Development of the pulmonary surfactant system in two oviparous vertebrates

Reactive oxygen species as mediators of oxygen signaling during fetal-to-neonatal circulatory transition

Reactive oxygen species (ROS) are frequently seen as pathological agents of oxidative stress. However, ROS are not always deleterious and can also act as cell signaling molecules. Vascular oxygen sensing and signaling during fetal-to-neonatal circulatory transition is a remarkable example of the physiological regulatory actions of ROS. The fetal relative hypoxic environment induces hypoxic pulmonary vasoconstriction (HPV) and ductus arteriosus (DA) relaxation favoring the presence of high pulmonary vascular resistance and right-to-left ductal shunt. At birth, the increase in oxygen tension causes relaxation of pulmonary arteries (PAs) and normoxic DA vasoconstriction (NDAV), thus diverting blood flow to the lungs. Although the response to changes in oxygen tension is diametrically opposite, the mechanisms responsible for HPV and NDAV appear to be the result of a similar interaction between triggering and modulating factors that lead to an increase in cytosolic Ca²⁺ concentration and Ca²⁺ sensitization of the contractile apparatus. Growing evidence points to an increase in ROS (mitochondria- and/or NADPH-derived superoxide and/or H₂O₂), leading to inhibition of voltage-gated K⁺ channels, membrane depolarization, and activation of voltage-gated L-type Ca²⁺ channels as critical events in the signaling pathway of both HPV and NDAV. Several groups of investigators have completed this pathway adding other elements such as neutral sphingomyelinase-derived ceramide, the sarcoplasmic/endoplasmic reticulum (through ryanodine and inositol 1,4,5-trisphosphate receptors), Rho kinase-mediated Ca²⁺ sensitization, or transient receptor potential channels. The present review focus on the role of ROS as mediators of the homeostatic oxygen sensing system during fetal and neonatal life not only in the PAs and DA but also in systemic arteries.

Developmental regulation of chicken surfactant protein A and its localization in lung

Surfactant Protein A (SP-A) is a collagenous C-type lectin (collectin) that plays an important role in the early stage of the host immune response. In chicken, SP-A (cSP-A) is expressed as a 26 kDa glycosylated protein in the lung. Using immunohistochemistry, cSP-A protein was detected mainly in the lung lining fluid covering the parabronchial epithelia. Specific cSP-A producing epithelial cells, resembling mammalian type II cells, were identified in the parabronchi. Gene expression of cSP-A markedly increased from embryonic day 14 onwards until the time of hatch, comparable to the SP-A homologue chicken lung lectin, while mannan binding lectin and collectins CL-L1 and CL-K1 only showed slightly changed expression during development. cSP-A protein could be detected as early as ED 18 in lung tissue using Western blotting, and expression increased steadily until day 28 post-hatch. Our observations are a first step towards understanding the role of this protein in vivo.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Plasticity in the timing of physiological development: Physiological heterokairy — What is it, how frequent is it, and does it matter?

The study of developmental sequences of physiological traits could be an important way of placing comparative developmental physiology (CDP) within the research agenda being forged by work on developmental plasticity. Here we focus on the concept of heterokairy defined by Spicer & Burggren in 2003 as changes in the timing of physiological development in an individual. The role of this concept in the future of the CDP is discussed. First we provide an historical perspective of the ideas that have led to the investigation of sequences in CDP. This is followed by a re-examination and clarification of the definition of physiological heterokairy before empirical case studies that (explicitly or implicitly) demonstrate physiological heterokairy are reviewed. We suggest that physiological heterokairy can be demonstrated through a wide range of invertebrate and vertebrate examples. However, care must be taken when inferring that heterokairy as a pattern is always the result of heterokairic processes as there is evidence that physiological heterokairy could result from the altered timing of both homologous or analogous physiological mechanisms. We conclude by discussing the potential link between heterokairy and heterochrony and suggest that the investigation of this link should be a major goal for workers in both CDP and developmental plasticity.

The safety and immunogenicity of an in ovo vaccine against Newcastle disease virus differ between two lines of chicken

Newcastle disease virus is a major threat to poultry and in ovo vaccines are needed. A live in ovo vaccine for Newcastle disease virus, which was licensed but not marketed, was unsafe. It killed 32% of line 0 chicks and 10% of vaccine Lohmann (VALO) chicks using the maximum recommended dose that infected about 40% of the embryos. VALO's made more antibody than line 0's whether infected in ovo or by contact. The vaccine interrupted the massive development of the air capillaries between injection and hatch 3 days later. Cytokines, delivered as DNA in plasmids, did not function as adjuvants. IFN-gamma prevented infection. IL-4 or IL-18 had little or no effect. Line 0 chicks that had been infected by contact were protected and so the unsafe in ovo vaccination of a minority could protect the majority.

Early development of chick embryo respiratory nervous system: an immunohistochemical study

The extrinsic and intrinsic respiratory nervous systems receive specific contributions from the vagal and sympathetic components. Using specific markers for vagal and sympathetic structures, we studied the distribution patterns of immunoreactivity to galanin (GAL), pituitary adenylate cyclase-activating polypeptide-27 (PACAP) and the tachykinin substance P in extrinsic and intrinsic nerve of chick embryo respiratory system, during development from the very early age to hatching. All peptides studied appeared in the intrinsic and extrinsic nervous systems early. We found substance P in both the vagal and sympathetic systems, PACAP in vagal components alone and GAL mainly in the sympathetic system. The intrinsic nervous system showed high immunoreactivity for all peptides studied. These data accord with the well known early trophic functions that peptides have on the development of nervous networks and modulatory activity on the intrinsic nervous system. The GAL again proves to be the main peptide in chick embryo sympathetic respiratory system.

Dipalmitoylphosphatidylcholine is not the major surfactant phospholipid species in all mammals

Pulmonary surfactant, a complex mixture of lipids and proteins, lowers the surface tension in terminal air spaces and is crucial for lung function. Within an animal species, surfactant composition can be influenced by development, disease, respiratory rate, and/or body temperature. Here, we analyzed the composition of surfactant in three heterothermic mammals (dunnart, bat, squirrel), displaying different torpor patterns, to determine: 1) whether increases in surfactant cholesterol (Chol) and phospholipid (PL) saturation occur during long-term torpor in squirrels, as in bats and dunnarts; 2) whether surfactant proteins change during torpor; and 3) whether PL molecular species (molsp) composition is altered. In addition, we analyzed the molsp composition of a further nine mammals (including placental/marsupial and hetero-/homeothermic contrasts) to determine whether phylogeny or thermal behavior determines molsp composition in mammals. We discovered that like bats and dunnarts, surfactant Chol increases during torpor in squirrels. However, changes in PL saturation during torpor may not be universal. Torpor was accompanied by a decrease in surfactant protein A in dunnarts and squirrels, but not in bats, whereas surfactant protein B did not change in any species. Phosphatidylcholine (PC)16:0/16:0 is highly variable between mammals and is not the major PL in the wombat, dunnart, shrew, or Tasmanian devil. An inverse relationship exists between PC16:0/16:0 and two of the major fluidizing components, PC16:0/16:1 and PC16:0/14:0. The PL molsp profile of an animal species is not determined by phylogeny or thermal behavior. We conclude that there is no single PL molsp composition that functions optimally in all mammals; rather, surfactant from each animal is unique and tailored to the biology of that animal.

The development of the pulmonary surfactant system in California sea lions

Pulmonary surfactant has previously been shown to change during development, both in composition and function. Adult pinnipeds, unlike adult terrestrial mammals, have an altered lung physiology to cope with the high pressures associated with deep diving. Here, we investigated how surfactant composition and function develop in California sea lions (Zalophus californianus). Phosphatidylinositol was the major anionic phospholipid in the newborn, whereas phosphatidylglycerol was increased in the adult. This increase in phosphatidylglycerol occurred at the expense of phosphatidylinositol and phosphatidylserine. There was a shift from long chain and polyunsaturated phospholipid molecular species in the newborn to shorter chain and mono- and disaturated molecular species in the adult. Cholesterol and SP-B concentrations were also higher in the adult. Adult surfactant could reach a lower equilibrium surface tension, but newborn surfactant could reach a lower minimum surface tension. The composition and function of surfactant from newborn California sea lions suggest that this age group is similar to terrestrial newborn mammals, whereas the adult has a "diving mammal" surfactant that can aid the lung during deep dives. The onset of diving is probably a trigger for surfactant development in these animals.

Hypoxic control of the development of the surfactant system in the chicken: evidence for physiological heterokairy

The surfactant system, a complex mixture of lipids and proteins, controls surface tension in the lung and is crucial for the first breath at birth, and thereafter. Heterokairy is defined as plasticity of a developmental process within an individual. Here, we provide experimental evidence for the concept of heterokairy, as hypoxia induces a change in the onset and rate of development of surfactant, probably via endogenous glucocorticoids, to produce individuals capable of surviving early hatching. Chicken eggs were incubated under normoxic (21% O(2)) conditions throughout or under hypoxic (17% O(2)) conditions from day 10 of incubation. Embryos were sampled at days 16, 18, and 20 and also 24 h after hatching. In a second experiment, dexamethasone (Dex), tri-iodothyronine (T(3)), or a combination (Dex + T(3)) was administered 24 and 48 h before each time point. Both hypoxia and Dex accelerated maturation of the surfactant lipids by increasing total phospholipid (PL), disaturated phospholipid (DSP), and cholesterol (Chol) in lavage at days 16 and 18. Maturation of surfactant lipid composition was accelerated, with day 16 %DSP/PL, Chol/DSP, and Chol/PL resembling the ratios of day 20 control animals. The effect of Dex + T(3) was similar to that of Dex alone. Hypoxia increased plasma corticosterone levels at day 16, while plasma T(3) levels were not affected. Hence, exposure to hypoxia during critical developmental windows accelerates surfactant maturation, probably by increasing corticosterone production. This internal modulation of the developmental response to an external stimulus is a demonstration of physiological heterokairy.

Pulmonary surfactant: the key to the evolution of air breathing

Pulmonary surfactant controls the surface tension at the air-liquid interface within the lung. This system had a single evolutionary origin that predates the evolution of the vertebrates and lungs. The lipid composition of surfactant has been subjected to evolutionary selection pressures, particularly temperature, throughout the evolution of the vertebrates.

The role of extrinsic and intrinsic factors in the evolution of the control of pulmonary surfactant maturation during development in the amniotes

Pulmonary surfactant is a mixture of lipids and proteins that is secreted by alveolar Type II cells. It reduces alveolar surface tension and hence the work of breathing. Despite the tremendous diversity of lung structures amongst the vertebrates, the composition of surfactant is highly conserved. Conserved elements of the surfactant system amongst distantly related species are likely to be crucial factors for successful lung development. Understanding the mechanisms by which the surfactant system becomes operational in animals with dramatically different birthing strategies and in distantly related species will provide important information about the role of the surfactant system in the commencement of air breathing and the processes regulating surfactant maturation and secretion. In mammals, the embryonic maturation of the surfactant system is controlled by a host of factors, including glucocorticoids, thyroid hormones, and autonomic neurotransmitters. Here we review the mechanisms controlling the maturation of surfactant production, including birthing strategy, phylogeny, lung structure, and posthatching environment. Using four species of egg-laying amniote (chicken, dragon lizard, sea turtle, and crocodile) previously described in detail and the large amount of information available for mammals, we examine the hypothesis that the control of surfactant production is dependent on glucocorticoids (dexamethasone [Dex]), thyroid hormones (T3), and autonomic neurotransmitters (epinephrine and carbachol). We also examine whether the overall intrinsic pattern of the control of surfactant maturation is conserved throughout the vertebrate radiation and then how the environment (extrinsic factors) may account for the observed differences in the patterns of development. We also discuss the utility of a coculture system of embryonic Type II cells and fibroblasts to determine the evolutionary pattern behind the control of surfactant and to demonstrate that the surfactant system matures under multihormonal control. We demonstrate that Dex and T3 are stimulators of surfactant production during embryonic development, but they lose their efficacy closer to hatching or birth. Epinephrine stimulates surfactant secretion beyond 75% of development and also after hatching or birth. Carbachol stimulates surfactant secretion in the bearded dragon and saltwater crocodile but not in the sea turtle, chicken, or mammals. It is likely that the differences in control of surfactant development are likely to be primarily related to metabolic activity and the duration of incubation (i.e., the "speed" of development). Moreover, the hormones examined appear important in promoting development and therefore appear conserved within the amniotes. However, the autonomic neurotransmitters induced different responses in different species. Hence, some factors are crucial for the proper maturation of the surfactant system, whereas others vary throughout evolution without being detrimental to the overall function of the system.

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 among the vertebrates. Here we hypothesize 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 hypothesize that the increased maturation of the type II cells in response to hormone pretreatment will result in enhanced responsiveness of the cells to surfactant secretagogues. We sampled embryos at days 60, 68, and 75 of incubation and after hatching. We administered dexamethasone (Dex), 3,5,3'-triiodothyronine (T(3)), or a combination of both hormones (Dex + T(3)), 48 and 24 h before each prehatching time point. Lavage analysis indicated that the maturation of the phospholipids (PL) in the lungs of embryonic crocodiles occurs rapidly. Only T(3) and Dex + T(3) increased total PL in lavage at embryonic day 60, but Dex, T(3), and Dex + T(3) increased PL at day 75. The saturation of the PLs was increased by T(3) and Dex + T(3) at day 68. Swimming exercise did not increase the amount or alter the saturation of the surfactant PLs. Pretreatment of embryos with Dex, T(3), or Dex + T(3) changed the secretion profiles of the isolated type II cells. Dex + T(3) increased the response of the cells to agonists at days 60 and 68. Therefore, glucocorticoids and thyroid hormones regulate surfactant maturation in the crocodile.

Ontogeny of the pulmonary surfactant and antioxidant enzyme systems in the viviparous lizard, Tiliqua rugosa

The antioxidant enzyme (AOE) system protects the lung from oxidative damage. The pulmonary surfactant (PS) system lowers the interfacial pressure within the lung, improving lung compliance and aiding lung clearance. In mammals, the AOE and PS systems develop in tandem during the final 10%-20% of gestation. Here, we investigated the development of these systems in the viviparous skink, Tiliqua rugosa. The content of total phospholipid (PL), disaturated phospholipid (DSP), and cholesterol (Chol) increased in lung washings from foetal lizards with advancing gestational age. Similarly, the relative saturation of the PLs increased throughout gestation, with mid-stage 40 foetuses having a DSP/PL equivalent to newborns and adults. Maternal lizards had significantly less total PL, DSP, and Chol than nongravid and newborn lizards; however, the relative composition did not differ from nongravid animals. This presumably results from compression of the lungs under the bulk of the developing foetus. The Chol/PL and Chol/DSP ratios declined early in development such that mid-stage 40 embryos had comparable ratios to both newborns and adults. Thus, it appears that the PS system matures in a similar manner in skinks and in mammals. However, the composition of surfactant is complete some weeks before parturition, probably to enable improved survivorship of the precocial young in the event of premature birth. Unlike the surfactant lipids, the AOEs, catalase, superoxide dismutase, and glutathione peroxidase did not differ appreciably throughout gestation. It appears therefore that like the surfactant lipids the AOE system is in readiness for air breathing throughout the latter stages of gestation, possibly in preparation for premature birth. Unlike mammals, the PS and AOE systems develop independently from one another.

Vascular reactivity in intrapulmonary arteries of chicken embryos during transition to ex ovo life

The present study aimed to characterize pulmonary vascular reactivity in the chicken embryo from the last stage of prenatal development and throughout the perinatal period. Isolated intrapulmonary arteries from non-internally pipped embryos at 19 days of incubation and from internally and externally pipped embryos at 21 days of incubation were studied. Arterial diameter and contractile responses to KCl, endothelin-1, and U-46619 increased with incubation but were unaffected by external pipping. In contrast, the contractions induced by norepinephrine, phenylephrine, and electric field stimulation decreased with development. No developmental changes were observed in endothelium-dependent [acetylcholine (ACh) and cyclopiazonic acid] or endothelium-independent [sodium nitroprusside (SNP)] relaxation. These relaxations were abolished by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Endothelium-dependent relaxation was unaffected by blockade of cyclooxygenase or heme oxygenase but was significantly reduced by nitric oxide (NO) synthase inhibitors. Reduction of O2 concentration from 95 to 5% produced a marked reduction in ACh and SNP-induced relaxations. Chicken embryo pulmonary arteries show a marked endothelium-dependent relaxation that is unaffected by transition to ex ovo life. Endothelium-derived NO seems to be the main mediator responsible for this relaxation.

The pulmonary surfactant system matures upon pipping in the freshwater turtle Chelydra serpentina

Pulmonary surfactant (PS), a mixture of phospholipids (PL), neutral lipids and surfactant proteins (SP), lowers surface tension within the lung, which increases lung compliance and improves the removal of fluid at birth. Here, we have examined the expression of thyroid transcription factor-1 (TTF-1) and the surfactant protein SP-B, and also the composition of pulmonary surfactant lipids in the developing lung of the turtle Chelydra serpentina. Lavage and lung tissue were collected from late embryonic, pipped and hatchling turtles. TTF-1, a regulator of gene expression of surfactant proteins and cell differentiation in mammals, was detected using immunohistochemistry in epithelia of the gas-exchange area and conducting airways during late development. Expression declined in hatchlings. SP-B was detected in subsets of cells within the respiratory epithelium at all stages sampled. The same cell types also stained for TTF-1. Turtle surfactant lipids matured toward the end of incubation. Maximal secretion of both total phospholipids and disaturated phospholipid (DSP) occurred at the time of pipping, coincident with the onset of breathing. The DSP/PL ratio increased after pipping, whereas cholesterol levels (Chol) increased prior to pipping. This resulted in a decrease in the Chol/PL and Chol/DSP ratios after pipping. Thus, TTF-1 and SP-B appear to be highly conserved within the vertebrates. Maturation of surfactant phospholipid content occurred with the commencement of pulmonary ventilation.

Antioxidant enzymes in the developing lungs of egg-laying and metamorphosing vertebrates

The activities of the pulmonary antioxidant enzymes (AOE), superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase, increase in the final 10–20 % of gestation in the mammalian lung, to protect the lung from attack by increasing levels of reactive oxygen species at birth. Whether the increase occurs as a normal ‘preparation for birth’, i.e. by a genetically determined mechanism, or in response to increased levels of oxygen, i.e. in response to the environment, is not completely understood. We examined the activities of catalase, SOD and GPx in the developing lungs of two oviparous vertebrate species, the chicken (Gallus gallus) and an agamid lizard (Pogona vitticeps), and in a metamorphosing vertebrate, the anuran Limnodynastes terraereginae. During in ovo development embryos come into contact with higher levels of environmental oxygen, and at a much earlier stage of development, compared with the intrauterine development of mammals. Furthermore, in metamorphosing frogs, the lungs are inflated at an early stage to aid in buoyancy, although the gas-exchange function only develops much later upon final metamorphosis. Here, we hypothesise that the activity of the AOE will be elevated relatively much earlier during development in both oviparous and metamorphosing vertebrates. We also examined the effect of mild hypoxia (17 % oxygen) on the development of the pulmonary AOE in the chicken, to test the hypothesis that these enzymes are responsive to environmental oxygen. In the normoxic lung of both Gallus gallus and Pogona vitticeps, catalase and GPx activities were significantly increased in late incubation, whereas SOD activity decreased in late incubation. Catalase and SOD activities were virtually identical in hypoxic and normoxic embryos of the chicken, but GPx activity was significantly affected by hypoxia. In the developing frog, the activities of all enzymes were high at stage 30, demonstrating that the system is active before the lung displays any significant gas-exchange function. SOD and GPx activity did not increase further with development. Catalase activity increased after stage 40, presumably correlating with an increase in air-breathing. In summary, catalase expression in the two oviparous vertebrates appears to be completely under genetic control as the activity of this enzyme does not change in response to changes in oxygen tension. However, in tadpoles, catalase may be responsive to environmental oxygen. SOD also appears to follow a largely genetically determined program in all species. Under normoxic conditions, GPx appears to follow a genetically determined developmental pattern, but this enzyme demonstrated the largest capacity to respond to environmental oxygen fluctuations. In conclusion, it appears that the AOE are differentially regulated. Furthermore, the AOE in the different species appear to have evolved different levels of dependency on environmental variables. Finally, the late developmental increase in AOE activity seen in mammals is not as pronounced in oviparous and metamorphosing vertebrates.

Avian air sac and plasma proteins that bind surface polysaccharides of Escherichia coli O2

Some serovars of Escherichia coli, mainly O2 and O78, are responsible for air sac and systemic infections in farm-raised turkeys (Meleagris gallopavo) and chickens (Gallus gallus). We looked in air sac surface fluid from young turkeys to identify proteins that bind surface polysaccharides of pathogenic respiratory E. coli O2. Turkey air sac surface fluid was subjected to affinity chromatography on Toyopearl AF-Epoxy-650M, coupled with either lipopolysaccharide (LPS) or lipid-free polysaccharide (LFP) purified from an avian pathogenic E. coli O2 isolate. A multimeric protein termed lipid-free polysaccharide binding protein-40 (LFPBP-40) composed of six covalently associated subunits of approximately 40 kDa was isolated by elution from LFP by EDTA or L-rhamnose. An analogous protein in air sac fluid proteins bound to intact E. coli O2 and eluted with L-rhamnose or N-acetylglucosamine (GlcNAc). The N-terminal amino acid sequence of LFPBP-40 DINGGGATLPQHLYLTPDV was related to the N-terminus of fragment 3 of a partially characterized human protein possessing T cell stimulation activity in synovial membrane of rheumatoid arthritis patients. However, endogenous amino acid sequences were unrelated to other known proteins. LFPBP-40 was immunoreactively distinct from pulmonary collectins and ficolins. These studies demonstrate a novel avian respiratory soluble lectin that can bind surface polysaccharides of pathogenic E. coli responsible for respiratory disease.

Dexamethasone and epinephrine stimulate surfactant secretion in type II cells of embryonic chickens

Pulmonary surfactant (PS), a mixture of phospholipids and proteins secreted by alveolar type II cells, functions to reduce the surface tension in the lungs of all air-breathing vertebrates. Here we examine the control of PS during lung development in a homeothermic egg-laying vertebrate. In mammals, glucocorticoids and autonomic neurotransmitters contribute to the maturation of the surfactant system. We examined whether dexamethasone, epinephrine, and carbamylcholine hydrochloride (agonist for acetylcholine) increased the amount of PS secreted from cultured type II cells of the developing chicken lung. In particular, we wanted to establish whether dexamethasone would increase PS secretion through a process involving lung fibroblasts. We isolated and cocultured type II cells and lung fibroblasts from chickens after 16, 18, and 20 days of incubation and from hatchlings (day 21). Epinephrine stimulated phosphatidylcholine (PC) secretion at all stages, whereas dexamethasone stimulated secretion of PC at days 16 and 18. Carbamylcholine hydrochloride had no effect at any stage. This is the first study to establish the existence of similar cellular pathways regulating the development of surfactant in chickens and eutherian mammals, despite the vastly different birthing strategies and lung structure and function.

The ontogeny of pulmonary surfactant secretion in the embryonic green sea turtle (Chelonia mydas)

Pulmonary surfactant, consisting predominantly of phosphatidylcholine (PC), is secreted from Type II cells into the lungs of all air-breathing vertebrates, where it functions to reduce surface tension. In mammals, glucocorticoids and thyroid hormones contribute to the maturation of the surfactant system. It is possible that phylogeny, lung structure, and the environment may influence the development of the surfactant system. Here, we investigate the ontogeny of PC secretion from cocultured Type II cells and fibroblasts in the sea turtle, Chelonia mydas, following 58, 62, and 73 d of incubation and after hatching. The influence of glucocorticoids and thyroid hormones on PC secretion was also examined. Basal PC secretion was lowest at day 58 (3%) and reached a maximal secretion rate of 10% posthatch. Dexamethasone (Dex) alone stimulated PC secretion only at day 58. Triiodothyronine (T(3)) stimulated PC secretion in cells isolated from days 58 and 73 embryos and from hatchling turtles. A combination of Dex and T(3) stimulated PC secretion at all time points.