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Alveolar Type II Cell Diagram


Tubular myelin and surfactant .


Figure 2 : Alveolar type II cell. The apical plasmalemma has microvilli, lateral membranes are indicated by hash marks, and the basal plasmalemma has adjacent gray area representing basement membrane. The nucleus (nucleus) is shown in blue with areas of condensed and open chromatin. A double-helix icon calls attention to telomerases/telomeres, which can play a role in alveolar type II cell function, surfactant production, and human disease. The nuclear membrane is continuous with the rough endoplasmic reticulum (RER), and the pathway of surfactant production progresses from the RER to the Golgi (dotted arrows), to vesicles, to multivesicular bodies/late endosomes (MVB/LEs), and fi nally to lamellar bodies (LBs). Secretion of surfactant occurs at the apical membrane. Other colors and symbols indicate the following: mitochondria (purple); lysosomes and proteosomes (red); surfactant protein A (SP-A); pro-surfactant proteins B and C, (proSP-B, proSP-C). Different symbols for membrane transporters highlight their dissimilar functions. Surfactant is indicated by myelin-like swirls, and the re-uptake of surfactant into macrophages (partial cell bottom right) is enhanced by the presence of granulocyte macrophage stimulating factor (GM-CSF).


the nucleus includes the addition of just a few ribosomes in Figure 2 and a relabeling as rough ER (RER). In addition, one profile of ER in Figure 1 spans nearly three-quarters of the circumference of the nucleus, not at all representative of what is seen with TEM. That profile was divided into smaller segments in Figure 2 , and the frequency of continuities between the RER and outer nuclear membrane and perinu- clear space was increased, along with the addition of a few vesicles leading to the cis face of the Golgi. Golgi . The Golgi apparatus was shown near the nucleus, the latter with gray shading in both figures, but without vesicles, of which there are many in type II cells. An arrow from the ER in Figure 1 indicates that surfactant protein A (SP-A), pro-surfactant protein B and C (proSP-B, and proSP-C) move through the cytoplasm from RER through Golgi with a mandatory stopover within the multive- sicular body/late endosome (MVB/LE). There are enough unanswered questions about the movement of surfactant proteins, particularly SP-A, from their respective production sites into LBs, that such a narrow representation may not be justified. In addition, direct fusion of the RER with LBs, bypassing Golgi and MVB/LE altogether, occurs commonly. Dotted arrows now in Figure 2 imply that route is variable. A few structures depicting lysosomes and proteasomes were added in Figure 2 .


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T e depiction of tubular myelin in Figure 1 was misleading since routine TEM almost always shows a heteroge- neous tangled mix of surfactant lipid layers, lamellar structures, proteins, and debris in the alveolar space, with only infrequent portions of orderly tubular myelin lattice. Many studies suggest that the lattice structure is dependent on the presence of SP-A and is not actually required for surfactant function [ 8 ]. T ere is little need for an electron micrograph here, and it gives a skewed emphasis to a small part of the overall diagram. Figure 2 uses tracings of actual randomly selected heterogeneous surfactant. T is also emphasizes the uptake of surfactant by alveolar type II cells and macrophages, as well as maintaining the “vector” quality of the entire diagram. T e image of a generic lipid bilayer in Figure 1 is not truly representative of surfactant. Mature surfactant has been shown in countless publications to have various protein components necessary for structure, function, and many other purposes; therefore, lipid bilayer image in Figure 1 was deleted in Figure 2 .


Uptake of spent surfactant from the alveolar space can occur at the apical membrane (shown on the right with a microvillar-studded surface), progressing, perhaps, through late endosomes (LE) and proteasomes. Separate arrows for recycling and catabolism in Figure 1 without descrip- tions of how these pathways might differ within the same MVB/LE beg debate and are combined in Figure 2 , with uptake (recycling and catabolism) considered as one event involving MVB/LE. Likely the major uptake of surfactant from the alveolar space occurs via the alveolar macrophage (a portion of a macrophage is now present in Figure 2 ). The arrow and label depicting reuptake in Figure 1 was made bold in Figure 2 , emphasizing that surfactant reuptake is facilitated by granulocyte-macrophage colony-stimulating factor GM-CSF.


Discussion


While cells and organelles can appear to be of various shapes when single 2D images of sections are analyzed, the features described above can be observed in most TEM images of alveolar type II cells. Figure 3 shows an example TEM image of such a cell. Most of the features described above in relation to Figures 1 and 2 can be seen in Figure 3 . Indeed, the information to produce the former fi gures came from careful study of hundreds of images similar to Figure 3 .


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