Retinoids in lung development.

The "retinoid revolution" has had considerable impact on research activity in the field of lung development. Postnatal lungs are very sensitive to dietary retinol (R) deprivation. The trachea and the bronchopulmonary tree in R-deficient animals show a striking change in morphology. The columnar epithelium undergoes keratinizing metaplasia, which is reversed when R is supplied. The fetal lung develops postnatally to become one of the most complex organs, characterized by 40 different cell types. The fetal lungs can accumulate retinyl esters. Before birth an unknown signal causes the lowering of these esters, which may be the source of retinoic acid (RA). The administration of glucocorticoids speeds up this process, which in turn is followed by acceleration of lung maturation, ending postnatally. It is not clear what role, if any, the cellular retinoid binding proteins play in this phenomenon. Fetal lung branching leading to the development of the alveolar tree is accelerated by RA, and so is the expression of some fetal genes coding for surfactant proteins and the enzymes that produce their lipid components. It is not clear whether RA influences the development of pulmonary epithelium by interacting directly with the epithelial cells or whether its effects require interaction with other cels like mesenchymal cells. Nuclear RA receptors are intimately involved in lung development. Despite the wealth of evidence on the effect of retinoids on lung development, the application of vitamin A to clinical work has been limited to prematurely delivered infants who have immature lungs. Here attempts are made, by improving their R status, to aid in the development and healing of lungs injured by oxygen therapy.