Myelin tubular

Poulain FR, Allen L, Williams MC, et al. Effects of surfactant apolipoproteins on liposome structure implications for tubular myelin formation. Am J Physiol 1992 262(6 Pt 1) L730-L739. 

The appearance of tubular myelin-like structures in swollen lecithin was observed by light microscopy well before the systematic investigation of liposomes [351-352]. Similarly, it was also demonstrated some time ago that the addition of calcium ions converted phospholipid liposomes to cochleate cylinders [353]. Subsequent studies have, however, revealed that the system is extremely complex. For example, examination of the phase-transition behavior of synthetic sodium di-n-dodecyl phosphate [(C12H2sO)2PO2Na+ or NaDDP] and calcium di-n-dodecyl phosphate [Ca(DDP)2] showed the presence of many diverse structures [354]. In particular, hydrated NaDDP crystals were shown to form lyotropic liquid-crystalline phases which transformed, upon heating to 50 °C, to myelin-like tubes. Structures of the tubes formed were found... 

All three of these proteins are present in the lung. The lamellar bodies are secreted into alveolar lumen where they are transformed into an extracellular form of surfactant that has a quadratic lattice structure called tubular myelin. The three-dimensional tubulin-myelin structures spread in a monolayer at the air-liquid interface. This spreading decreases the surface tension, prevents alveolar collapse at the end of expiration, and confers mechanical stability to the alveoli. The surfactant system is in a continuous state of flux, and surfactant is recycled by uptake... 

Surfactant convertase, a diisopropylfluorophos-phate (DFP)-binding and DFP inhibitable carboxyl-esterase (Gross et al. 1997) of 70 kDa (reduced), is required for conversion of heavy density (tubular myelin) to fight density (small vesicle) subtypes of surfactant. Carboxylesterase ES-2 mRNA was localised to type II pneumocytes and alveolar macrophages but not to Clara cells (Clark et al. 1997). Lamellar bodies contain both glycosylated (70 kDa)... 

Fig. 98. Tubular myelin in the alveolar space of a 296 g female white rat (No.3 breeder Winkelmann Borchen-Kirchbor-chen) which inhaled 10 mg powdered aluminium/m 4 h per day, 5 days per week from August 16 to October 27, 1967 for a total of 51 days. Fixed on October 30,1967 under methitural anaesthesia by intratracheal instillation of 2.5 % glutaralde-hyde in phosphate buffer (pH 7.4) before opening die thorax. Postfixation with 1 % osmium tetroxide in phosphate buffer (pH 7.4). Contrasted en bloc for 12 h with 0.5% uranyl acetate in 70% ethanol. Embedded in a 2 8 mixture of methyl and butyl methacrylate. Sectioned at 50 nm. Lead citrate after Reynolds (1963). Plate 18/08. (from Schiller 1971)...

Various other functions have been attributed to the surfactant proteins. Primarily among them is their role in tubular myelin formation in presence of calcium. It has been proposed that tubular myelin is the precursor to the surface active phospholipid monolayer, but investigations regarding this are incomplete. Collectively the results of studies on surfactant proteins suggest that these proteins may play a prominent role in the organization of the lipids in the ainivay. 

The formation of tubular myelin requires the presence of phosphatidylglycerol and both SP-A and SP-B (24,25). Gene knockout studies indicate that SP-B is essential for the functioning of normal surfactant (26,27). SP-A is essential... 

The lamellar body contents are secreted from the apices of the type 11 cells into the alveoli. Secretion appears to involve membrane fusion and exocytosis into the hypophase between the cell and the air surface. After their release, the lamellar bodies become hydrated in the aqueous subphase and unravel into a matrix of tubular myelin (Fig. 2). Tubular myelin is hypothesized to release phospholipid spontaneously to the air-liquid interface where the phospholipids reside in thermodynamically favorable orientation with the acyl chains extending into the air phase, whereas the head groups remain submerged in the aqueous phase. After one or several respiratory cycles, the surfactant material is then forced back into the aqueous phase, where the surfactant forms bilayer vesicles. The vesicles may then enter into either a recycling or degradative pathway (31). 

Tubular myelin is often observed in alveolar spaces and is one of the most interesting forms of surfactant. This form is characterized by a highly ordered lattice structure that consists of long tubules, each a square when viewed in cross section (see Fig. 2). Under high-power electron nucroscopy, the tubules appear... 

Figure 2 Transmission electron micrograph showing the formation of tubular myelin (TM) from a secreted lamellar body (LB) in the alveolar space of a guinea pig The unravelling of the lamellar body to form the tubular myelin can be seen. There is a fuzzy zone within each interstice of the lattices, which has been shown to contain SP-A (magnification X 40,000).

Although SP-A is important for the formation of tubular myelin, it is not essential for the formation of the surface film at the air-liquid interfaee. Other surfactant materials (i.e., SP-B or SP-C-lipid mixtures) ean generate surfaetant films at the air-liquid interface, with rapid adsorption times and high surfaee pressures (low surface tension) that closely resemble the characteristics of natural pulmonary surfactant (39). 

An aqueous hypophase lies beneath the surface film and above the epithelium. Its thickness may vary depending on its location within the alveolus (74). The hypophase has an amorphous structure and contains proteins, proteoglycans, lipoproteins, lipid micelles, and tubular myelin (78). 

Suzuki Y, Fujita Y, Kogishi K. Reconstitution of tubular myelin from synthetie lipids and proteins assoeiated with pig pulmonary surfactant. Am Rev Respir Dis 1989 140 75-81. 

Voorhout WF, Veenendaal T, Haagsman HP, Verkleij AJ, van Golde LM, Geuze HJ. Surfactant A is localized at the eomers of the pulmonary tubular myelin lattiee. J Histochem Cytochem 1991 39 1331-1336. 

Williams MC. Ultrastructure of tubular myelin and lamellar bodies in fast-frozen adult rat lung. Exp Lung Res 1982 4 37-46. 

Hassett RJ, Engleman W, Kuhn C. Extramembranous particles in tubular myelin from rat lung. J Ultrastruct Res 1980 71 60-67. 

Nakamura H, Tonosaki A, Washioka H, Takahashi K, Yasui S. Monomolecular surface film and tubular myelin figures of the pulmonary surfactant in hamster lung. 

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