The Nature of Cytoplasm

The nature of cytoplasm has been scientifically studied for at least a century and a half. That history was described by Beth Burnside (1974) in a landmark meeting devoted to the cytoskeleton at the New York Academy of Sciences.

An early French observer of cellular material, Felix Du Jardin proposed in 1835 that all cells were composed of a motile material called "sarcode" that had both structural and contractile properties. In 1861, Austrian E. Brucke linked the mechanical and physiological properties of cells to a fundamental organization or architecture of cytoplasm as distinguished from purely chemical or physical properties. Early Dutch microscopist van Leeuwenhoek observed that red blood cells became deformed passing through capillaries Hand then sprang back into shape, demonstrating the elasticity of cytoplasm. The variety of elaborate forms that cells assume and maintain also require some cytoplasmic rigidity, properties which led nineteenth century biologists to conclude that cytoplasm is not merely a liquid nor an emulsion nor an aqueous suspension of life bearing granules. Rather, cytoplasmic architecture and contractility could be explained by the proposal of a mesh-like "reticular" or "fibrous" substructure whose interstices were filled with fluid. To some early biologists, the structure of cytoplasm therefore consisted of a continuous reticular network of delicate fibrils extending through the cell (reticular theory). Others claimed that the fibrils forming the cytoplasmic substratum were unbranched and discontinuous (fibrillar theory). Both of these theories were initially supported, but later deflated, by microscope preparation techniques of fixation and staining which arose between 1870 and 1890. Fibrillar and reticular frameworks appeared everywhere in many types of fixed cells, particularly in muscle, nerve, cartilage and epithelial cells.

Electron Microscopy Fodrin
Figure 5.1: Cytoskeletal network in a rat kangaroo kidney cell (ptK2) illustrated by tubulin antibody and light microscopy. Microtubules emanate from a dense MTOC region near the nucleus (N). With permission from Marc DeBrabander (1982).
Microtubules Electron Microscopy

Figure 5.2: Microtubules from PtK2 cells double labeled with antibody and immunogold under high magnification. Large circles (10 nanometer gold particles, arrows) tag glutamated tubulin subunits, small circles (5 nanometer gold particles) label tyrosinated tubulin subunits. With permission from Geuens, Gundersen, Nuydens, Cormellisen, Bulinski, and DeBrabander (1986), courtesy of Marc DeBrabander and Janssen Pharmaceutica Research Laboratories.

Figure 5.2: Microtubules from PtK2 cells double labeled with antibody and immunogold under high magnification. Large circles (10 nanometer gold particles, arrows) tag glutamated tubulin subunits, small circles (5 nanometer gold particles) label tyrosinated tubulin subunits. With permission from Geuens, Gundersen, Nuydens, Cormellisen, Bulinski, and DeBrabander (1986), courtesy of Marc DeBrabander and Janssen Pharmaceutica Research Laboratories.

Figure 5.3: Schematic of cellular cytoskeleton/membrane. M: cell membrane, MP: membrane protein, GP: glycoprotein extending into extra-cellular space, MT: microtubules, MF: microfilaments (actin filaments or intermediate filaments), MTL: microtrabecular lattice. Cytoskeletal proteins which connect MT and membrane proteins include spectrin, fodrin, ankyrin, and others. By Fred Anderson.

Figure 5.3: Schematic of cellular cytoskeleton/membrane. M: cell membrane, MP: membrane protein, GP: glycoprotein extending into extra-cellular space, MT: microtubules, MF: microfilaments (actin filaments or intermediate filaments), MTL: microtrabecular lattice. Cytoskeletal proteins which connect MT and membrane proteins include spectrin, fodrin, ankyrin, and others. By Fred Anderson.

Fischer and Hardy (1899) showed that the new fixatives and stains induced artificial coagulation of gelatin, egg albumin, and other proteins. The coagulation gave rise to beautiful reticular and fibrillar formations and even produced strikingly real, but phoney mitotic spindles! These studies discouraged support for the fibrillar and reticular theories to the extent that many cytologists denied that meshworks seen in fixed material had any validity in the living cell. Butschli added his alveolar foam theory of cytoplasmic structure, claiming that the reticular appearance seen in fixed materials and sometimes in living cells resulted from a honeycomb of vacuoles of one substance crowded together in a continuous phase of another. This theory died because it failed to account for the fixation observations and because vacuolated cytoplasm was uncommon. Meanwhile, observations of true fibrillar formation in living cells were accumulating. The use of polarized light microscopy revealed "birefringent" submicroscopic rods in the cytoplasm of muscle, nerve, sperm, and spindles of dividing cells. Biologists began to consider cytoplasm as a "gel," in which rod-shaped filaments formed cross linkages. Gel aptly describes the mechanical properties of cytoplasm: an elastic intermeshing of linear crystalline units giving elasticity and rigidity to a fluid while allowing it to flow.

The protoplasmic rods revealed by birefringence in polarized microscopy were thought to be proteins, or linear aggregates of proteins, which were held together by hydrogen bonding. Seifriz concluded that the fibrillar "artifacts" produced in cells or protein solutions upon fixation were significant. He suggested that the relatively coarse microscopic threads in fixed materials may be artifactually produced aggregates of important submicroscopic threads, probably linear proteins. To account for the elastic and tensile properties of cytoplasm,

Seifriz proposed the "brush heap theory" of interlacing fibers. He and others later suggested that cytoplasmic fibers could exist in the brush heap form or could form lateral associations by hydrogen bonding into paracrystalline aggregates, and that switching between the two states accounted for cytoplasmic behavior observed in living cells. We now know that subunits of the protein actin can polymerize in a variety of forms including interlacing gels or filamentous bundles. Cytoplasmic "gel" states occur due to crosslinking of actin filaments and other cytoskeletal proteins, and can be converted to more aqueous "sol" states by calcium ions and other factors.

Development of the electron microscope through the 1960's initially did not illuminate the substructure of cytoplasm. Portions of cells which were optically empty by light microscopy persisted in being empty in electron micrographs. The cell was perceived by many to be a "bag of watery enzymes." However, in some cases the fibrillar structures seen with light microscopy appeared as fine tubular filaments with the electron microscope. They comprised the internal structure of cilia, flagella, centrioles and basal bodies, and were prominent in the mitotic apparatus of dividing cells. Tubular filaments were also frequently encountered in protozoa, nucleated red blood cells and the dendrites of neurons. Ironically, the fixative then used in electron microscopy, osmium tetroxide, had been dissolving filamentous elements so that their presence was observed only sporadically. Like the 19th century fixative induced coagulation which induced superfluous fibrillar structures, the use of osmium tetroxide delayed recognition of the cytoskeleton. Later, with the advent of glutaraldehyde fixation, delicate tubular structures were found to be present in virtually all cell types and they came to be called microtubules. Bundles of microtubules seen with the electron microscope corresponded to birefringent fibers seen in living cells. Their ubiquity suggested that they were the fibrillar substructure of cytoplasm previously predicted on theoretical grounds. After some skepticism, microtubules became generally accepted as "household organelles" in nearly all cells studied. Subsequent characterization of actin, intermediate filaments, the microtrabecular lattice or "ground substance," and the structure of centrioles led to the recognition that cells were comprised of dynamic networks of connecting filaments and brought the cytoskeleton out of the closet.

0 0

Responses

  • Chris Morrison
    How is the nature of cytoplasm?
    2 years ago

Post a comment