Structure of the Lattice:
Conventional transmission electron microscopes have accelerating potentials of several thousand volts and produce an electron beam that penetrates tissue and cell slices (i.e., sections) having thicknesses of about 0.2 nm.
The development in the 1970s of high-voltage electron microscopes capable of accelerating electrons over a potential of one million volts has made it possible to study specimens having the thickness of whole cells. Such microscopes are quite spectacular, usually occupying two floors of a laboratory (they are about 30 feet tall) and weighing about 20 tons.
Because the electron beam traverses the thickness of an entire cell, the interior of the cell is revealed in depth.
Principally as a result of the high-voltage electron- microscopic studies of K. R. Porter, M. Schliwa, J. J. Wolosewick, and J. B. Tucker, the cytoplasm or ground substance of eukaryotic cells has been shown to be divided into two major phases: the microtrabecular lattice and the intertrabecular spaces. A portion of the lattice is seen in the high-voltage photomicrograph of Figure 23-1 and is depicted diagrammatically in Figure 23-2.
The lattice is not apparent in electron photomicrographs of conventional thin sections because the sections lack the depth necessary to reveal the network. The micro-trabecular are rich in protein, whereas the inter-trabecular space is aqueous and serves to dissolve or suspend the great variety of small molecules involved in cellular metabolism (e.g., glucose, amino acids, oxygen, and inorganic salts).
At its margins, the lattice is attached to the plasma membrane of the cell; but it also interconnects many of the cytoplasmic organelles (e.g., mitochondria, endoplasmic reticulum, ribosomes, polysomes, etc.) as well as the ctyoplasmic filaments and microtubules that comprise the cytoskeleton (Fig. 23-2).
The micro-trabecular lattice is not a rigid or static structure. Rather, it varies in response to changes in cell activity and shape. Its disposition in a cell also varies according to the cellular environment. For example, when cells are cultured at a low temperature (e.g., 4°C), they round up, becoming spherical. The change in cell shape is due to a change in the cytoskeleton. First, the microtubules disassemble; then the cytoplasmic filaments disappear; and, finally, the microtrabeculae are modified although they do not decompose entirely.
Most notable is the separation of some of the microtrabeculae from the lattice and their aggregation into gobbets (Fig. 23-2). The loss of parts of the lattice produces gaps that allow the cell organelles to move about more freely and also exhibit Brownian motion. If the cells are returned to their normal (higher) temperature, the microtrabecular lattice and the cytoskeleton reform, and the organelles are again confined in their movement.
Changes in the chemical composition of the environment of a cell also produce reversible changes in the lattice. Cytochalasin B (Fig. 23-3), a drug obtained from the mold Helminthosporium dematoiderum, causes the microtrabecular to thicken. High or low osmotic pressure, changes in the concentration of ions (e.g., Mg2+ and Ca2+), and the presence of certain metabolic inhibitors also cause reversible changes in the lattice.
Chemistry of the Lattice:
Chemically, the most important constituents of the lattice are proteins. Two-dimensional electrophoresis of extracts of the lattice suggest that more than 100 different proteins comprise this polymeric network. Among the specific proteins that are thought to be present are actin, myosin, and tubulin. These proteins are also major constituents of cytoplasmic filaments and microtubules . A basic difference in the composition of microtrabeculae and the cytoskeletal elements (i.e., microtubules and cytoplasmic filaments) is revealed when both are treated with organic detergents such as Triton X-100.
Cells treated with Triton X-100 lose the microtrabecular lattice along with membranous structures such as mitochondria, endoplasmic reticulum, plasma membrane, and nuclear envelope, but they retain their cytoskeleton (Fig. 23-4). This behavior suggests that microtrabeculae have certain physical properties (and therefore a chemical composition) that are similar to those of membranes but unlike those of the cytoskeletal elements.
The observation that nonionic detergent extraction of cells leaves behind a residue consisting of microtubule and cytoplasmic filament proteins of the cytoskeleton has made it possible to study the cytoskeleton using conventional– transmission electron microscopes. When the cytoskeletons of extracted cells are freeze-dried and replicas are produced using platinum, the cytoskeleton is revealed in considerable detail (Fig. 23-5).
Functions of the Microtrabecular Lattice:
The microtrabecular lattice apparently serves as an intracellular scaffolding that helps suspend and ‘ organize the diverse structural components of the cytoplasm, including many of the cellular organelles. Acting in concert with the cytoplasmic filaments and microtubules, the lattice plays an important role in maintaining cell shape and in cellular movements.
Evidence also exists that suggests that a number of the enzymes of intermediary metabolism including those of glycolysis are bound to the lattice. It is possible that these enzymes are associated with the lattice in an ordered fashion; for example, the enzymes of a particular metabolic pathway may be bound to the lattice in such a manner that successive reactions of the pathway are spatially coordinated.