The Lung Develops in Three Stages, and Pulmonary Surfactant Must Be Present at Birth
By the time of birth, the lung must be ready to assume the gasexchange functions of the placenta. The lung develops in three stages of equivalent duration. Beginning as an outgrowth of the foregut, the lung bud invades the mesenchyme of the thorax and divides into all the major airway branches during the first third of gestation.
Because these primordial airways are lined with a cuboidal epithelium and look like a gland in cross section, this stage of development is known as the Pseudoglandular stage. In the second phase of development, the bronchioles develop, and the lung is invaded by blood vessels (the canalicular stage). In the final stage, or alveolar sac stage, alveolar sacs, and in some species alveoli, develop. The stage of maturity of the lung at birth in general matches the maturity of the fetus. Lambs and piglets have well-developed alveoli, but humans and especially rodents have thicker-walled alveolar sacs. In these latter species, alveoli develop as the animal grows post- natally.Pulmonary surfactant is essential if the lung is to remain inflated after birth (see Chapter 45). Beginning at about midgestation, there is an increase in the synthesis of surfactant components, such as lecithin, within the lung. This increase in lecithin synthesis coincides with the appearance of type Il alveolar cells (the source of surfactant) and with an increase in pulmonary blood flow. Some of this lecithin is secreted into the alveolar lumens and appears in the amniotic fluid, where it can be measured as an indicator of the state of lung maturity. Lung maturity coincides with an increase in serum cortisol levels in the fetus.
Until the time of birth, the vascular resistance of the fetal pulmonary circulation is high, for several reasons. The fetal lung is not inflated; therefore the large vessels are not pulled open by the surrounding alveolar septa. In addition, the hypoxia of the fetus maintains the pulmonary vascular smooth muscle in a state of contraction that narrows the arteries. The first few breaths alleviate both these conditions.
The fetal lung continuously secretes fluid until about 2 days before birth. This fluid, which is rich in chloride and low in bicarbonate and protein, travels up the trachea and through the fetus’s mouth into the amniotic cavity. The fluid in the alveolar spaces and airways is in part squeezed out of the lung as the thorax is compressed during birth. The majority is reabsorbed into lymphatic and blood vessels shortly after birth.
Beginning at about one third of gestation, the fetus makes breathing movements, although it moves little of the viscous fluid to and fro in the airways. These movements apparently prepare the respiratory muscles for their postnatal function.