The control of the development of the pulmonary surfactant system in the saltwater crocodile, Crocodylus porosus

Pulmonary surfactant is a mixture of lipids and proteins that controls the surface tension of the fluid lining the inner lung. Its composition is conserved amongst the vertebrates. Here, we hypothesise that the in ovo administration of glucocorticoids and thyroid hormones during late incubation will accelerate surfactant development in the saltwater crocodile, Crocodylus porosus. We also hypothesise that the increased maturation of the type II cells in response to hormone pre-treatment will result in enhanced responsiveness of the cells to surfactant secretagogues. We sampled embryos at days 60, 68 and 75 of incubation and following hatching. We administered dexamethasone (Dex), triiodothyronine (T3) or a combination of both hormones (Dex + T3), 48 and 24h before each pre-hatching time point. Lavage analysis indicated that the maturation of the phospholipids (PL) in the lungs of embryonic crocodiles occurs rapidly. Only T3 and Dex + T3 increased total PL in lavage at embryonic day 60, but Dex, T3 and Dex + T3 increased PL at day 75. The saturation of the PLs was increased by T3 and Dex + T3 at day 68. Swimming exercise did not increase the amount or alter the saturation of the surfactant phospholipids. Pre-treatment of embryos with Dex, T3 or Dex + T3 changed the secretion profiles of the isolated type II cells. Dex + T3 increased the response of the cells to agonists at days 60 and 68. Therefore, glucocorticoids and thyroid hormones regulate surfactant maturation in the crocodile.