Showing papers in "International Review of Cytology-a Survey of Cell Biology in 1981"

Ultrastructure, Chemistry, and Function of the Bacterial Wall

Abstract: Publisher Summary This chapter describes the ultrastructure, chemistry and function of the bacterial wall. Cell walls are dynamic and change to fulfill functions dictated by the cell in response to the environment. Most bacteria respond unequivocally to the Gram reaction; that is, some retain the large crystal violet-iodine complex (Gram-positive), whereas others are decolorized by the alcohol treatment (Gram-negative) and can be counterstained. Cell age, autolysin levels, and growth conditions can affect the Gram reaction. Unlike Gram-positive bacteria, freeze-cleaved and -etched Gram-negative cells present a number of cleavage sites within the wall that is an indication of multilayering. This wall is chemically and structurally more complex than its Gram-positive counterpart. Each of the layers of capsule, slime layers, and surface arrays reside above the wall and may be singular or in combination with one another. Each presents unique problems for preservation and visualization by electron microscopy. The chapter also discusses the functional aspects of walls that include (1) interaction with metals; (2) β-lactam drugs and low-level antibiotic resistance; and (3) functional discontinuities in the wall fabric.

Cell Replacement in Epidermis (Keratopoiesis) via Discrete Units of Proliferation

Abstract: Publisher Summary This chapter reviews cell replacement in epidermis (keratopoiesis) via discrete units of proliferation. Dorsal or ear epidermis from mice is studied. For mouse epidermis, the data indicate that the tissue is subdivided into discrete units of proliferation, each with a certain autonomy but not independent of the neighboring units because they have to interact to produce an effective surface barrier. Each unit is maintained and dependent upon a group of about ten basal cells, one of which would appear to be a stem cell. The size and topographical relationship between the cells is illustrated in the chapter. The chapter also discusses a two-tier proliferative system; with this system, controls may operate at many levels. Any of the controls could potentially be changed; the chapter discusses several related consequences. The rate of progression of the stem cell through the cycle could be altered because of changes in its position or as a consequence of changes in the levels of, or susceptibility to, those factors that may act as on/off switches. Several possible changes might occur in the transit population, which can have dramatic effects on the total cell output.

The Differentiated State of Normal and Malignant Cells or How to Define a “Normal” Cell in Culture

Abstract: Publisher Summary This chapter discusses the differentiated state of normal and malignant cells. The first step of cell separation (trypsinization and collagenase treatment), by removing membranous receptors and matrix elements and structures, produces a discontinuity between the cell and its environment. Thus, if the outside directs what (and how much) the cell should or should not produce, the flow of information may never be exactly the same. There are two exceptions to this. The extent and the composition of the extracellular matrix is an important differentiated trait of tendon cells. The tendon in vivo produces an extensive and organized matrix consisting primarily of collagen bundles. In the presence of ascorbic acid, these cells produce an extensive matrix also in culture. The organization of the matrix, however, is not analogous to the tendon in vivo. Both in scanning and transmission electron micrographs, the matrix appears as a mesh-like network where collagen bundles have a smaller diameter than the bundles observed in intact tendon. This may be because tendon cells in vivo are lined up in orderly arrays, while in culture they have little or no orientation especially at subconfluent stages. If cells in culture are made to line up by providing them with a preformed collagenous matrix, it may be possible to achieve an organized deposition of a de novo synthesized matrix.

Mechanisms of Intralysosomal Degradation with Special Reference to Autophagocytosis and Heterophagocytosis of Cell Organelles

Abstract: Publisher Summary This chapter describes the mechanisms of intralysosomal degradation with a special reference to autophagocytosis and heterophagocytosis of cell organelles. Autophagy is the process of sequestration of intracellular components and their subsequent degradation by the lysosomes. The process of degradation is of fundamental importance in cell function because under steady-state conditions subcellular components are broken down and resynthesized many times during the life span of the cells. Autophagy contributes to the turnover of cell constituents during physiological cell conditions. Autophagy is induced by numerous treatments, conditions, or agents that cause cell dysfunction. Biological components from the extracellular space can also enter the lysosomal compartment and become degraded through the process of heterophagy. The chapter discusses an experimental model in which intracellular organelles are introduced into the lysosomal apparatus of Kupffer cells by means of heterophagy. The study of the heterophagic model has advantages over induced autophagy because the degradation of each membrane constituent or cell organelle can be separately studied. The heterophagy model is also useful to evaluate the capacity of lysosomes to degrade various membrane components in vivo .

Metabolic cooperation between cells.

Abstract: Publisher Summary Metabolic cooperation is the exchange of molecules among cells through permeable junctions formed at sites of cell contact. The phenomenon is alternatively termed “contact feeding” and is different from cross feeding, where material is transferred from one cell to another via the extracellular medium. This chapter reviews the various techniques used to demonstrate metabolic cooperation and the evidence that these techniques share a common mechanistic basis. It focuses on the techniques based on the phenotypic modification of cells consequent upon cell contact. The chapter also discusses the properties of permeable junctions, the factors controlling their formation, and the possible functions of metabolic cooperation in vivo . Metabolic cooperation has features that make it an attractive candidate for the transmission of many kinds of intercellular signals.

The Cells of the Gastric Mucosa

Abstract: Publisher Summary This chapter describes the structure of the normal adult mammalian gastric mucosa, The interior wall of the gastric mucosa is thrown into folds, rugae, or plicae gastricae, which as a rule are longitudinally oriented and sometimes branching. These folds are most prominent along the lesser curvature and are more marked in the empty stomach than in the filled one. Minor furrows divide the surface of the mucosa into irregularly outlined gastric areas, which are a few millimeters in diameter. A very large number of funnel-shaped gastric pits, foveolae gastricae, can be seen all over the mucosal surface; quite often, these pits are interconnected by tiny grooves. The gastric mucosa is divided into three different zones: the cardiac zone, the fundus-corpus zone, and the pyloric zone. These zones differ from each other with respect to the depth of the gastric pits and the organization of their glands.

The Chloroplast Endoplasmic Reticulum: Structure, Function, and Evolutionary Significance

Abstract: Publisher Summary This chapter discusses the structure, function, and evolutionary significance of the chloroplast Endoplasmic Reticulum (ER). In majority of the classes of algae a unique association exists between the rough endoplasmic reticulum of the cytoplasm and the cell's chloroplasts. The cell's chloroplasts are completely enclosed by a continuous sheet of ER that has ribosomes on its outer surface. A layer of tubules and vesicles, the periplastidal reticulum is present in the narrow space between the double-membraned chloroplast envelope and the nuclear envelope. Some algae have a specialized cytoplasmic ER cisternum that is continuous with chloroplast ER. The main function of the chloroplast ER is to synthesize nuclear-coded proteins that are destined for the chloroplast. The chapter discusses the intriguing question of biochemical composition of the nucleomorph, and if it is the vestigial nucleus of a chloroplast-containing eukaryotic symbiont. ER cisternae that are, at times, associated with the plastids of bryophytes and vascular plants are very different from chloroplast ER in algae. The clue to the theory of evolution of chloroplast ER is the nucleomorph of cryptomonads. The chapter proves that the chloroplast of cryptomonads arose from a eukaryotic endosymbiont; and the nucleomorph has the major characteristics of a nucleus.

Microtubule-Membrane Interactions in Cilia and Flagella

Abstract: Publisher Summary This chapter discusses the microtubule-membrane interactions in cilia and flagella. Cilia and eukaryotic flagella are specialized organelles that project from the cell surface. They are responsible for the movements of whole cells and for the movements of materials across cell surfaces. All eukaryotic cilia and flagella possess the same uniform substructure of nine doublet microtubules (the outer doublets) surrounding two single central microtubules. The microtubule-membrane bridges, along the long axis of the doublet microtubule, are the second major sites to which membranes are attached. These bridges are responsible for the attachment of the doublet microtubules to the ciliary necklace and ciliary granule plaques at the ciliary base, to accessory fibers in sperm, protozoans, and ctenophores, to extraciliary structures such as mastigonemes, and to adjacent ciliary or plasma membranes as in ctenophores, mussel gill laterofrontal cilia, and in trypanosomes. The morphological studies of cilia in a wide variety of organisms revealed that the microtubules comprising the axonemes are attached to the membrane where they are enveloped.

Nucleus-Associated Organelles in Fungi

Abstract: Publisher Summary This chapter discusses the nucleus-associated organelles (NAOs) of fungi. In many fungi, at various stages in the life cycle, the nuclei are associated with Golgi bodies, flagella root systems, mitochondria, microbodies, and nuclear caps. Nucleus-associated organelle is a variously shaped, primarily osmiophilic structure, which is located outside of, or within, the nuclear envelope and typically lies at the spindle poles during nuclear division. Fungal NAOs are involved with three main activities; nuclear division, nuclear movements including karyogamy, and spore delimitation. Centrioles are involved in flagellum production and the consequent cell movements. The NAO is involved in both mitosis and meiosis; the replicated NAOs separate and the spindle develop between them such that there is always at least one NAO at each spindle pole. The NAO commonly undergoes some morphological change during the transition from interphase to mitosis and meiosis. One of the most common changes is enlargement, which is most clearly seen in basidiomycetes, such as Boletus and Trametes .

Regulation of the Cell Cycle in Eukaryotic Cells

Abstract: Publisher Summary This chapter examines several models and mechanisms that are proposed to describe the regulation of the cell cycle in eukaryotic cells. It discusses how cell growth is coordinated with cell division and how entry into S phase is controlled in cycling cells. The chapter also describes the ways that the cell cycle is slowed down or arrested in quiescent cell populations. It focuses on the regulation of the mammalian cell cycle. Studies with yeast are discussed; they contribute to the interpretation of data concerning the mammalian cell cycle. The mitotic cell cycle of Saccharomyces cerevisiae can vary from seventy-five minutes to nine hours depending on the cell strain and the culture conditions. For a given cell strain, the G1 interval between nuclear division and initiation of DNA synthesis is quite variable in length. The minimum length of G1 is difficult to measure precisely, but several experiments indicate that it is less than 10–12 minutes.

Ultrastructure and Biology of Female Gametophyte in Flowering Plants

Abstract: Publisher Summary This chapter describes the ultrastructure and biology of female gametophyte in flowering plants The ovular primordium is generally delimited into dermatogen, one- or two-layered subdermatogen, and a central corpus of cells arranged in longitudinal rows The archesporial cell differentiates in a hypodermal position at the summit of a central column of nuclear cells because of the propitious nutritive conditions in this region Megasporocytes of Lilium candidum exhibit a peculiar behavior of endoplasmic reticulum (ER) During young stages, the membranes are sparse, but soon, plenty of them form parallel strands around the nucleus, aggregate, and then become spirally or concentrically twisted Subsequently, the lamellae disintegrate into small vesicles, which are freely scattered in the cytoplasm The megasporocyte wall differs from adjacent nuclear walls in having rich distribution of randomly oriented fibrillar material In Lilium, small vesicles and tubules containing electron-dense material occupy the space between the cell wall and plasma membrane These structures, called “paramural bodies,” increase during the maturation of megasporocyte and are involved in cell wall synthesis Besides, the paramural bodies store polysaccharides for subsequent use

Tight Junctions in Arthropod Tissues

Abstract: Publisher Summary This chapter describes the tight junctions in the arthropod tissues. The tight junctional particles are aligned in ridges shared by fusion of the plasma membranes of adjacent cells, thereby leading to closure of the intercellular space. Tight junctions or zonulae occludentes provide a seal among the epithelial cells, thereby creating a diffusion barrier to the intercellular movement of ions and molecules. The en face views of membranes are revealed, and then the intramembranous patterns assumed by junctional particles may be studied using the freeze-cleaved replicas. The plasma membrane of vertebrate cells is divided by circumferential junctions that establish and maintain cellular polarity. Tight tight junctions have a complex reticulum of tight junctional ridges, while leaky tight junctions have fewer strands and a less complex network. In the arthropods, which exhibit many physiological differences from vertebrate species, there has been a search for tight junctions in tissues known to possess permeability barriers. Analysis of the development of tight junctions can occur in vivo only during embryonic and very early hatchling stages in arthropods or possibly during the regeneration of injured tissues. The chapter also summarizes the features that various junctions share with tight junctions, together with the respects in which they differ.

Membrane ultrastructure in urinary tubules.

Abstract: Publisher Summary This chapter describes the freeze-fracture ultrastructure of the plasma membrane of epithelial cells at the level of the glomerulus, proximal tubule, loop of Henle, distal tubule, and collecting duct. The chapter provides a qualitative description of the membrane faces and junctional differentiations. A quantitative analysis of the number and size of intramembrane particles representing protein-containing structures in the membrane is also provided. Freeze-fracturing involves freezing a tissue specimen at a low temperature of -150°C, fracturing the frozen specimen with a cooled microtome blade, and then shadow-casting the frozen and fractured surface with platinum and carbon. The systematic application of the freeze-fracture technique to the kidney revealed an impressive mosaic of membrane structure among the epithelial cells lining the urinary tubules. There are variations in the density, size, and shape of intramembrane particles among membrane domains of the same cell, among different cells, and in the degree of development of intercellular junctions between cells.

Polytene Chromosomes of Plants

Abstract: Publisher Summary This chapter describes polytene chromosomes of plants. Polytene chromosome structures occur in highly endopolyploid interphase nuclei. They are cable-like bundles of sister chromosomes that are the result of endocycles, that is, chromosome-replication cycles without nuclear division. The chapter reviews the findings of polytene chromosomes in plants and their development and structure in comparison with giant chromosomes in Diptera. The functional and evolutionary aspects of polyteny are also discussed. Polytene chromosomes help to elucidate the biological significance of non-protein-coding DNA. Within the cell nucleus, the huge amount of never expressed DNA exhibits different conformations due to its sequence diversity that is reflected by conformational differences of the chromatin. Thus, different chromatin domains, or polytene chromosome bands, can react to different electrodynamic environments, and hence release specific patterns of domains into active conformation. Polyteny can be utilized as a model system for understanding the role of chromatin patterns in determining the direction of differentiation. Plant polytene nuclei with their high structural variability are useful in the study of the interrelationships among environment, DNA content, cell function, and in relation to the key role of chromatin organization.

Gene expression and cell cycle regulation.

Abstract: Publisher Summary The ability to grow and divide is a fundamental property of living organisms. The duplication of cellular contents and their distribution to two daughter cells is an orderly process of changes in the structure and function of cellular constituents, which is coordinated and controlled by a sequential genetic program. The major control point for the initiation of this sequence lies in the period prior to DNA replication in most cell types and may be defective in transformed and tumorigenic cells. Cells growing in vitro usually arrest in the prereplicative phase in response to high population density or serum or nutrient deprivation. This quiescent G 0 state is characterized by decreased metabolic activity, in particular, a decrease in ribosome content and protein synthetic capacity and prolonged viability in the absence of growth and division. A return to the division cycle may be initiated by supplying the missing nutrient or by surface acting agents including polypeptide growth factors, carbohydrate binding plant lectins, and proteases and other enzymes. These agents initiate a series of surface and cytoskeletal rearrangements and transmit a proliferation signal to the interior of the cell.

Genetics and Aging in Protozoa

Abstract: Publisher Summary This chapter discusses the genetics of aging in protozoa. Protozoa can exhibit a finite growth potential and can escape extinction by fertilization and formation of successive generations. Aging in ciliates exhibits predictable, progressive, species-specific rates of decline in function and, finally, death. In the absence of fertilization, the cells undergo repeated cell divisions and produce cells that are progressively less vigorous than the progenitor cell. Fertilization in ciliates represents a risk that can result in immediate death, reduced life span, or resetting the age clock to zero. The outcome of the process is dependent on breeding behavior, individuals participating in the cross, and their ages. Protozoa present an untapped reservoir for an evolutionary approach to the identification of fundamental mechanisms that are utilized in the assurance of longevity. Similarities between age-associated changes in protozoa and multicellular organisms are discussed, and the similarities among unicells and multicells are described. Molecular mechanisms that can direct the determinations of gene function are emerging rapidly. The presence of repressors or derepressors in the cytoplasmic environment of the developing nucleus can turn on or off certain genes, and, once activated or repressed, perpetuate that state.

Morphological and Biochemical Aspects of Adhesiveness and Dissociation of Cancer Cells

Abstract: Publisher Summary The cell surface-associated adhesive factor from tumor cells may play a key role in tumor cell adhesiveness. The factor can be synthesized by well-differentiated tumor cells (characterized by development of junctional complexes) but not by undifferentiated tumor cells. This finding suggests that this adhesive factor may represent an expression of tumor cell differentiation. The emphasis is placed on the hepatoma cells of rat. Its demonstration in embryonic rat liver cells suggests that this adhesive factor might be one of carcinoembryonic proteins; the factor is also associated with the embryonic cell adhesiveness (characterized by development of junctional complexes). The adhesive factor, when characterized, may provide an important model system in the study of the phenomenon associated with gene expression. Any adhesive substance associated with adhesiveness in adult rat liver cells can be separated from the cells, because the cell-to-cell contact is also characterized by development of junctional complexes.

Endosperm—Its Morphology, infrastructure, and Histochemistry

Abstract: Publisher Summary This chapter discusses the morphology, ultrastructure and histochemistry of endosperm. Endosperm is formed by the repeated divisions of the primary endosperm nucleus. It is the main source of food for the embryo in angiosperms. The central cell is a highly active cell that performs several important functions in the embryo sac. The fusion of the secondary nucleus or the polar nuclei with the sperm nucleus gives rise to the primary endosperm nucleus, which is generally triploid. The primary endosperm nucleus is associated with aggregates of dense material in the perinuclear cytoplasm. The primary endosperm nucleus divides rapidly to form the endosperm tissue. The endosperm nuclei increase in size as the development proceeds. In angiosperms there are three types of endosperm development— namely, nuclear, cellular, and helobial. The mature endosperm in some plants shows rumination that is caused by the seed coat activity or by the endosperm itself. The endosperm haustoria are of widespread occurrence in angiosperms. The haustorial cells undergo enlargement and aggressive growth during development that results in the destruction of ovular tissues adjacent to the invading haustoria. When endosperm cells are actively dividing, the haustoria become involved in the nutrition of the endosperm cells. The cytoplasm of the aleurone cell is characterized by many small to large electron-transparent osmiophilic lipid bodies/droplets, also known as, spherosomes. The reserve materials of endosperm tissue undergo autolysis to yield simpler components that are used up during the growth and germination of embryo.

Cycling ⇄ Noncycling Cell Transitions in Tissue Aging, Immunological Surveillance, Transformation, and Tumor Growth

Abstract: Publisher Summary This chapter discusses the model for cell and tissue proliferation, which interrelates the problems of tissue aging, immunological surveillance, transformation, and tumor growth. The chapter illustrates diagrams that depict cycling and noncycling cell proliferative transitions as they apply to the problems of aging, immunological surveillance, transformation, and tumor growth. It also describes the cycling to noncycling cell transitions, which lead to the establishment of normal tissues composed of four major categories of cycling, noncycling G 1 -, G 0 -, and G 2 -blocked cells, which are arrested at different temporal and biochemical points in the G 1 and the G 2 periods of the cell cycle. Tissue proliferative aging and immunosenescence (age-related decline in immune function) are depicted as being caused by impaired release of noncycling cells to the proliferative cycling state. A primary tumor may arise from the immunological release and proliferation of preexisting, previously transformed dormant noncycling tumor cells, which had been held in restraint by immunological suppression.

The Diptera as a Model System in Cell and Molecular Biology

Abstract: Publisher Summary The cells of Diptera have been used as a model system in a wide variety of experiments designed to probe the organization of the eukaryotic genome. The cells of these organisms lend themselves readily to both cytological and biochemical investigations using specific techniques, such as light and electron microscopic autoradiography, immunofluorescence, salt or sucrose density gradient and gel electrophoretic analyses, hybridization in situ, and molecular cloning-recombinant DNA studies. This chapter reviews the research carried out in past years using dipteran model systems, specifically three families—the Drosophilidae, the Chironomidae, and the Sciaridae. Six separate areas of research are considered. They are chromatin structure, middle repeated DNA (MR DNA), highly repeated DNA (HR DNA), satellites, and heterochromatin, puffs, heat shock, and mechanisms of gene control: the nonhistone proteins.

Protoplasts of Eukaryotic Algae

Abstract: Publisher Summary This chapter describes the protoplasts of eukaryotic algae and discusses the advantages and problems of working with them. Protoplast is a viable cell whose wall and other materials external to the plasmalemma have been removed, but it retains all internal components. Algae, from which the protoplasts are induced, are always grown as liquid cultures. There is no uniformity among the media and all are chemically undefined, containing either seawater or soil extracts, and sterilized by filtration or autoclaving. The exogenous hydrolytic enzymes are the main inducers of protoplast formation. The induced protoplasts of fresh-water algae lose the mechanical barrier that maintains their internal osmotic pressure. They retain their integrity and viability in an osmotically protective medium of equal or greater tonicity than that of the normal internal cellular osmolarity. Protoplast viability is assessed by the exclusion of vital dyes by living cells. The dyes most commonly used are trypan blue, crystal violet, neutral red, and fluorescein diacetate. Regeneration of an outer wall followed by cell division is the desired outcome of most protoplast experimental work. Protoplast formation and regeneration are theoretically ideal tools for the understanding of cell wall structure and formation.

The Relationship of in Vitro Studies to in Vivo Human Aging

Abstract: Publisher Summary This chapter discusses the relationship of in vitro studies to in vivo human aging. Human diploid fibroblasts have a limited proliferative capacity when placed into tissue culture. This limited life span of fetal human fibroblasts has led to their extensive employment for studying human cellular aging. Cells at early passage are designated as “young” and at late passage as “old” or “senescent.” These cells are used as a model for human aging. There are several approaches to examine whether alterations in cells in culture reflect human aging in vivo. Comparison of in vivo and in vitro fibroblast populations is currently not possible because these cells are difficult to identify and isolate in vivo. This approach may be possible in the future with certain differentiated cell types, such as epidermal keratinocytes where in vivo identification of the cell is possible. The chapter examines human diploid fibroblasts derived from donors of different ages at the same level of early in vitro passage to determine if the alterations observed as a function of in vitro serial passage (in vitro aging) occur as a function of the age of cell culture donor (in vivo aging).

Cell Interactions and the Control of Development in Myxobacteria Populations

Abstract: Publisher Summary This chapter discusses the cell interactions and the control of development in myxobacteria populations. Although the myxobacteria are prokaryotic in cell structure, chemistry, and mode of cell division, the cell population itself engages in cell-cell interactions and behavioral responses resembling those of eukaryotic organisms. Several aspects of cell behavior are accessible to genetic and biochemical analysis in the myxobacteria. These include cell-cell signaling both by diffusible molecules and via cell contact, cell-cell recognition, programmed changes in cell cohesiveness, photomorphogenesis, and the regulation of cell differentiation. Under the appropriate conditions of nutrient depletion, cells aggregate by gliding into numerous aggregation centers, preparatory to the construction of fruiting bodies. Cell differentiation may begin very early during starvation, perhaps at or near the beginning of the aggregation period. The aggregating cell expresses new phenotypic characteristics consistent with functions required for aggregation, and for rnyxospore development. Myxospores are formed in the dark than in the light, and their appearance in the dark coincides with the formation of the ridges. The guanosine polyphosphates serve as intracellular signals of nutrient limitation but may also be the triggers for the initiation of development.

Chloroplast DNA Replication in Chlamydomonas reinhardtii

Abstract: Publisher Summary Chlamydomonas reinhardtii occupies a central position in cytoplasmic inheritance because it is the simplest organism that contains a chloroplast and non-Mendelian inheritance. Being unicellular and heterothallic, Chlamydomonas can be handled in the laboratory with the same ease as bacteria and yeast. The vegetative, haploid life cycle and the diploid meiotic sexual cycle can be physiologically controlled in large synchronous populations to facilitate either cell cycle biochemistry or genetic analysis. This chapter discusses the replication of the chloroplast DNA in vivo. Chloroplast replication usually is initiated 1–3 hours after the onset of the light period and continues for six hours, whereas nuclear DNA synthesis is initiated 2–3 hours into the dark period and continues for six hours. If cell division results in the production of four daughters, then two rounds of replication of chloroplast DNA synthesis precede two rounds of nuclear DNA synthesis. Both DNAs replicate by a semiconservative mechanism. Chlamydomonas does not appear to have dark repair reactions, so the chloroplast chromosome is stable over the entire cell cycle

The role of phosphorylated dolichols in membrane glycoprotein biosynthesis: relation to cholesterol biosynthesis.

Abstract: Publisher Summary The participation of certain phosphorylated long-chain polyisoprenols (dolichols) as saccharide carriers and donors in the biosynthesis of complex carbohydrates has been among the most actively pursued discoveries in the field of membrane biosynthesis and glycoprotein metabolism in the last decade. Dolichols are major long-chain polyisoprenols found in animal cells. They mediate the assembly of asparagine-linked oligosaccharide units of exported and intracellular glycoproteins. This chapter focuses on the animal cell, particularly the cultured vascular smooth muscle cell. The chapter reviews the aspects of glycoprotein structure, the participation of phosphorylated dolichols in glycoprotein biosynthesis, and the interdependent regulation of biosynthesis of phosphorylated dolichols and cholesterol. HMG-CoA reductase is an inducible enzyme that, under steady-state cultural conditions, is in the suppressed state and behaves as a classic nonequilibrium enzyme. It has a rapid turnover rate and its activity is barely detectable in vitro. In some tissues it undergoes covalent modification by phosphorylation/dephosphorylation as a means of modulating its activity. These characteristics of HMG-CoA contribute to its position as a rate-controlling enzyme in polyisoprenol biosynthesis. The increased flux of substrates through this enzyme is useful in discerning regulatory enzymatic steps secondary to the reductase and specific to the pathways of cholesterol or other polyisoprenols such as the dolichols.

Insulin binding and glucose transport.

Abstract: Publisher Summary This chapter discusses the insulin binding and glucose transport. The insulin binding is explained with the help of recent data on cellular localization, cooperativity, regulation of receptors and receptors in normal and transformed cells. The action of insulin starts with the binding of the hormone to specific receptors on the plasma membrane of target cells. Studies of insulin receptors require a satisfactory reagent probe, one with the same biological properties as the native insulin. The monoiodinated insulin has a biological activity similar to native insulin, based on stimulation of glucose oxidation by fat cells. For glucose transport, the chapter reviews various regulatory aspects, such as cell growth, adaptation, and hormonal influences in normal and neoplastic cells. The chapter focuses on the mechanism and regulation of glucose transport in cells where glucose is transported by facilitated diffusion. This process is mediated by a specific glucose carrier but does not require energy; therefore, the direction of glucose flux is in accordance with its concentration gradient. The most common experimental design for measuring rates of substrate transport is the zero-trans influx procedure.

Integration of oncogenic viruses in mammalian cells.

Abstract: Publisher Summary This chapter discusses the integration of oncogenic viruses in mammalian cells. Cells transformed by either RNA- or DNA-containing oncogenic viruses retain the genomes of the transforming viruses and, generally, express antigens, coded by the viral genome. The genomes of oncogenic viruses can integrate in different sites in the same or different chromosomes in different viral-transformed cells. It is not clear, however, whether the viral genomes integrate into specific sites or at random in the mammalian cell genome. Nucleotide sequencing of the cellular DNA sequences flanking the viral integration sites and the use of recombinant plasmid DNAs containing a selectable gene (thymidine kinase) and the tumor virus genome should result in a better understanding of the molecular mechanism of the integration of oncogenic viruses in the cellular genome.

The structure and functions of the mycoplasma membrane.

Abstract: Publisher Summary The plasma membrane is the site of a great number of essential functions in mycoplasmas, as it is with all organisms, but the membrane assumes an even greater importance with mycoplasmas because of their simplicity and lack of a cell wall. This chapter describes the structure and functions of the mycoplasma membrane. Many mycoplasmas are pathogenic, disease being caused in a number of animal, plant, and insect hosts. Mycoplasma membranes are of particular interest not only because of their involvement in disease because of adhesion to host cells, but also because they provide a system that is most suited to many investigations into the structure and function of biomembranes. Two principal reasons explain their suitability in biomembrane research. First, mycoplasmas grow in synthetic culture media and controlled alterations in membrane composition can be introduced by manipulation of the medium content. Second, mycoplasma membranes are easily prepared free from other cellular material. The composition of a membrane can be altered by its method of isolation. Mycoplasma membranes contain all the cell lipid and one-quarter to one-half of the cell protein. The gross chemical composition of the membranes varies with species but generally falls within the range of 50–60% protein, 30–40% lipid, and 1–3% carbohydrate.

On the nature of oncogenic transformation of cells.

Abstract: Publisher Summary This chapter presents a class of genes in the genome of higher cells, which is called “growth genes.” The existence of the growth genes in multiplicity in a genome is envisaged as necessitated by the participation of each growth gene in a distinct pathway or related pathways of differentiation. The primary function of growth genes is to elicit cell growth; they are also intricately involved in programs of gene expression, which operate during development of the organism. After transient expression during development, they become permanently repressed for the remainder of the organism's life cycle except in certain cases of tissue regeneration and cell renewal. Given the proper conditions, postdevelopmental expression of any growth gene in a cell can lead to oncogenic transformation. To consummate the oncogenic process, mutations alone do not appear to be sufficient. Additional steps involving genomic reprogramming are necessary.

Histone gene expression: hybrid cells and organisms establish complex controls.

Abstract: Publisher Summary Complexes of histones with DNA dictate the fundamental structure of chromatin in eukaryotic cells. As visualized by electron microscopy, this structure is monilifom (such as, beads on a string) and consists of a self-associating histone complex around which DNA is wrapped. The histone complexes may package DNA in a manner that is protective yet permissive of both replication and transcription. Alternatively, the association of the histone complexes with DNA may be so precise as to control the transcription of specific genes. Whether one chooses to view the biological role of histones as nearer one or the other of these views, it appears that the DNA of eukaryotic cells is packaged and that histones are indispensible for the packaging process. The genome structure and function are related. There is an evolutionary conservation of a specific number of chromosomes with certain characteristic morphological features in a given species. The addition or deletion of chromosomes, or the unbalanced translocation of parts of chromosomes, often results in specific pathological states.