Membrane

MEMBRANE (cell coat). The protoplasm of any cell, plant or animal, is separated from the surrounding environment by a denser layer. This layer may be very thin, almost imperceptible, but it may also develop as a structure separate from the internal parts of the protoplasm. In the first case, the densest, thinnest layer is called the surface film, 'hapto-genic membrane' or surface M.; in the second, one speaks of M. in the proper sense of the word. The surface M. (Protoplasmahaut) cannot be opened by ordinary microscopic methods of investigation, even with the application of the finest stains, and therefore its significance is negligible: each cell in the preparation appears devoid of any M. In some cases it can be seen only in the ultramicroscope (Gaidukov, Seifriz, Napsteen Cranner), but not in all objects. The physiological significance of the surface M. is enormous; it depends on the regulation of the penetration of various substances into the cell. Pfeffer was the first to understand its significance and began to study its permeability. Despite solid evidence of the existence of a surface M., many authors (Lepeshkin, M. Fischer, etc.) deny its presence, basing themselves on 1) model experiments with selective absorption of ions by gelatinous jellies devoid of any coat, and 2) observations on plasmolysis and movement of particles in the surface layer of cells (Lepeshkin); during plasmolysis in many cases strands remain connecting the protoplasm that has separated from the walls to the walls, but in consistency they differ in nothing from plasmolized cytoplasm. The majority of modern authors, recognizing the existence of a surface M., consider it a layer of the cell's cytoplasm, qualitatively different from it. This layer, i.e. the surface M., cannot be considered a constant structure—it can change and even disappear, but is formed every time cells come into contact with the surrounding environment; for example, when a cell is crushed, a surface M. forms immediately around the oozing droplets, reacting to dyes (i.e. permeable to some dyes and impermeable to others), just like the M. of living intact cells. The most convincing data have been obtained in experiments with a micromanipulator. It is possible to show, for example, that in amoebae the surface layer is dense (in a state of gel) and it can be carefully pulled away from the liquid cytoplasm by micromanipulator needles. Experiments with microinjection of salts are even more convincing (Chambers and his students; 1915-28). When immersed in NaCl or KCl solutions, amoebae die due to the destruction of the surface layer; in CaCl₂ or MgCI₂ solutions they remain alive. When injected with the same salts NaCl and KCl cause a rapidly disappearing liquefaction, while CaCl₂ causes a non-disappearing condensation of the cytoplasm. With physical methods it is also possible to confirm the existence of a surface coat, for example by studying the electrical conductivity of erythrocytes. Apparently the surface coat is very thin; based on the values of the dielectric constant of a suspension of erythrocytes, one can think that it consists of only one layer of molecules (see Monomolecular layer) or of two (studies of Gorter and Grendel on human erythrocytes). Opinions on the composition of the surface M. are contradictory. According to Overton's theory it is formed from lipoids, mainly lecithin and cholesterol, which as substances lowering surface tension accumulate on the surface of the cell. But this assumption is contradicted by the permeability of cells for water, sugars and their small permeability for salts. To explain the penetration of these substances into the cell, one must admit that in addition to fatty substances the M. also contains protein substances. Bechhold could convince himself of this during research in the ultramicroscope; this is also indicated by the change in permeability of erythrocytes in acidic and alkaline media, which is possible only in the presence of substances, e.g. proteins, changing their charge to one and the same side of the isoelectric point. The most probable assumption is that the M. of cells is built mosaically from lipoids and proteins. The surface M. is specific for each cell, and its selective properties cannot be explained entirely based on model experiments with artificially made membranes. To the type of surface M. one must also refer to double-contour M. of the simplest (the so-called pellicle in most infusoria, flagellates, and in some epithelial cells). M. in the proper sense. Some cells are equipped with a M. differing from the rest of the cytoplasm not only in structure but also in chemistry (plant cells are covered with a M. of cellulose, crustacean cells with a M. of chitin). Depending on the position of the cell in the system of other cells, as well as on function, the M. develops either around the whole cell, e.g. the egg cell, or only at one of its poles in the form of the so-called cuticle (cells of the intestinal epithelium, kidneys, etc.). The method of formation of these M. is still not elucidated in many cases. They appear either as a secretion of the cytoplasm or as a result of condensation of its surface layer and finally can arise by condensation of substances surrounding the cell. In many cases (cell wall of plant cells, chitinous M. of crustaceans, etc.) the M. possesses birefringence, i.e. is built crystallinely. The cuticle of cells in a single-layered flat epithelium, in the epithelium of the bladder and in the covering epithelium of amphibians is homogeneous, very thin and apparently also has the significance of a supporting structure (Studnicka, Heidenhain). In cylindrical epithelia the cuticular membrane reaches great power and complexity of structure, forming the so-called 'rugosities' of the cell. The latter is formed by thin outgrowths of ectoplasm, cemented together by some substance (Zimmermann, Heidenhain). Whether epithelial cells have a membrane surrounding them on all sides is still not decided, although some authors consider this very probable. Of connective tissue cells, only the erythrocytes of amphibians have a true M. (the so-called marginal ring), although according to some data (Lepeshinskaya) a true M. also exists in the erythrocytes of mammals. The other cells of connective tissue have only a surface condensed layer. The question of the M. of fibers is not yet finally decided, but the prevailing opinion is that they have no own M. (mucoid, as was thought before). The M. of muscle cells, the so-called sarcolemma, is transparent, thin and follows all changes of the cell during contraction, sometimes fibrillarity opens in it; since it differs in its chemical properties from sarco- plasm and myofibrils, many authors (Prenant, etc.) consider it a product of the secretion of connective tissue binding the muscle fibers. On the M. of nerve fibers see Nerve Fibers.