaller airways are formed. Fetal breathing movements are identified as early as eight weeks, and by the end of this phase, the two lobes of the left lung and three lobes of the right lung can be identified. Cartilage and smooth muscle cells are present, and about half of the epithelial cell types that will eventually comprise the mature lung are identifiable. The canalicular phase of development extends from weeks 17 through 26. It is during this phase that the airway branching is completed. Interstitial tissue decreases, and prospective gas exchange regions begin to appear. A differentiation process occurs in the cuboidal epithelial cells, and Type I and II pneumocytes appear. Type I pneu-mocytes are the functional exchange unit of the lung, while Type II pneumocytes produce surfactant, a phospholipid substance that serves to decrease surface tension within the Type I cell and prevent it from collapsing. Vascularization of the lung occurs throughout development, and, at this point, the capillaries can be found in close proximity to the pneu-mocytes. The saccular phase of development extends from 24 weeks through 36 weeks gestation. During this time, there is continued thinning of the connective tissue between the potential air spaces, further maturation of the Type II pneumocytes and increased surfactant production. Primitive alveoli, lined by both Type I and Type II pneumocytes, can now be identified as pouches in the walls of the sac-cules and respiratory bronchioles. The alveoli phase of development extends from 36 weeks gestation through about 3 years of age. At birth, there are few alveoli present, but potential airspaces are present as smooth-walled ducts and saccules with thickened septa. Inflation of the saccules occurs at birth. The septa become thin and grow into air spaces, forming partitions within the pouches. Within a few months the infant's alveoli resemble those of the adult, with greatly increased surface area available for gas exchange. Completion of the vascularization process during this time results in single capillary networks associated with each area of gas exchange [105,136]. After birth, minor structural changes continue to occur. The alveolar surfaces become more complex, and the alveoli become more numerous with the in crease in body size. The most rapid phase of pulmonary growth probably occurs within the first years of life, followed at 4-6 years of age by a slow growth phase. The number of alveoli that will be present in adulthood is reached at approximately eight years of age, after which the alveolar surface face enlarges by increases in volume or size [57, 135]. Radiographic measurement of lung diameters demonstrates linear growth during childhood and a spurt at puberty .
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