Development of the pulmonary surfactant system in non-mammalian amniotes

Pulmonary surfactant (PS) is a complex mixture of phospholipids, neutral li pids and proteins that lines the inner surface of the lung. Here, it modula tes surface tension thereby increasing lung compliance and preventing the t ransudation of fluid. In mammals, the PS system develops towards the end of gestation, characterized by an increase in the saturation of phospholipids in lung washings and the appearance of surfactant proteins in amniotic flu id. Birth, the transition from in utero to the external environment, is a r apid process. At this time, the PS system is important in opening and clear ing the lung of fluid in order to initiate pulmonary ventilation. In ovipar ous vertebrates, escape from an egg can be a long and exhausting process. T he young commence pulmonary ventilation and hatching by 'pipping' through t he eggshell, where they remain for some time, presumably clearing their lun gs. This paper relates changes in the development of the pulmonary surfacta nt system within the non-mammalian amniotes in response to birth strategy, lung morphology and phylogeny in order to determine the conservatism of thi s developmental process. Total phospholipid (PL), disaturated phospholipid (DSP) and cholesterol (Chol) were quantified from lung washings of embryoni c and hatchling chickens, bearded dragons (oviparous), sleepy lizards (vivi parous), snapping turtles and green sea turtles throughout the final stages of incubation and gestation. In all cases, the pattern of development of t he pulmonary surfactant lipids was consistent with that of mammals. PL and DSP increased throughout the latter stages of development and Chol was diff erentially regulated from the PLs. Maximal secretion of both PL and DSP occ urred at 'pipping' in oviparous reptiles, coincident with the onset of airb reathing. Similarly, the amount of DSP relative to total PL was maximal imm ediately after the initiation of airbreathing in chickens. Th;relative timi ng of the appearance of the lipids differed between groups. In the oviparou s lizard, surfactant lipids were released over a relatively shorter time th an that of the sleepy lizard, turtles, birds and mammals. Thus, despite tem poral differences and vastly different lung morphologies, birth strategies and phylogenies, the overall development and maturation of the PS system is highly conserved amongst the amniotes. (C) 2001 Elsevier Science Inc. All rights reserved.