Recent laboratory data have shown that all-trans-retinoic acid, a derivative of vitamin A, can regenerate alveoli in adult rats with elastase induced emphysema (Massaro, GD and Massaro D. Retinoic Acid Treatment Abrogates Elastase-induced Pulmonary Emphysema in Rats. Nature Med 3:675-677, 1997). Based upon the findings that prior to septation, rats have fibroblasts rich in vitamin A storage granules, high concentrations of cellular retinol binding protein, and lung nuclear retinoic acid binding receptors, all of which diminish after septation and the fact that retinoic acid increases the number of alveoli in rats, the investigators reasoned that retinoic acid plays a key role in septation. Tracheal instillation of elastase into adult rats resulted in an increase in lung volume, a decrease in surface area, and large alveoli as in human emphysema. Intra peritoneal injection of all-trans retinoic acid in the elastase-treated rats for 12 days prior to sacrifice reduced the lung volume and increased the surface area to normal.