Figure 1. Schematic diagram of the life cycle of pulmonary surfactant. Surfactant proteins and lipids are synthesized in the rough endoplasmic reticulum (RER) and Smooth ER (SER), respectively, and transported to the Golgi apparatus (Golgi). SP‐B and SP‐C and the PLs are transported via vesicles from the trans Golgi network to multivesicular bodies (MVB) before being packaged into lamellar bodies (LBs). LB secretion into the liquid lining of the alveoli (hypophase) occurs via exocytosis across the AECII plasma membrane, and is stimulated by various secretagogues. SP‐A and SP‐D are secreted constitutively via a non‐LB pathway. Within the hypophase, LBs swell and unravel, forming tubular myelin (TM), consisting of lipids and proteins (particularly SP‐A and SP‐B); TM is contained within the large aggregate surfactant (LAS) fraction that can be isolated from lung lavage. This fraction is surface active and supplies the lipids to the air‐liquid interface as well as the surfactant reservoir, which is a multilayer structure associated with the surface film (also known as the surface‐associated phase). The adsorption of lipids to the air‐liquid interface is mediated by the hydrophobic surfactant proteins, SP‐B and SP‐C. As the mixed molecular film is compressed, some of the lipid is squeezed out into the multilayer reservoir and the film undergoes a restructuring, rendering it capable of reducing surface tension (ST) to near 0 mN/m. Upon reexpansion, some lipids from the reservoir reenter the surface film. Lipids from the surface film and the reservoir become inactive, forming part of the small aggregate surfactant (SAS) fraction and are eventually taken back up by the AECII via endocytosis. These lipids are then recycled via the endocytic pathway to MVBs and combined with new lipids and proteins from the Golgi to form new LBs. Figure reproduced with permission from Elsevier from Orgeig et al. (414).

Figure 2. Schematic diagram of the evolutionary sequence of air breathing organs among the fishes. The ontogenetic origin (i.e., ventral or dorsal) and the evolution of lungs, swim bladders, and their blood supply and the loss of respiratory function are indicated in italics. Resp. = respiratory; fn = function. Figure modified with permission from The University of Chicago Press from Perry et al. (442) and reproduced with minor modifications with permission from Elsevier from Ref. (108).

Figure 3. Relative proportions of unsaturated phospholipids (USP), disaturated phospholipid (DSP) and cholesterol (Chol) in the two different types of surfactant. The protosurfactant is dominated by USP and Chol and is present in the Actinopterygian fishes and in the basal Sarcopterygian, the Australian lungfish which have simple, bag‐like lungs that are smooth, noncompartmentalized and largely avascular. This surfactant is poorly surface active and highly spreadable, suggesting that it is adapted to function as an antiadhesive. The tetrapod surfactant is present in the two derived species of Sarcopterygian lungfish, the African and South American lungfish and in the tetrapods. This surfactant has a much higher proportion of DSP and a lower proportion of Chol. This surfactant is highly surface active which suggests that it is able to support and stabilize small respiratory units.

Figure 4. Relationship among surfactant cholesterol and the PLs during the evolution of the vertebrates. (A) The Cholesterol/Phospholipid ratio (Chol/PL) expressed as a ratio of μg/μg, and (B) the Disaturated Phospholipid/Phospholipid ratio (% DSP/PL) expressed as a percentage of total PL of lavage material obtained for a range of air‐breathing vertebrates. The species are: the Teleost fish, the goldfish C. auratus (C. aur) (106); the air‐breathing Actinopterygian fish P. senegalensis (P. sen), Calamoicthys calabaricus (C. cal), and L. osseus (L. oss) (106); the Australian and African lungfish N. forsteri (N. for) and P. annectens (P. ann) (403); the tiger salamander Ambystoma tigrinum (A. tig) (106); the amphibians A. tridactylum (A. tri), Siren intermedia (S. int), Bufo marinus (B. mar) and Xenopus laevis (X. lae) (106); the rattlesnake Crotalus atrox (C. atr) (112); the lizard Ctenophorus nuchalis (C. nuc) (99); the chicken Gallus gallus (G. gal) (266); the rat Rattus norvegicus (R. nor) (106); the human (H. sap) (106); the fat‐tailed dunnart Sminthopsis crassicaudata (S. crass) (305); the microchiropteran bats Nyctophilus geoffroyi (N. geoff); and Chalinolobus gouldii (C. goul) (81). The lizard, the dunnart, and the bats were at their warm‐active body temperature (33‐37°C). Data expressed as mean ± SE, n usually between 4 and 9. Figure reproduced with permission from Daniels and Orgeig (104).

Figure 5. Relationship between cholesterol and disaturated PL during the evolution of the vertebrates. The Chol/DSP ratio is expressed as mean ± SE (μg/μg). All abbreviations, data sources, and other details are as for Figure 4. Figure reproduced with permission from CSIRO Publishing from Orgeig et al. (407).

Figure 6. Schematic diagram of the alveolar interdependence model, illustrating the structure‐function behavior of lung parenchyma in response to alterations in surface tension (γ). The functional unit is the alveolar duct (or a set of ducts forming an acinus) embraced by peripheral connective tissue fibers. The peripheral fibers (PF) are connected to the pleura, are the main force‐bearing element, and are largely independent of changes in surface tension (γ). The axial fibers (AF) are rings of tissue forming the entrance of alveoli; they are influenced by the surface tension of the air‐liquid interface, which is continuous along the alveolar wall. The 2D alveolar walls represent a negligible mechanical component. Low surface tensions allow a large alveolar surface area between slightly stretched axial fibers. However, when surface tension is abnormally high the axial fibers become more stretched resulting in duct enlargement, flattening of alveoli, and a decreased alveolar surface area. Figure reproduced with permission from Elsevier from Bachofen and Schürch (9).

Figure 7. Schematic diagram of an alveolar wall illustrating the movement of fluid (arrows) between the fluid lining the air spaces (hypophase) and the interstitial space. The small radius of curvature of corners and crevices leads to a large negative fluid pressure in the alveolus, which tends to draw fluid into the alveolus from the interstitium. Furthermore, under hydrostatic pressure, net fluid movement occurs out of the capillaries into the surrounding tissue and the alveolus. Sodium pumps in the AECIIs remove sodium from the hypophase and transport it into the interstitium, causing a net passive fluid movement out of the alveolus, thereby preventing fluid buildup. Excess fluid in the interstitium is removed by the lymphatic circulation. BM, basement membrane; LB, lamellar bodies; N, nucleus. Figure reproduced with permission from Elsevier from Orgeig et al. (414).

Figure 8. Opening pressure of the lungs of nonmammalian vertebrates. (A) Schematic representation of the pressure required to open and then fill a completely collapsed lung with air infused at a constant rate (usually 1 mL/min). Note that initially the pressure increases before the lung begins to inflate, then once open the lung inflates with little change in pressure. (B) The opening pressure before and after surfactant removal by lavage of collapsed lungs from a range of vertebrates: C. aur = Carassius auratus (goldfish, Teleostei, and Actinopterygii), P. sen = Polypterus senegalus (bichir, Polypteriformes, and Actinopterygii), C. cal = Calamoicthys calabaricus or Erpetoichthys calabaricus (rope‐fish, Polypteriformes, and Actinopterygii), L. oss = Lepisosteus osseus (gar, Lepisosteiformes, and Actinopterygii), A. tig = Ambystoma tigrinum (tiger salamander, Caudata, and Amphibia), C. nuc = Ctenophorus nuchalis (lizard, Lacertilia, Squamata, and Reptilia), and T. ord = Thamnophis ordinoides (Garter snake, Serpentes, Squamata, and Reptilia). Data are expressed as mean ± SE, n between 4 and 8. Figure reproduced with permission from Oxford University Press and The Society for Integrative and Comparative Biology from Daniels et al. (110).

Figure 9. Three‐dimensional models of trimeric SP‐A (A) and SP‐D (C) and octadecameric SP‐A consisting of six SP‐A trimers forming a bouquet structure (B) and dodecameric SP‐D consisting of four trimers forming a cruciform structure (D). The four structural domains of the human SP‐A polypeptide chain (A) are shown: (I) NH₂‐terminal segment; (II) collagen‐like domain with a sequence irregularity, which divides the collagen‐like domain in two parts: NH₂‐terminal (II_N) and COOH‐terminal (II_C) portions; (III) neck region between the collagen and the globular domain; and (IV) COOH‐terminal globular domain. A similar domain structure pattern is shown for the SP‐D polypeptide chain (C). Figure modified with permission from Portland Press from Sanchez‐Barbero et al. (494) and from Elsevier from Haczku (209) and Orgeig et al. (411).

Figure 10. Schematic diagram summarizing the major factors and signaling pathways that stimulate surfactant secretion from AECII. Several beta‐2 adrenergic agonists, including isoproterenol, adrenaline, and noradrenaline stimulate the β‐2 adrenergic receptor. The receptor is coupled to a heterotrimeric G protein (Gs), which stimulates adenylate cyclases (AC), to produce cyclic AMP (cAMP), which in turn stimulates the cAMP‐dependent PKA. Similarly the adenosine A_2B receptor stimulates PK‐A via the same intermediates. Another pathway involves the direct or indirect stimulation of PKC. The synthetic surfactant secretagogue tetra‐decanoylphorbol acetate (TPA) and cell‐permeable diacylglycerols (DAGs) are potent direct stimulators of PK‐C. ATP and UTP bind to the purinergic receptor (P2Y₂) which is coupled to a heterotrimeric G protein Gq, which stimulates phospholipase C (PLC)‐β3. Activation of PLC‐β3 leads to the formation of inositol triphosphate (IP₃) and DAG. The latter stimulates PK‐C, while IP₃ feeds into the third secretory mechanism, which involves the elevation of intracellular Ca²⁺ levels by IP₃, by calcium ionophores or by stretch of the basement membrane. Calcium in turn stimulates CaMK. The stimulatory effect of stretch is mediated indirectly by the P2Y₂ receptor, and therefore the PK‐C and/or CaMK signaling pathways. Specifically stretch is mediated by the AECI which are connected to AECII via gap junctions. Cytoplasmic Ca²⁺ increases in either cell can spread via gap junctions, and the intercellular transmission of Ca²⁺ triggers the secretion of ATP from AECI into the extracellular spaces via stimulation of the specific P2 purinergic receptor (P2X₇R), which in turn activates Ca²⁺ signaling pathways in AECII via P2Y₂ purinergic receptors to induce surfactant secretion from AECII in a paracrine manner. However, physiological stimulation of surfactant secretion by ventilation or labor may be mediated via the β‐2 receptor. The exact subsequent mechanisms leading to lamellar body exocytosis are not well understood, but are thought to involve PK‐stimulated protein phosphorylation, which presumably activates contractile proteins to move lamellar bodies to the apical surface to fuse with the plasma membrane. Figure reproduced with permission from Elsevier from Orgeig et al. (414).

Figure 11. Comparison of the development of the human, sheep, rabbit and rat lung indicating the relative gestational period for each of the five phases of lung development, both in terms of the percent of gestation and the weeks/days of gestation. Also indicated are the approximate appearance of mature lamellar bodies (LBs, shaded circles) and PLs in the lavage or amniotic fluid (white circles). Figure reproduced with permission from Elsevier from Orgeig et al. (414).

Figure 12. Schematic diagram indicating the relative timing of the first appearance of mRNA of the surfactant proteins, SP‐A, SP‐B, and SP‐C, in lung tissue of human, sheep, rabbit, and rat both in terms of the percent gestation as well as the lung developmental period. Figure reproduced with permission from Elsevier from Orgeig et al. (414).

Figure 13. Schematic diagram summarizing neurohormonal regulation of surfactant maturation in fetal alveolar epithelial type II (AECII) cells. Glucocorticoids (black arrows) and thyroid hormones (gray arrows) increase surfactant production as well as the number of adrenergic receptors, rendering AECIIs more responsive to surfactant secretagogues. Glucocorticoids stimulate surfactant synthesis by AECII indirectly, possibly via KGF produced by fibroblasts, a pathway that is enhanced by T₃ (528). Thyroid hormones act directly on nuclear receptors of AECII to increase surfactant synthesis, but the pathway is unclear. While glucocorticoids increase gene expression of surfactant proteins (SPs) as well as that of the enzymes CTP CCT and FAS to increase PLs synthesis, thyroid hormones increase the activity of these enzymes (indicated by thick grey arrows). Together, increased synthesis of SPs and PLs leads to an increase in lamellar bodies (LBs) and hence surfactant secretion. In addition, glucocorticoids stimulate surfactant secretion via downstream signaling factors of the PKA signaling pathway and via a GR‐dependent pathway [blocked by the GR antagonist RU‐486 (257)]. Several beta adrenergic agonists, including isoproterenol, adrenaline and noradrenaline stimulate the β‐2 receptor to increase surfactant secretion. The receptor is coupled to adenylate cyclase (AC), which produces cyclic AMP (cAMP) via a trimeric GTP‐binding protein (G), to stimulate cAMP‐dependent PK‐A. cAMP administered to isolated AECII also acts via PK‐A to increase β‐2 receptor number (135). Other factors including hormones, growth factors, and physical factors are described in the text. Dashed lines indicate pathways that are not fully elucidated. Figure reproduced with permission from Elsevier from Orgeig et al. (413,414).

Figure 14. Diagrammatic representation of the mechanism by which endogenous (circulating or locally produced) cortisol and antenatal glucocorticoids act in the lung to increase the gene and protein expression of surfactant protein. GC: glucocorticoid; GR: GC, receptor; TTF‐1: thyroid transcription factor‐1; TBE: TTF‐1 binding element; x, y, and z indicate cofactors for different SP genes. Figure reproduced with permission from Morrison et al. (377).

Figure 15. Fetal arterial oxygen content (mmol O₂/L blood) in two sheep models of human IUGR. The uteroplacental embolization (UPE) model (A) of IUGR in the sheep fetus results in periods of fluctuating hypoxemia over the 20‐d experimental period starting at 110 d of gestation. In contrast, placental restriction (PR, n = 28; control, n = 31) in sheep (B) results in chronic hypoxemia that is maintained throughout late gestation (271). Control, open circles; UPE (A) or PR (B), closed circles. Figure reproduced with permission from Elsevier from Orgeig et al. (412) with original data from Morrison (375) and Murotsuki et al. (381).

Figure 16. Aligned amino acid sequences of the mature SP‐C protein inferred from the translation of the nucleotide sequences published in Potter et al. (455). The genus names are listed, except in the case of U. americanus and U. maritimus, for which the genus is Ursus and in the cases of M. brevicaudata and M. domestica, where the genus name is Monodelphis. Heterothermic species are underlined. Amino acid position is indicated by the numbers along the top of the figure. The black vertical line indicates the boundary between the N‐terminal extramembrane and the C‐terminal transmembrane domains at amino acid position 12. Dots indicate conserved amino acids relative to the first sequence, that is, Rattus. Question marks indicate residues that were not obtained. “X” at site 29 in Ursus americanus represents heterozygosity for the amino acids arginine (R) or glycine (G) (151). Figure reproduced with permission from Elsevier from Potter et al. (455).

Figure 17. Fluorescence images of solvent‐spread surfactant film from warm‐active (A) and torpid (B) dunnarts measured at 23°C at 5, 10, 15, 20, 25, and 30 mN/m surface pressure (indicated inside the images). Subphase: 0.15 M NaCl, 1.5 mmol/L CaCl₂, 1.0 mmol/L TRIS‐HCl buffer at pH = 7.0. (C) Quantitative fluorescence image analysis of solvent‐spread surfactant films of warm‐active (solid symbols) and torpid (open symbols) dunnarts at different surface pressures. (a) Percentage of area of coverage of the probe‐excluded regions; (b) diameter (μm) of the probe‐excluded regions. (D) AFM images of solvent‐spread surfactant films from warm‐active (a, b) and torpid (c, d) dunnarts at 30 mN/m surface pressure. The films were transferred onto freshly cleaved mica by the Langmuir‐Blodgett transfer technique. Area of scan a, b: 2.5 × 2.5 μm²; c, d: 10 × 10 μm². a and c were taken in height mode, while b and d were taken in phase mode. LE, liquid‐expanded phase; LC, liquid‐condensed phase. Figure reproduced with permission from Oxford University Press and The Society for Integrative and Comparative Biology from Orgeig et al. (401).

Figure 18. AFM images of surfactant films from hibernating and summer‐active ground squirrels. Representative AFM images of summer‐active (left Panels A1‐A6) and hibernating samples (right panels B1‐B6) were taken from films compressed to 35 mN/m and transferred onto mica supports. The images cover a scan size of 50 μm² (top), 5 μm² (middle), and 1 μm² (bottom). In each pair of panels the left image is a topographic scan and the right image is a phase scan. The black arrows in A3 and B3 point to very large domains/3D structures. Black arrows in phase images A6 and B6 point to three different regions coexisting in summer‐active and hibernating samples, respectively. Hibernating samples had significantly smaller domains than those of summer‐active samples in 50 μm² scan size images. Figure reproduced with permission from Elsevier from Suri et al. (553).

Figure 19. (A) Aligned amino acid sequences of the mature SP‐C protein inferred from translation of the nucleotide sequences published in (151). Species indicated in black are terrestrial, in orange are semiaquatic and in blue are divers. Dots indicate conserved amino acids between the sequence of interest and Canis. Dashes indicate an alignment gap due to the insertion of an extra amino acid in Mirounga. The light blue box indicates the two positively charged amino acids, lysine and arginine at the boundary of the N‐ and C‐terminal domains. The light green box indicates the PCCP motif, which in the case of the carnivores is a PCFP domain (pink box). Amino acids indicated in light green with a black box surrounding them are those that were identified as being under positive selection in the site models. Positively selected amino acids are only indicated in the diving species of each contrast. Figure reproduced with permission from Elsevier from Foot et al. (150). (B) Schematic diagram of an SP‐C molecule in a surfactant PL film. The α‐helix within the C‐terminal transmembrane domain is embedded at an angle in the PL fatty acid tails and the N‐terminal extramembrane domain is associated with the hydrophilic PL head groups. The two palmitate groups that are covalently linked to two cysteine residues (C) are anchored within the PL fatty acid tails. On either side of the cysteine residues is a proline residue (P) which changes the orientation of the protein chain, enabling correct orientation of the palmitates and the N‐terminal segment with the head groups. The two + signs indicate two positively charged residues, Lys (K) and Arg (R) at the boundary between the more polar N‐terminal segment and the hydrophobic C‐terminal segment. The two red circles indicate the location of site 2 and sites 9 and 10 that are evolutionarily labile and are under positive selection in the diving mammals. In cetaceans and sirenians, the charge and polarity of these residues is important, possibly leading to stronger binding of the N‐terminal tail to the PL head groups, leading to greater stability of the lipid‐protein complex during the high compression forces during diving. In pinnipeds, the hydrophobicity at sites 9 and 10 is crucial, possibly leading to greater interactions with the palmitic acid residues linked to the cysteines (C), and possibly aiding in the adsorption of SP‐C to the air‐liquid interface upon resurfacing after a dive. Figure reproduced with permission from Elsevier from Foot et al. (150).

Figure 20. (A) Aligned amino acid sequences of the mature SP‐C protein inferred from translation of the nucleotide sequences published in (151). Species indicated in black are terrestrial, in orange are semiaquatic and in blue are divers. Dots indicate conserved amino acids between the sequence of interest and Canis. Dashes indicate an alignment gap due to the insertion of an extra amino acid in Mirounga. The light blue box indicates the two positively charged amino acids, lysine and arginine at the boundary of the N‐ and C‐terminal domains. The light green box indicates the PCCP motif, which in the case of the carnivores is a PCFP domain (pink box). Amino acids indicated in light green with a black box surrounding them are those that were identified as being under positive selection in the site models. Positively selected amino acids are only indicated in the diving species of each contrast. Figure reproduced with permission from Elsevier from Foot et al. (150). (B) Schematic diagram of an SP‐C molecule in a surfactant PL film. The α‐helix within the C‐terminal transmembrane domain is embedded at an angle in the PL fatty acid tails and the N‐terminal extramembrane domain is associated with the hydrophilic PL head groups. The two palmitate groups that are covalently linked to two cysteine residues (C) are anchored within the PL fatty acid tails. On either side of the cysteine residues is a proline residue (P) which changes the orientation of the protein chain, enabling correct orientation of the palmitates and the N‐terminal segment with the head groups. The two + signs indicate two positively charged residues, Lys (K) and Arg (R) at the boundary between the more polar N‐terminal segment and the hydrophobic C‐terminal segment. The two red circles indicate the location of site 2 and sites 9 and 10 that are evolutionarily labile and are under positive selection in the diving mammals. In cetaceans and sirenians, the charge and polarity of these residues is important, possibly leading to stronger binding of the N‐terminal tail to the PL head groups, leading to greater stability of the lipid‐protein complex during the high compression forces during diving. In pinnipeds, the hydrophobicity at sites 9 and 10 is crucial, possibly leading to greater interactions with the palmitic acid residues linked to the cysteines (C), and possibly aiding in the adsorption of SP‐C to the air‐liquid interface upon resurfacing after a dive. Figure reproduced with permission from Elsevier from Foot et al. (150).