Showing papers in "International Review of Cytology-a Survey of Cell Biology in 1984"

The muscle satellite cell: a review.

Abstract: Since the first reports of satellite cells in 1961, considerable knowledge has accumulated concerning their phylogenetic distribution and their location, morphology, and function. There is no doubt that satellite cells are capable of undergoing mitosis and that they have considerable motility. These cells function as the progenitors of the myofiber nuclei that must be added during normal (postnatal) growth of muscle. In muscle undergoing or attempting to undergo regeneration, the satellite cell functions as a myogenic stem cell to produce myoblasts that line up and fuse within the scaffolding of the remnant basal lamina or migrate into the interstitium to produce neofibers . A number of problems remain to be solved concerning the regulation of satellite cell function. At this time it is equivocable whether or not the presumptive myoblast and the satellite cell are functionally identical and at the same stage of myogenic differentiation. Apparently there is species variation in terms of the ability of myotubes from embryonic myogenic cells and satellite cells to synthesize protein. The mechanism(s) by which a wide variety of stimuli activate satellite cells is not known, nor is the mechanism(s) by which satellite cells become inactive during the latter stages of growth and adulthood known. Mitogenic factors are present in damaged muscle; but the specific characteristics of these factors and their mechanism of activation are also unknown. Hormones are certainly involved in the regulation of proliferation and differentiation of myogenic cells, but whether presumptive myoblasts and satellite cells or their myotubes respond similarly to hormones in culture has not been adequately examined. Greater understanding of these mechanisms will increase the possibility of total muscle recovery from severe injury or disease. Such knowledge would also have particular application to the production of meat animals and to a greater understanding of the growth process in general.

Development of the Cotton Fiber

Abstract: Publisher Summary This chapter discusses the various facets of cotton-fiber development and focuses on some currently critical areas of investigation. In addition to its commercial importance, cotton fiber has several attributes that make it an experimental system of choice for the investigation of physiological and biochemical changes accompanying cell elongation and/or maturation. The fiber originates and ends as a single cell, and thus elongation can be studied free of any complications from cell division. The variability in fiber length and secondary wall thickness of fibers among different cotton varieties depends upon the interplay of a complex pattern of metabolic processes and regulating mechanisms that govern fiber growth. The in vitro methodology of cotton ovule culture holds considerable promise for investigating the effects of a number of plant growth substances on cell growth. The developing cotton fibers provide a rich source of homogeneous single cells, which should be preferred for structural and biosynthetic studies of the cell wall.

Cell Biology of Trypanosoma cruzi

Abstract: Publisher Summary Among the protozoa of the Trypanosomatidae family, a large number of species represent agents of diseases, such as Chagas' disease. This chapter reviews some aspects of the cell biology of Trypanosoma cruzi, giving emphasis to those aspects related to the ultrastructure of pathogenic protozoa. Protozoa of the Trypanosomatidae family show, during their, life cycle, several forms which can be easily identified by light microscopy in Giemsa-stained preparations. The chapter also explains the life cycle of T. cruzi. In the life cycle of T. cruzi, there are forms which are able to divide. There is one form, considered to be highly differentiated and responsible for the infectivity of these protozoa, which does not divide. It is highlighted that the trypomastigote form can transform into a rounded form which possesses a free flagellum. This form, which appears in the stomach, is able to transform into either short epimastigotes that start a process of multiplication in the intestinum or into long epimastigotes which move to the more posterior region of the digestive tract of the bug. Cell surface is also emphasized in the chapter.

Capping and the Cytoskeleton

Abstract: Publisher Summary This chapter summarizes the results obtained in capping/cytoskeleton research. There are two types of capping phenomena: ligand-dependent and ligand-independent processes. A number of different reagents, such as colchicine and hypertonic media, can cause the formation of cap structures in the absence of any externally added ligand. Morphological studies, using both transmission and scanning electron microscopy, of cells forming caps in hypertonic medium have identified a strong association between cap structures and microvilli. External ligand (antibody, lectin, or some hormones) mediated receptor patching and capping is responsible for important immune-related responses: ( 1) proliferation and differentiation of the cells into antibody-secreting plasma cells, (2) the development of an increased number of precursor cells which are responsible for immunological memory, and (3) the activation and proliferation of T cells which carry out cell-mediated cytotoxic killing. In nonimmune cells, capping may also be involved in important cellular processes, such as endocytosis, chemotaxis, mitogenesis, and general cell–cell recognition.

Maturation and Fertilization in Starfish Oocytes

Abstract: L'ignorance qui estoit naturellement en nous, nous l'avons, par longue estude, confirmee et averee. (MONTAIGNE)Sorrow is knowledge; they who know the most Must mourn the deepest o'er the fatal truth The Tree of knowledge is not that of Life. (BYRON)

DNA sequence amplification in mammalian cells.

Abstract: Publisher Summary This chapter describes DNA sequence amplification in mammalian cells. The synthesis of a chromosomal DNA fiber occurs through the agency of thousands of tandemly arranged replicons, each of which usually functions only once in a given S period to ensure that the fiber is exactly duplicated along its entire length. The result of this process is that the two identical daughter chromatids lay side-by-side, connected by a centromere, until the separation of chromatids occurs at mitosis and the ploidy of each daughter cell is restored to the original configuration. There are exceptions to this mode of replication, however, in which, parts of chromosomes or the entire chromosomal complement are rereplicated prior to a cell division event, with the consequence that the genetic constitution of the cell can be increased in total, or only at selected loci. This chapter illustrates examples of selective DNA amplification in both prokaryotic and eukaryotic systems. It discusses the cytological manifestations of gene amplification and the stability of the process.

Toward a dynamic helical model for the influence of microtubules on wall patterns in plants

Abstract: Publisher Summary This chapter marshals evidence that leads to discuss hypotheses on relevant microtubule behavior. The predominating impression in the chapter is that microtubules and nascent microfibrils are parallel; where they are not, this can be attributed to reorientations preparing the way for a new wall lamella. The molecular basis of parallelism is pursued but the parallelism is, regardless of mechanism, insufficient to explain the diversity of wall textures that are encountered. This is partly because there is a discrepancy of scale: wall textures being discussed in micrometer terms of entire cell lengths but models for the transmembrane parallelism usually being constructed over the narrower range which is measured by the nanometers that separate the two major elements. The chapter further discusses microtubule patterns and the patterns of cellulose in algae.

The modeling approach.

Abstract: Publisher Summary Simple (steady-state) models can aid in the interpretation of radioactive tracer data, provide information on reaction rates in vivo , and make predictions about intracellular and intercellular compartmentation. More complex (transition) models can be used to integrate, over time, data on enzyme mechanisms, metabolite concentrations, and flux. These models can help define those events, which are most critical to a particular metabolic transformation, for example, in development, aging, malignancy, dimorphic fungi, or a fermentation process. The modeling approach is also useful in the analysis of specific areas of metabolism in higher animals, for example, glycogen metabolism in liver or glycolysis in the perfused heart. This chapter summarizes the data required for and the kind of information obtained from a steady state model and compares these to a transition model.

Neuronal secretory systems

Abstract: Publisher Summary This chapter discusses the neuronal secretory systems to critically examine the processes in the magnocellular neurons, and considers to what extent they are variations of Palade's model, and whether they display any unique characteristics, particularly in comparison with other types of neurons. Vasopressin (VP) and oxytocin (OT) are designated as neurohypophysial hormones, for the hypothalamic neurohypophysial system (HNS), which was the first neuronal secretory system in which they are identified in profusion. The specific enzymes could vary the cleavage products of propressophysin and prooxyphysin in different central nervous system (CNS) sites. In addition, modifying influences are probably exerted by the different co-peptides and co-transmitters within VP and OT neurons. The rich efferent input from other neuronal systems, the feedback relays, and the selective coordination between neurosecretory cells contribute to the extended repertoire of the so-called neurohypophysial peptides. The diversity and versatility of the peptidergic neurosecretory cells that produce VP and OT reveal affinities with other neurons in the CNS.

Cellular Organization for Steroidogenesis

Abstract: Publisher Summary This chapter provides an overview of cellular organization for steroidogenesis. There are two approaches: purification and reconstitution, which resemble the classical steps of organic chemistry—namely, isolation, and synthesis. It is clear that the methods of cell biology must be applied to steroidogenesis—that is, it is necessary to understand how the steroidogenic cell is organized in such a way as to permit the steps of steroidogenesis to proceed in an orderly fashion, so that a regulated output of the completed hormones can be achieved. In reviewing the regulation of steroidogenesis, the chapter points out two major factors—the properties of individual enzymes and their arrangement in lipid membranes; these are important in the microsomal process. The major steroidogenic organs are the adrenal cortex, testis, ovary, and placenta. Although each organ produces its own characteristic profile of steroid hormones, the enzymes involved and the organization of the process are fundamentally similar in every case. Differences in the hormones produced by the organs result from the differences in the amounts of the various steroidogenic enzymes. The chapter also discusses the organization of the steroidogenic cell.

Membranes in the Mitotic Apparatus: Their Structure and Function

Abstract: Publisher Summary Membranes are universal components of the mitotic (or meiotic) apparatus (MA). The nuclear envelope (NE) is present in cells of all eukaryotic organisms from the most primitive to the most advanced and it becomes closely associated with the spindle fibers during their formation and/or function. In addition to the NE, there are other membranes—the endoplasmic reticulum (ER) and the Golgi apparatus that occur in the MA of many different cells. During the process of mitosis, these membranes undergo transformations and changes in disposition that correlate with the formation and function of the microtubular spindle fibers. During prophase, the nucleus, although surrounded by an envelope, is devoid of membranes. However, upon breakage and dispersal of the NE, in cells of higher eukaryotes, membranes enter the nuclear region, and some elements accumulate at the spindle perimeter and create a boundary between the MA and the rest of the cytoplasm. This chapter highlights the structural contribution of membranes to the MA and discusses the variability in this morphology that exists in different cell types.

Protein diffusion in cell membranes: some biological implications.

Abstract: Publisher Summary This chapter discusses the biological implications of protein diffusion in cell membranes. Current efforts to capture and quantitate the motion of proteins in membranes are predicated on the assumption that this phenomenon has important biological implications. The chapter reviews the bases of that assumption and discusses how passive diffusion of membrane proteins contributes directly to certain biological functions. It explains how cells might deal with the potentially negative consequences of protein lateral motion. The chapter also explains how lateral diffusion of membrane proteins may directly facilitate a number of membrane-mediated biological functions, including self-assembly, cell–cell recognition and adhesion, enzymatic reactions, hormonal response, and other signaling processes. It highlights the fact that the diffusion of a membrane protein is a reflection of the fluid lipid environment in which it resides.

Membrane Heterogeneity in the Mammalian Spermatozoon

Abstract: Publisher Summary This chapter discusses membrane heterogeneity in the mammalian spermatozoon. Mammalian spermatozoa, after leaving the testis, are subjected to a sequence of environmental changes involving passage through the epididymis, temporary storage, and further transport through the female reproductive tract. Testicular spermatozoa, or spermatozoa isolated from proximal regions of the epididymis, are unable to participate in fertilization, but because of their epididymal transport, those from the cauda epididymidis or from an ejaculate can be induced to fertilize an egg. Epididymal transit confers only the potential to participate in fertilization, further modifications being necessary before gamete fusion occurs. In addition, spermiogenesis moulds the spermatozoon into the highly specialized shape required for reaching and penetrating the egg; upon completion of this initial stage of development, the main structural characteristics of the sperm cell are readily apparent. In essence, the spermatozoon consists of two functionally independent compartments—the head and the tail. The head contains all the genetic material and carries an apparatus—the acrosome—for penetrating the egg investments and fusing with the vitelline membrane. Structural differentiations in sperm membranes are highlighted in the chapter.

Endocytosis and exocytosis: current concepts of vesicle traffic in animal cells.

Abstract: Publisher Summary Animal cells have specific pathways to transport macromolecules from their surrounding environment to their interior and from internal compartments to the cell surface or other intracellular locations; many of these movements appear to be receptor-dependent processes in which specific membrane receptors bind macromolecules, segregate them into discrete membrane-limited compartments, and move the molecules to new locations. Such processes include the clustering and internalization of receptor-bound ligands at the cell surface in clathrin-coated pits, the formation of endocytic vesicles (receptosomes) from coated pits, the movement of receptosomes by saltatory motion to the Golgi system, the concentration of materials in the coated pits of the Golgi system that are destined for delivery to lysosomes, and the directed traffic of materials destined for exocytosis out of the Golgi to the cell surface. This chapter describes some experiments, which have led to current understanding of the various organelles involved in traffic and some of the biochemical mechanisms involved.

Pollen—Pistil Recognition: New Concepts from Electron Microscopy and Cytochemistry

Abstract: Publisher Summary This chapter elucidates the nature and characteristics of pollen information and pistil read-out systems Pollen information is generally regarded as being housed within the two domains of the pollen wall—that is, in the pollen coat or within the intine polysaccharide matrix On the female side, there is increasing evidence that the pellicle of dry-type stigmas, the remarkably plastic surface layer, appears to have many of the properties of a membrane, if not its structural characteristics These two surface layers provide the possibility for direct contact and interactions These new concepts in plant cell biology are especially valuable in interpreting several techniques used empirically in plant breeding to overcome incompatibility responses These methods involve modifications to the surfaces of pollen or pistil partners, either by physical or chemical treatments

Effect of microtubule inhibitors on invasion and on related activities of tumor cells.

Abstract: Publisher Summary Invasion marks the onset of malignancy in solid tumors. A consequence of invasion is metastasis leading to the death of the host in many cases. Microtubule inhibitors disturb the equilibrium of microtubule assembly/disassembly and, therefore, interfere with the structure and function of the mitotic spindle and of the cytoplasmic microtubule complex. In addition, immunocytochemistry with antibodies against tubulin—the building block of microtubules—is the method of choice to evaluate the status of microtubule complexes inside cells. Microtubule inhibitors arrest invasion of malignant cells in organ culture and in vivo. This chapter reviews the data about the effect of microtubule inhibitors (MTIs) on invasion, on directional migration, and on other cellular activities that are possibly involved in invasion. It discusses the hypothesis that MTIs inhibit tumor invasion because they interfere with directional migration through disturbance of the cytoplasmic microtubule complex (CMTC). Biochemical experiments (in the test tube), experiments with cultured cells (in vitro), and experiments using whole organisms (in vivo) are described.

The neuronal organization of the outer plexiform layer of the primate retina.

Abstract: In the primate retina at the level of the first synapse in the visual system, the outer plexiform layer, processes from 15 different types of neurons have so far been described. These are the synaptic spherules of rods, the pedicles of three spectral types of cones, dendrites and axons of two types of horizontal cell, dendrites of seven types of bipolar cell, processes of interplexiform cells, and the outwardly coursing dendritic extensions of biplexiform ganglion cells. The interconnections of these neurons as studied by electron microscopy and Golgi-EM are presented in a summary diagram (Fig. 27). Basal processes from cone pedicles contact the cone pedicles, and rod spherules forming gap junctions. The dendrites of both types of horizontal cell (hI and hII) connect only to cone pedicles and form lateral elements of triads at the ribbon synaptic complex. The HI axon terminals end as lateral elements at rod spherules while the axons of HII horizontal cells connect with cones in a manner similar to their dendrites. Interplexiform cells (ipc) do not contact either rod or cone synaptic endings. Rod bipolar cell (rb) dendrites end as central elements at the ribbon synaptic complex of rod spherules. The dendrites of flat midget (fm), flat top (fb), and giant bistratified bipolar (gb) cells all form basal junctions with cone pedicles. Ending as central elements of triads at cone pedicles are the dendrites of invaginating midget (im), diffuse invaginating cone (ib), and blue-cone (bb) bipolar cells. Biplexiform ganglion cells (bgc) connect to rods as central elements opposite the synaptic ribbon in the spherules. As compared to an earlier summary diagram of the outer plexiform layer (Kolb, 1970), the primate retina is now known not to be as simply organized as was once thought. Although our knowledge of the types of neurons contributing processes to this first synaptic layer, and the nature of their connections with other neurons has been broadened, especially within the past few years, this summary diagram is not intended to represent the complete or final "picture." Undoubtedly, future investigations along the lines of research outlined here will provide additional details to this wiring diagram so that we may better understand the processing of visual information by neurons in the retina.

Lysosomal functions in cellular activation: propagation of the actions of hormones and other effectors.

Abstract: Publisher Summary This chapter evaluates the aspects of the individual steps in the staging of vectorial pathway and assesses their potential metabolic consequences in the processing and execution of information delivered by agonist. The chapter determines from the analysis an array of effectors and target cells as possible, whether such a pattern is generalized or unique to only certain classes of effectors or target cells. The independent data and the implications of lysosomal function in propagation and coordination of transcellular events are also considered. The integration of information is derived from the analysis of interlocking problems that contributes to the understanding of critical molecular events associated with triggering of cellular responses to tropic hormones and other effectors.

Structure and Function of the Crustacean Larval Salt Gland

Abstract: Publisher Summary This chapter explores that halobionts are organisms that can live, reproduce, and complete their life cycle in concentrated brines. These organisms can be phylogenetically quite diverse, ranging from the primitive halobacteria and unicellular algae to complex vascular plants and animals. A harsh environmental habitat, such as is found in inland salt lakes and coastal salterns, usually demands that organisms evolve special adaptations to osmotic stress to maintain water and electrolyte balance compatible with their life cycle. For halobionts which seek “osmotic conformity,” life is sustained in large measure by the fact that the internal osmotic pressure within individual cells is controlled by accumulating a compatible organic solute. The chapter discusses that halobacteria achieve similar control over intracellular osmotic pressures by genetic manipulation of macromolecules that comprise the subcellular organelles. Many of the cytoplasmic proteins of halobacteria have undergone extensive amino acid substitutions, primarily in the acidic amino acids. These substitutions enable cytosolic proteins to be more functional in the presence of high intracellular inorganic osmolyte. Finally, it focuses on two classes of aquatic arthropods, the insects, and crustaceans that are among the most successful animal halobionts.

Cytology of the secretion in mammalian sweat glands

Abstract: Publisher Summary This chapter explains the cytology of the secretion in mammalian sweat glands. The mammalian skin glands are repeatedly used as material for the study of cytology of secretion. Among the different kinds of skin glands, the sweat glands are widely distributed through almost all the species of mammals and vary remarkably in fine structure according to species. The human sweat glands are best differentiated as compared with those of nonhuman mammals. The secretory portion of the apocrine sweat glands is a glomerulum composed of secretory tubules whose cross diameters are different from place to place and often form strong dilatations. The secretory tubules sometimes send out a branch, which may end blindly or anastomose to the main tubule to form a ring. In the human skin, the apocrine sweat glands in the axillary region are best developed and have the most complicated glomerulum. The wall of the secretory tubule of the apocrine sweat glands consists of two different kinds of cells, the glandular secretory cells and the contractile myoepithelial cells. Another characteristic of the sweat glands is wide variations in structure and function, which are explained in the chapter.

Cytobiology of the Ovulation-Neurohormone Producing Neuroendocrine Caudo-Dorsal Cells of Lymnaea stagnalis

Abstract: Publisher Summary This chapter discusses the morphological, neurophysiological, endocrinological, biochemical, and pharmacological characteristics of the caudo-dorsal cells (CDC), with particular reference to structure–function relationships. Three main factors have contributed to the present knowledge of the nature of the peptidergic CDC of Lymnaea stagnalis: the suitability of the cells for experimental studies, the multidisciplinary approach with which they have been investigated, and the detailed knowledge of various related aspects of the snail's biology, such as endocrinology, reproduction, the sensory system, the muscular system, the blood and connective tissue, and hydromineral regulation. Some of the results may increase the insight into structure and functioning of nervous systems in general. The quantitative electron microscopic analyses of the cyclic dynamics of CDC activity show the way in which a neuroendocrine system can meet the physiological demand for a sudden high titer of neurohormone to induce ovulation by synthesizing, transporting, and degrading secretory material in a timed, cyclic, and quantitative fashion. The clear relation between the electrical discharge and massive release of ovulation neurohormone of Lymnaea stagnalis (CDCH) demonstrates the significance of the simultaneous neural and endocrine character of the neuroendocrine cell.

DNA Methylation in Eukaryotic Cells

Abstract: Publisher Summary This chapter discusses DNA methylation in eukaryotic cells. DNA methylation pattern undergoes discrete changes, especially during the process of differentiation, and these changes are intimately related to gene expression. DNA methylation in eukaryotic cells serves as a gene silencing mechanism. This function is accomplished by a tissue-specific DNA methylation pattern, which is established in the embryo and subsequently inherited in the somatic tissue by a maintenance methylase. The formation of the pattern involves changes in the methylation of specific sites, which is achieved by interplay between replication and transcription rates, on the one hand, and the intracellular level of the DNA methylase on the other. The chapter describes DNA methylation and gene expression with correlation between methylation and gene activity, its cause and effect, mechanisms of blocking gene activity, and differentiation.

Stability of the cellular translation process.

Abstract: Publisher Summary This chapter discusses the stability of the cellular translation process. It also describes models of error propagation. To make a full study of the theoretical dynamics of error propagation requires that each level of macromolecular information transfer be considered. These include transcription and translation as well as DNA replication and repair because errors in the synthesis of DNA polymerases and repair enzymes can give rise to mutations in genes coding for information handling molecules. The chapter outlines the evolutionary considerations in the context of cellular translation. Various models have been proposed based on properties of the genetic code, while others have developed from the assumption that there existed an initially random autocatalytic system comprising polypeptides and polynucleotides. The chapter explains the measurement of error rates and the turnover of proteins and messages.

Supramolecular Cytology of Coated Vesicles

Abstract: Publisher Summary This chapter discusses the advent of thin sectioning techniques for electron microscopy in erythroblasts and during the next decade they were shown to be present in many different cells and tissues from animals and plants although several different names were used to describe them. It is likely that they constitute a ubiquitous and unique organelle in essentially all eukaryotic cells. A partially purified preparation has been isolated from guinea pig brain and demonstrated that the coat structure was a regular array of pentagons and hexagons producing an icosahedral structure. The closed lattice has been proposed that was formed from a hexagonal network by conversion of 12 hexagons to pentagons. The proposed lattice structure has been amply supported by Heuser's rapid-freeze deep-etch morphological studies. The chapter discusses the role of coated vesicles in endocytosis in many cell types. Finally, it has been indicated that coated vesicles are involved in the genesis of lysosomes.

Nitrate Assimilation in Eukaryotic Cells

Abstract: Publisher Summary This chapter presents an overview of nitrate assimilation in eukaryotic cells Nitrate is the predominant form of combined nitrogen available within oxidative environment Its assimilation is achieved through its biological reduction to ammonium The subsequent utilization of ammonium to form amino and amido-N compounds provides the link among the plethora of pathways of organic nitrogen metabolism and the pathways of inorganic nitrogen assimilation The capability to assimilate nitrate is possessed by certain bacteria, some fungi, and virtually all algae and higher plants It is absent from the animal kingdom Nitrate assimilation represents substantial energy expenditure by the cell when compared with ammonium utilization because eight reducing equivalents are consumed in the reduction of nitrate to ammonium Consequently, cells that assimilate nitrate, regulate this pathway to avoid wasteful use of reducing power when the product, ammonium, is available The form of regulation adopted varies in accordance with the metabolic pattern and status of the cell type but the fundamental purpose of the regulation is the same: to affect an economy of existence The chapter also explains genetic regulation of nitrate assimilation in fungi

Cellular Clocks and Oscillators

Abstract: Publisher Summary The work and viewpoint represented in this chapter is concerned with the dynamic and temporal properties of cell growth and proliferation as they are manifested in cultured cells and intact organisms, and encompasses circadian rhythms, cellular clocks, and chemical oscillators. It also reviews the evidence for high frequency oscillations and emphasizes their manifestations and function as a clock in the cell cycle. A distinction can be made between a cellular clock and a chemical oscillator based on their capacity to maintain a reasonably constant sense of time. The chapter provides a distinction between the chromosome replication cycle, the cell growth cycle, and the cellular clock. Cell cycle arrest, division delay, division setback, and cell cycle phase response have variable and overlapping meanings to workers in the field. The growth and division cycle (GDC) equates with the accumulation of necessary cellular constituents, with cell mass, with size, and in general with the developmental processes of the cell. The chapter also examines oscillators in the cell cycle.

Pathways of endocytosis in thyroid follicle cells.

Abstract: Publisher Summary This chapter discusses that the thyroid gland maintains constant levels of thyroid hormones, thyroxine and triiodo thyronine, in the circulation of most mammalian species. This ability of the thyroid gland is based on the highly regulated sequence of steps in the secretory process: Thyroglobulin, the macromolecular secretory product, and the precursor of thyroid hormones, is released and stored in the follicle lumen where it remains segregated from the extrafollicular space by a tight epithelial monolayer. Studies on endocytosis in thyroid follicle cells require experimental access to the luminal cell surface. This has been accomplished in the past by microinjection of tracers into the follicle lumen. The chapter discusses that suspensions of inside-out follicles were used in which the apical plasma membranes of follicle cells were directed toward the culture medium and separated by tight junctions from the lateral cell surfaces. The tightness of the epithelial monolayer and the stable inside-out polarity are prerequisites to study endocytosis selectively from the apical plasma membrane. The chapter describes the pathways of endocytosis in thyroid follicle cells with particular reference to results obtained from work with inside-out follicles.

Aging of cells in culture

Abstract: Publisher Summary The chapter discusses experimental results obtained using fibroblast-like cells that give some indications of the possible mechanisms that limit the division capacity of normal cells in culture. It focuses on the newer cell-culture models involving cells that retain differentiated functions in culture and their possible contribution to determining the relationship between in vitro and in vivo cellular aging. The in vitro growth of normal somatic cells is characterized by an inherent limitation of their proliferative potential. The number of population doublings a cell strain can undergo is a distinct phenotypic characteristic independent of the chronological age of the culture. Although there is evidence that specific additions to the culture medium—such as hydrocortisone and fibroblast growth factor (FGF)—may extend the proliferative capacity of human and bovine cells, respectively, they ultimately cease division. The population-doubling level of the cultures at the time of hydrocortisone or FGF addition is critical in obtaining the maximum lifespan extension. The lower the population doubling level of the culture when treated, the larger the cumulative number of population doublings accrued. The remarkable consistency of normal cells in culture to express a limited in vitro replicative lifespan, which is inversely related to the age of the donor from which the cell culture was initiated, has led to their utilization as models for cellular aging.

Histochemistry of Zinc and Copper

Abstract: Publisher Summary This chapter discusses the available histochemical procedures for the cellular localization of zinc and copper, allowing a deeper insight into their roles in physiological and biochemical processes. The importance of zinc and copper in biochemical and physiological processes at all levels of cellular complexity is well established and their roles in bacteria, fungi, plants, animals, and man have been studied intensively. Zinc plays a significant role in DNA, RNA, protein, and lipid syntheses, and zinc and copper are intimately involved as cofactors in a number of important enzyme systems, nonenzymatic proteins, and other molecules. The concentrations of the two elements in different tissues can be measured quantitatively by various analytical procedures. However, the amounts of these trace elements do not permit their precise localization at cellular and subcellular levels. Quantitative analytical and histochemical methods demonstrate that a relatively large amount of zinc is present in the hippocampus of the adult rat. The dithizone method and Timm's staining suggest that the zinc is present in the axon terminals of the mossy fibers.

Neural Organization and Cellular Mechanisms of Circadian Pacemakers

Abstract: Publisher Summary This chapter focuses on two aspects of current research: (1) the organization of neural systems that subserve circadian pacemakers in animals, using gastropod eyes as the primary example and (2) the cellular and molecular mechanisms of circadian pacemakers, with major emphasis on in vitro model systems, such as the isolated gastropod eyes, isolated vertebrate pineals, Gonyaulax cultures, and Neurospora . The remarkable ability of all eukaryotes to precisely time daily physiological and behavioral events arises from an inherent biological oscillator or pacemaker composed of cellular components. In animals, specific populations of endogenously active neurons usually associated with a visual pathway to the brain function as controlling circadian pacemakers. Neurons in gastropod eyes are the best understood circadian pacemakers. In them, the populations of about 100 or 1000 endogenously active neurons produce robust circadian rhythms of neuronal activity, even in eyes isolated for weeks in organ culture. The population is synchronized apparently by extensive electrical coupling among the neurons through neuropilar gap junctions. The pacemaker neurons send axons to the brain and distribute temporal information to neural centers controlling locomotion and other behaviors.