| Chapter 4: | Stages of Lung Development |   |
Fetal lung development occurs in five distinct phases, each named after the major events associated with them. They include:
- During the embryonic phase, which begins at about 4-6 weeks
of gestational age, the lung begins as a bud from the foregut. That
bud branches into a right and left primary bronchi. The branching continues
forming the proximal airways. Common malformations originating in this
phase are the laryngeal cleft and tracheoesophageal fistula. There are
few structural pulmonary abnormalities that occur during the embryonic
phase because an embryo damaged during this period does not usually
survive.
- During the pseudoglandular phase, which lasts from about week 7 to week 16, the predominant feature involves the formation of conducting airways. At about week, the diaphragm is also formed. During the pseudoglandular phase, the mucus glands, cilia, goblet cells, and cartilage also begin to appear in the conducting airways. Respiratory epithelium begins to differentiate during this phase, so injuries during this phase can result in abnormal bronchial positions, connections, or number of bronchi. If the diaphragm does not form sufficiently to separate the thoracic and abdominal cavities, diaphragmatic hernia may result.
- During the canalicular phase, weeks 17 to 28, the gas exchanging
area of the lung develops. Multivesicular and lamellar bodies associated
with surfactant production begin to appear at about 20 weeks, and differentiation
of Type I and Type II pneumocytes begins during this phase. Pulmonary
capillaries are near alveoli at this point, but not near enough for
effective gas exchange. Alveolar wall thickness is approximately 45
microns at 20 weeks gestation, and it decreases to about 20 microns
at 32 weeks, then eventually to 15 microns at term. In comparison, adult
thickness is about 1 micron.
Surfactant produced during the canalicular phase is immature and easily destroyed. Its chemical composition (thus its functional capability) changes dramatically in the latter stages of gestation. The alveolar surface area at the end of this phase is approximately 1 square meter, and it increases to about 4 square meters at birth. Injuries to the fetus in this phase can cause damage to the gas-exchanging area of the lung, causing a deficiency in alveoli, which may be severe enough to produce pulmonary hypoplasia. The gas-exchanging portion of the lung matures during the final two phases: - During the saccular phase, which lasts from weeks 29 to 35, interstitial tissue space decreases and airspace walls narrow. They become more compact and lateral projections extending from the walls to divide the airspaces into smaller units. At 32 to 36 weeks alveoli are present.
- The alveolar phase, from week 36 on, is devoted to final development and maturation of the alveoli. The number of alveolar at birth has been estimated to be anywhere from 10 to 150 million, and the number increases after birth till completion by the time the neonate is 2 to 3 years old. At this point, the structure of the lung is usually sufficient to survive injuries, but injuries in this phase may interfere with alveolarization and the lung's function.