[Recent findings in fetal lung development: structure, surfactant, lung fluid].

A great deal of lung development takes place after birth; new alveoli continue to develop until 8-11 years. However, the differentiation of epithelial cells is characteristic of the fetal lung from 24 weeks of gestation onwards: this is the point at which the surfactant containing type II cells can first be identified. Lung blood flow and the metabolic rate of type II cells increase in parallel rates the last 20% of the gestation period. The timely synthesis of surfactant depends on the availability of the fetal hormones T3, cortisol and prolactin, whereas this synthesis is inhibited by insulin and testosterone. Endogenous surfactant consists of 80% phosphatidylcholine and 10% protein. A sufficient quantity of surfactant is only available at term. Nowadayx, surfactant deficiency can be treated successfully with various exogenous surfactant preparations. Fetal lung liquid contributes about one half to the amniotic fluid. It is partly secreted by an active transport system. Secretion is inhibited by the stimulation of beta-adrenergic receptors in the lung tissue. The epithelial surface of the alveoli is a barrier which limits protein penetration considerably; lung liquid contains minimal amounts of protein. Under pathological conditions (RDS, haemorrhagic lung oedema) the alveolar barrier is disturbed so that plasma protein penetrate into the air spaces and form hyaline membranes.