Patterns of lung aeration in the perinatal period of domestic fowl and Brush Turkey

Abstract

During the transition from chorioallantoic to pulmonary respiration in domestic fowl, the embryonic lungs are aerated by displacement of fluid and lung expansion due to growth. Initial lung aeration may occur initially by convection of fluid from the parabronchial lumina. The total lung volume increases almost twice as fast as the rest of the body in this period, but the air-free volume changes little. Thus the proliferation of parabronchial air capillaries appears to be responsible for aspiration of air to the gas exchange surfaces. Aeration is most rapid during the interval between internal and external pipping, but continues through hatching; at hatching, the total lung organ volume. relative to yolk-free embryo mass, is 17.7 ml · kg⁻¹ and air represents 44% of this volume.

Eggs of the Brush Turkey, a megapode, lack an air cell because they are incubated in mounds where high humidity restricts evaporative water loss. Therefore the embryos neither pip internally nor ventilate the lungs prior to hatching. There is no air in the lungs before the chorioallantois is destroyed and the first breath taken. Hatching is rapid and the lungs become aerated during the first hours after hatching; O₂ consumption rises in parallel with lung aeration. Megapodes may be able to hatch ‘viviparously’ because the abnormally thin shell makes hatching quick and energetically inexpensive. Other birds probably require the development of higher capacity for pulmonary gas exchange before they can successfully hatch from thicker shells.