Development of the fetal lung.

The development of the fetal lung compared to that of other organs is unusual in the degree of its dependence on extrinsic stimuli. When the space available to the growing lung is limited by space-occupying lesions or when the diaphragm is paralysed, lung growth is markedly impaired. The relationship of lung volume to growth may depend on lung distension. Lung hypoplasia associated with experimental procedures causing inhibition or blunting of fetal breathing movements suggests that the distending forces may be generated by these movements. Maturation is less dependent on distension and more dependent on the hormonal environment. Distensibility and stability of the lung in fetal sheep develops rapidly within a few days of birth and correlates strongly with the plasma cortisol concentration. Hypophysectomy retards mutation which is restored by infusing adrenocorticotropin but not cortisol into the fetus. The hormones mainly responsible for controlling the various aspects of maturation probably include cortisol, iodothyronines and catecholamines but the interrelationships of these hormones and the extent of involvement of other hormones is uncertain.