At first, seven were breathing and had healthy, pink skin. But within 30 minutes, all seven had difficulty breathing and their skin turned blue. The researchers concluded that mice lacking Fgf9 survive embryonic development but die after birth, when they must get oxygen through their lungs instead of through the placenta. Because these mice appeared to die from pulmonary complications, the researchers investigated the role of Fgf9 in the lungs. They found two main differences between the mice lacking Fgf9 and the healthy animals.
Lungs have a complex series of airways that range from large tubes to small branches.
The airways in the mice lacking Fgf9 appeared normal during early embryonic development, when the larger tubes form. But later in development, the tubes failed to branch. Ornitz and colleagues found Fgf9 localized to the surface lining of the lung. They therefore propose that it controls the extent of tissue surrounding the airways and subsequent branch development. Second, the animals lungs did not grow large enough to fill the chest cavity.