Showing papers in "International Review of Cytology-a Survey of Cell Biology in 1991"

Development of Follicles in the Mammalian Ovary

Abstract: Publisher Summary This chapter discusses the development of follicles in the mammalian ovary. The unresolved issues in follicular development are focused. Folliculogenesis culminates in the production of fully ripe, preovulatory follicles visible to the naked eye as large bulges on the surface of the ovary. Each ripe follicle contains thousands of highly differentiated cells. This complex, functional miniature organ arises from the handful of cells that constitute a simple primordial follicle, a structure so small that it is invisible at the lower magnifications of a light microscope. All regulatory influences can only permit or prevent cells from completing the full maturation process; they cannot change the course of differentiation. A plethora of modulating influences act as permissive inducers, impeding or propelling the committed follicular cells through the process of clonal expansion. As each follicle progresses through its program of limited clonal expansion and maturation, its cells proliferate more and more rapidly. With every passing generation, the proliferative potential of the granulosa and theca cells continues to diminish, while the state of maturation increases.

Aging and immune function

Abstract: Since it is too difficult to study aging of the organism as a whole, most investigators try to focus on a specific physiological system that exhibits age-dependent functional changes, in the hopes that elucidation (in biochemical and developmental terms) of the mechanism of senescent change will provide insight into the aging process itself. The immune system is among the most maleable of such models, in that well-defined cell types will produce well-defined molecules with predictable functions in vitro and in vivo. The increasing power of basic immunological science should, in the next decade, permit an increasingly fine appreciation of how aging leads to immune decline. This expanding conceptual framework will then suggest new ideas about the role of immunosenescence in degenerative, infectious, and neoplastic illnesses and may also generate increasingly rational strategies for therapeutic intervention.

Plant Gravity Sensing

Abstract: This review of plant gravity sensing examines sensing in organ gravitropism, sensing in single-cell gravitropism, and nongravitropic sensing. Topics related to sensing in organ gravitropism are (1) identification of the gravitropic susceptors, including intracellular asymmetry in equilibrium position and after reorientation, susceptor signal-to-noise ratio, signal integration over threshold stimulation periods, intracellular asymmetry and gravitropic competence, and starch deficiency and gravitropic competence; (2) possible root statocytes and receptors, including identification of presumptive statocytes, cytology, and possible receptors and models of sensing; and (3) negatively gravitropic organs, including identification and distribution of presumptive statocytes and cytology and possible receptors. Topics related to nongravitropic sensing include gravitaxis, reaction wood, gravimorphogenesis, other gravity-influenced organ movements, and cytoplasmic streaming.

The replication, differentiation, and inheritance of plastids with emphasis on the concept of organelle nuclei

Abstract: Publisher Summary This chapter discusses the replication, differentiation, and inheritance of plastids. The process of organelle division comprises two main events: division of the organelle nucleus and organelle kinesis (analogue of cytokinesis). Almost all the mature chloroplasts of land plants and algae contain a pt-genome located in the specific regions of chloroplasts that develop from tiny proplastids. The proplastids also can differentiate both directly and indirectly from other plastids, such as etioplasts, chloroplasts, chromoplasts, leucoplasts, and amyloplasts in a tissue-specific manner or depending upon environmental factors, such as light and temperature. The pt-nuclear division occurs after the endoduplication of ptDNA or in the absence of prior DNA synthesis. A strong binding among specific sites of ptDNA and membrane systems is observed in chloroplasts and may be related to the dispersion of pt-chromosomes and the activation of photogenes after greening. The plastidkinesis is mediated by the plastid dividing (PD) ring in lower eukaryotes, in moss, and in gymnosperm, which is located on the cytoplasm outside plastids. In angiosperm, the plastid dividing ring is a doublet, which is composed of an outer ring and inner ring.

Molecular anatomy of the blood-brain barrier as defined by immunocytochemistry.

Abstract: Publisher Summary This chapter discusses the immunocytochemical aspects of the specific molecular setup of cerebral microvessels and the perivascular apparatus. The molecular characteristics of cerebral blood vessels that are relevant to the immunocytochemical characterization of the Blood–Brain Barrier (BBB) are described in the chapter. Recent developments and improvement of methods for immunolabeling and the production of specific probes such as affinity-purified polyclonal antibodies and monoclonal antibodies, have contributed to new discoveries about the molecular composition and morphogenesis of the BBB. The BBB exhibits a specific collection of structural and metabolic properties thatare also found in tight transporting epithelia. The implementation of antibodies that recognize proteins of non-BBB origin shows that these biochemical markers and the functions that they represent are localized in the BBB endothelium. The characterization of target molecules to which polyclonal or monoclonal antibodies that are generated to epitopes of the BBB endothelium or brain homogenates is also considered in the chapter.

Regulation of synthesis and transport of secreted proteins in cereal aleurone.

Abstract: Publisher Summary This chapter discusses the regulation of synthesis and transport of secreted proteins in cereal aleurone. It is one of the few digestive tissues found in plants, and the synthesis and secretion of hydrolytic enzymes by this tissue are controlled primarily by the plant hormones gibberellic acid (GA) and abscisic acid (ABA), and by the calcium ion. The aleurone cells from various cereals have the same basic structure and function and play the same role in germination-related processes. The most extensively studied aleurone enzyme is α-amylase. α-Amylase plays a role in the endosperm during germination; it is made in very large amounts in aleurone; it is easily assayed, purified, and detected on electrophoretic gels; it is very stable; and it consists of a single polypeptide chain. The mechanisms that regulate the intracellular transport and secretion of hydrolytic enzymes from the barley aleurone cell as well as the mechanisms involved in the targeting of proteins to subcellular compartments and the identity of these compartments are reviewed in the chapter.

Control of calcium regulating hormones in the vertebrates: parathyroid hormone, calcitonin, prolactin, and stanniocalcin.

Abstract: Publisher Summary This chapter discusses the control of calcium regulating hormones in the vertebrates. Control of ionic calcium levels of the extracellular fluids is mandatory for all vertebrates because only minor changes of calcium, in particular the ionized calcium fraction, have pronounced effects on the permeability of cell membranes to ions and, consequently, on important physiological processes, such as muscle contraction, nerve signal transduction, and control of cellular metabolism. Calcium uptake is balanced by renal calcium excretion, with exception of periods of body growth or, in females, periods of reproduction and parental care. Parathyroid hormone (PTH) dominates the control of the Ca2+ concentration of the extracellular fluid by its calcium-mobilizing action on bone and, indirectly, via its control of the synthesis of 1,25(OH)2D3, intestinal calcium absorption. Calcitonin (CT) reduces the extracellular calcium concentration by inhibiting osteoclastic bone resorption and by promoting calcium deposition in the skeleton. In the terrestrial vertebrates, the evidence for the involvement of prolactin (PRL) in calcium metabolism is limited to calcium handling during gestation and lactation in mammals.

Mitochondria-Rich Cells in the Gill Epithelium of Teleost Fishes: An Ultrastructural Approach

Abstract: Publisher Summary This chapter describes the ultrastructure of chloride cells in the gill epithelium of teleostean fishes. It also explores the extent to which changes in water salinity can affect this structure in various species. In teleosts, the gills consist of several branchial archs. Each branchial arch bears several primary lamellae (or filaments), which, in turn, give rise to rows of secondary lamellae. Primary and secondary lamellae are covered by an epithelial sheet, which is subdivided into two regions—namely, (1) primary epithelium covering the primary lamellae and (2) secondary epithelium covering the secondary lamellae. Among the more superficial cells in contact with the outside medium, two types of cells have been regularly identified: the flattened, sometimes piriform, pavement cells and the ovoid chloride cells. The chloride cells are voluminous, mainly encountered in the interlamellar regions of the primary epithelium and on the side of the primary lamella supplied with afferent blood. In the secondary epithelium, the presence of chloride cells is more erratic and may vary according to the fish species. The cytoplasm of chloride cells contain a highly developed membranous system made up of anastomosed tubules. The tubular system extends throughout the whole cytoplasm, except for the Golgi area and a narrow band located just beneath the apical surface.

Orientation of Cortical Microtubules in Interphase Plant Cells

Abstract: Publisher Summary This chapter examines the orientation of cortical microtubules (MTs) in interphase plant cells and explains the way interphase MTs orient by first establishing salient features of cortical microtubule arrays and then considering hypotheses for alignment against these features. Oriented cortical MTs are highly characteristic of plant cells. They are implicated in two functions of particular importance in plant morphogenesis: (1) the orientation of cellulose microfibrils when present as an interphase array and (2) the determination of the division plane when organized as a preprophase band. Interphase array is the MTs of all cells not actually undergoing mitosis or cytokinesis. These may range from meristematic cells, truly in an interphase among mitoses, to much older cells, which have expanded and differentiated and will not divide again in normal circumstances. It includes cells showing dispersed growth and those showing tip-localized growth. Based on studies using animal tubulins, MT assembly has been viewed as a two-stage process in which the product of a rate-limiting nucleation reaction extends at both ends by the reversible addition of tubulin through an equilibrium reaction. Microtubule-organizing centers (MTOCs) promote nucleation and permit assembly at levels of tubulin too low for a spontaneous assembly.

Molecular structure and function of microtubule-associated proteins.

Abstract: Publisher Summary This chapter reviews the structure and function of microtubule (MT)-associated proteins (MAP). The discovery of MAP with defined roles in MT-based motility, such as kinesin and cytoplasmic dynein, and important advances in the structural and morphological analyses of other MAP, including fibrous and nonfibrous MAP are discussed in the chapter. By using the tools of recombinant DNA technology, such as site-directed mutagenesis, transfection of truncated or otherwise mutated MAP genes into living cells, and synthetic peptides, the mechanisms of the various types of MAP-MT interactions and the influence of potential regulatory domains such as phosphorylation sites are determined. The proposed role of fibrous MAP as cross-linking elements mediating stable or transient interactions between MT and various other cell structures are summarized in the chapter. The only established function of MAP is the stabilization of MT. Increased stability of endogenous MT to depolymerizing agents is seen when purified tau proteins are microinjected into cells lacking tau. Another important cellular function of fibrous MAP, long proposed on grounds of in vitro experiments, is the promotion of MT assembly. The fibrous MAP also plays a role in morphogenesis by specifying the interaction partners of MT in different types and compartments of cells.

Structure of the mouse egg extracellular coat, the zona pellucida.

Abstract: Publisher Summary This chapter describes zona pellucida (ZP), which is a thick extracellular coat that surrounds the plasma membrane of all mammalian eggs. It highlights mouse egg ZP structure. Either three or four glycoproteins constitute the ZP of eggs from a wide variety of mammals. ZP is well organized with primary and secondary sperm receptors (two different ZP glycoproteins), arranged periodically along ZP filaments. During the process of fertilization in mammals, sperm must bind to and penetrate through the ZP to reach and fuse with egg plasma membrane. The ZP is responsible for both species specificity of sperm binding and establishment of a slow block to polyspermy following gamete fusion. The mouse egg ZP is a relatively thick translucent extracellular coat that contains ∼3–4 ng of protein and is extremely porous, permitting relatively large macromolecules and even small viruses to pass through it. The unfertilized mouse egg ZP is composed of three glycoproteins: ZP1, ZP2, and ZP3. These glycoproteins appear either as very broad bands following one dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS–PAGE), or as a very complex set of spots following high resolution two-dimensional SDS–PAGE.

New aspects of stroma-parenchyma relations in mammary gland differentiation.

Abstract: Publisher Summary This chapter reviews the morphological and biochemical evidence for stromal–parenchymal relationships in mammary gland development, mainly in mice, and discusses the possible cellular and molecular mechanism of these interactions. The mammary gland is a reproductive organ unique to the Class Mammalia. The mammary gland is composed of epithelial parenchyme and two different types of mesenchymal stroma—that is, dense mammary mesenchyme and fatty stroma. The dense mammary mesenchyme is present immediately surrounding the epithelium at the 14- to 15-day embryonic stage, in endbuds at puberty, and in cancers. This mesenchyme determines mammary epithelium and fixes the ability of the epithelium to interact with the fatty stroma. Androgen receptors appear in dense mesenchyme at the embryonic stage as a result of the inductive interaction with mammary epithelium. The fatty stroma is essential for typical mammary gland morphogenesis. Living mammary stroma with three-dimensional structure is essential for normal morphogenesis. The two types of mammary stroma synthesize different extracellular matrix proteins.

Melanin-concentrating hormone: a general vertebrate neuropeptide.

Abstract: Publisher Summary This chapter reviews the potential roles of Melanin-Concentrating Hormone (MCH) in mammals. The MCH has a striking effect in teleost fish, causing them to become pale in color. The endocrine repertoire to comparative studies on lower vertebrates shows that identical or highly homologous hormones occur in all vertebrate classes, their physiological roles may differ. A hormone is involved with several diverse functions even within one animal, but the emphasis on a particular function can shift during the course of evolution, so that homologous hormones in fishes and mammals may serve different roles in the two classes. A hormone or neuropeptide exerting an obscure effect in mammals might elicit a response in lower vertebrates, which is the cause of its discovery. There are many examples of physiological events that are regulated by two opposing hormones, an arrangement that gives rapid and finer control than changing titers of a single hormone. The distribution of MCH perikarya and fibers is examined in few species and suggests that the organization found in bony fish may differ slightly from that in other vertebrates.

The pre- and postnatal influence of hormones and neurotransmitters on sexual differentiation of the mammalian hypothalamus.

Abstract: Publisher Summary This chapter discusses brain structures and functions, which have developed differently in male and female animals. It describes some of the biological causes thatinduce such developmental disparities. A number of functions which are controlled by the brain are expressed differently in male and female mammalian organisms. These functions include the release of gonadotropic hormones from the pituitary gland, male and female sexual behavior, play and social behavior, agonistic behavior, learning behavior, gender-role behavior, and posture during urination. The most obvious functional differences between male and female animals are those involved in reproductive physiology and reproductive behavior. There is a sensitive developmental period in the brain during which sexual differentiation of neural substrates proceeds irreversibly under the influence of gonadal hormones. In the rat this period starts a few days before birth and ends approximately 10 days after birth. Female rats, treated during this sensitive period with androgens or estrogens, will permanently lose the capacity to release gonadotropin-releasing hormone (GnRH) in response to estrogenic stimulation, and will lose the capacity to show female lordosis behavior. The loss of female characteristics is termed “defeminization.” Female rats which are treated postnatally with androgens will develop the capacity to show the complete masculine sexual behavior pattern following administration of testosterone in adulthood. The acquisition of male characteristics is termed “masculinization.”

Regulation of intracellular pH in plant cells

Abstract: Publisher Summary This chapter reviews the basic principles of the techniques used for intracellular pH measurement in the main cell compartments—namely, cytoplasm and vacuole Alongwith being both substrate and product in numerous metabolic reactions, protons fulfill the regulatory role of coordinating the activities of enzyme-catalyzed pathways, membrane transport, and other regulators The protons connect cellular compartments and also play important roles in intercellular traffic The sudden pH shifts may impose critical loads on the cells The technical improvements that have been brought about since the previously published reviews are described in the chapter The technique involves the extraction of cell sap and the measurement of its pH with a glass electrode It is used for various types of plant materials The principle of this technique is based on three requirements: the probe molecule is metabolically inert, only the uncharged form is membrane permeant, and the probe is not to change the pH of the respective compartment The distribution of protons within a plant cell appears as a critical element of cell organization and function

Laser microbeam as a tool in cell biology.

Abstract: Publisher Summary This chapter highlights cellular and subcellular laser microsurgery and manipulation and describes the ablation of single cells, parts of single cells, individual organelles, and parts of individual organelles. The chapter discusses optical trapping along with cell-fusion and gene-insertion techniques. Laser microbeam is a significant tool in cell and developmental biology. The classic application of laser microsurgery to organelles, unicellular organisms, and cells in embryos has been adopted to probe the structure and function of cells (and organelles) and study the development of a variety of plant and animal organisms. Of the greatest number of studies conducted in invertebrates, the nematode Caenorhabditis elegans has provided an ideal model for the use of the laser microbeam as the lineages and anatomical fates of all somatic cells, from fertilization to adulthood, are completely known. The greatest number of laser microbeam studies has been conducted on the nuclear components. These have been related to various structural and functional problems—namely, (1) gene manipulation,(2) chromosome and nuclear organization, and (3) cell division or motility.

Cellular and molecular biology of neuronal intermediate filaments.

Abstract: Publisher Summary This chapter describes the structural properties of neuronal intermediate filaments (IFs) and how these properties influence neuronal-IF assembly dynamics and axonal-transport kinetics. IF proteins, including the neuronal IFs, belong to a complex, multigene family, which has been divided into six types of genes. IF proteins typically contain a central, α-helical rod domain of approximately 310 amino acids, which is subdivided into three coils by short, non-α-helical linker domains of more variable length and sequence. The rod domain is highly conserved in primary and secondary structure among the IFs, particularly at the ends. This rod domain confers a fibrous character upon IF proteins, and by virtue of its hydrophobic heptad repeats yields a hydrophobic strip along one side of the helix to promote dimerization. The amino-terminal head domains of the neuronal IFs exhibit only limited sequence homologies, including an 11-amino acid-stretch highly conserved between rat α-internexin and NF-M. Rich in arginine, they are highly basic, and the prevalence of glycine, proline, and serine residues predicts a structure dominated by β-turns and β-sheets.

Changes in Gene Expression during Cardiac Growth

Abstract: Publisher Summary This chapter reviews the changes that occur during development, hemodynamic overload, and after administration of various hormones. Cardiac hypertrophy results from various patterns of growth of the multiple-cell types which constitute the heart. Cardiac hypertrophy is an integral part of normal development. Cardiac myocytes are heterogeneous and include three main types, which differ by their phenotype and their function: atrial, ventricular, and conductive cells. Hypertrophy results from an increase in the overall RNA and protein synthesis and from quantitative changes in gene expression. The cardiac phenotype is dependent on quantitative modulation of individual genes, which results in alteration of the relative ratio of the different components of the myocyte. A modulation of the cardiac genome, which leads to a myocardial phenotype different from that of the normal heart, is superimposed on the global activation of protein synthesis that leads to hypertrophy.. The genetic organization of the main contractile proteins is elucidated and some of the new features involved in this organization are focused in the chapter.

Comparative biochemistry and biophysics of ribosomal proteins.

Abstract: Publisher Summary This chapter discusses the role of the ribosomal proteins and ribosomal functions and describes the extent of rRNA found to participate in these activities. Several of the proteins are also known to be closely associated with different functions. The ribosome is the site of protein synthesis in all organisms. It is a complex organelle composed of a few RNA molecules and a complement of a large number of relatively small proteins. In the analysis of macromolecular functions there is a need to identify the groups involved and to visualize the structures responsible for a certain function. One important method for the exploration of structural and functional relationships is in vitro mutagenesis. Three-dimensional structural information is used in the application of this technique. Protein synthesis is performed in three separate phases: initiation, elongation, and termination. The main events during these three phases for bacterial protein synthesis, which serve as a model for protein synthesis, are summarized in the chapter.

Divergence and differential expression of actin gene families in higher plants.

Abstract: Publisher Summary This chapter describes the diversity and differential expression of plant actin gene families and proteins. The plant actins are encoded by ancient and divergent classes of genes that may not have shared a common actin gene ancestor since the evolution of early vascular plants. The encoded actin proteins show remarkable sequence and charged residue variability within any single plant. The number of assorted roles proposed for plant actins in various cell processes and cell types, this complex gene family is required to meet the demands for actin regulation and/or novel actin protein function. The correlation of diverse actin genes and proteins with numerous proposed actin functions and the demonstration of a required role for actin in these processes are yet to be explored. Isolation or construction of plant actin mutants provides the critical proof of function for individual classes of actins. Analysis of plant actin gene sequences suggests that they contain extremely diverse protein isovariants.

Immunoreactive GnRH in Hypothalamic and Extrahypothalamic Areas

Abstract: Publisher Summary This chapter reviews the presence and the physiological significance of gonadotropin-releasing hormone (GnRH) inside and outside the brain of vertebrates; surveys comparative aspects that reveal similarities and differences, discussed from the evolutionary and adaptive viewpoint. Immunoreactive GnRH perikarya are widespread in the brain even outside the hypothalamus. Immunoreactive GnRH fibers project in many directions alongwith the Median Eminence (ME). In Elasmobranchs, unique among vertebrates, their fibers project in various directions except ME. The discharge into the ventricular system and from there into the venous system is also a primitive mechanism of transport observed in the lamprey and stickleback. The possibility of alternative routes of communication with pituitary or other targets is hypothesized in Chondrichthyes. The detection of ir GnRH in the blood of Chondrichthyes is attractive in explaining a puzzling situation in these fish, as much as the brain may communicate with the pituitary. The involvement of messengers other than cyclic adenosine monophosphate (cAMP) is a general feature of the mechanism of action of GnRH at pituitary and gonadal levels.

Cellular mechanisms of pattern formation in the developing limb.

Abstract: Publisher Summary This chapter describes limb development in the chick wing emphasizing the musculature and skeletal pattern. The chapter discusses the sequence of events during early limb development and limb outgrowth. The pattern formation along the different axes of the limb and the importance of specialized regions in the wing are discussed in the chapter. The chapter examines the process of cell differentiation and reviews several theoretical models that have been proposed to account for limb development. Limb development consists of a series of sequential processes. A limb site is determined, polarity is established, and outgrowth ensues. As the limb elongates, the skeletal pattern appears in a proximodistal sequence. The stabilization of each element follows after a period in which cell determination is labile and may be influenced by external factors. Pattern development in the limb is a highly intricate process involving cell–cell, cell–tissue, and tissue–tissue interactions that change with time and position in a complicated three-dimensional geometry, which also changes with time.

Cell division in diatoms

Abstract: Publisher Summary This chapter describes mitosis in diatoms and explores how the observations and inferences from these cells relate to mitotic mechanisms in general. Diatoms are among the most widespread, prolific organisms on the planet and they are significantly important in global ecology. There are two major groups of diatoms—the centrics and the pennates, both diploid in the vegetative state. The centrics are oogamous, differentiating uniflagellate sperm that fertilize a large, inert oogonium. In contrast, pennates usually have isogamous amoeboid gametes. Most diatoms studied possess a prominent granule tightly associated with the interphase nucleus. In some larger diatoms, the microtubule center (MC) is very conspicuous in vivo . MC is usually spherical during interphase but may change during the cell cycle. As the primorial spindle grows, the microtubule (MT) cytoskeleton associated with the MC becomes particularly striking. Then the nucleus moves to a predetermined position in the cell for mitosis, a movement orchestrated by the MT cytoskeleton enlarging around the MC. The MC is intimately involved with the creation of the spindle, after which it dissolves. In turn, new MCs arise close to the spindle poles.

Immunoarchitecture of regenerated splenic and lymph node transplants.

Abstract: Publisher Summary This chapter describes the immunoarchitecture of regenerated splenic and lymph node transplants. Regeneration refers to the natural renewal of a damaged tissue. The regeneration of splenic or lymph node tissue is different, as small segments of these organs are purposely placed at an unusual site where they regenerate by rebuilding the organ compartments. The process of splenic regeneration depends on few surviving reticular cells, which provide a suitable microenvironment for regeneration. During regeneration, granulocytes, monocytes, and lymphocytes immigrate into the implanted tissue. Thus, both the surviving cells and the immigrating cells of the host play a role in regeneration, forming the stromal elements and the free-moving cells, respectively. The regenerated splenic tissue performs most of the functions of a normal spleen but at a reduced level. It is possible to demonstrate the functional capacity of the regenerated lymph node tissue by recording the development of germinal centers after subcutaneous injection of killed bacteria. The regeneration of splenic and lymph node tissue provides interesting models for studying general biological phenomena in cellular and humoral interactions in regeneration. In addition, these tissues might be used in the clinical situation to reduce the incidence of lethal infections and relieve lymph edema for the benefit of future patients.

Calcium transport across epithelia.

Abstract: Publisher Summary This chapter discusses mechanisms by which cells transport calcium and modify calcium fluxes in the body. The major chemical functions that are dependent on calcium include: nerve excitation, muscle contraction, extrusion, and export processes. The level and regulation of the calcium content of body fluids involves millimolar concentrations. Food calcium may undergo solubilization and dilution with bile and intestinal fluids; calcium concentrations in the chyme can reach tens of millimoles. Thus, the body's calcium traffic—ingestion, digestion, absorption, plasma-calcium maintenance, cellular uptake and release, bone-calcium deposition and removal, i.e., resorption, urinary and fecal-calcium excretion—involves an unending manipulation of various calcium concentrations, as well as changes in state from liquid to solid and back to liquid. In the balance approach, the amount of calcium excreted in the stool is subtracted from the amount ingested during a comparable period. This is termed “net absorption.” The net amount of calcium absorbed represents the net load of calcium from the intestine. By using radioactive or stable isotopes of calcium, along with a balance procedure, one of the streams of calcium in the intestine can be labeled—either the endogenous or the food calcium—and thereby the true amount absorbed and the quantity of endogenous calcium lost in the stool can be quantitated.

Multiphasic Uptake Mechanisms in Plants

Abstract: Publisher Summary This chapter discusses the uptake mechanisms in plant roots with the influx of solutes across the plasmalemma and the plasma membrane of plant cells. Solute uptake in plants is mediated by multiphasic mechanisms. The uptake of a particular solute occurs by a single structure, a membrane channel, having carrier-like properties at low external solute concentrations and channel-like properties at high concentrations. With the recognition that this entity may exist in several different concentration-dependent states, the widely held assumption that carriers and channels represent separate entities are reexamined. The number of carriers or mechanisms for uptake of a particular solute is of fundamental importance for understanding the transport of that solute. Uptake of nutrient ions by plant roots is represented by multiphasic isotherms—namely, the multiphasic model. Uptake of organic solutes from the soil is of less importance than uptake of inorganic ions. The relation between the rate of solute uptake and the external solute concentration can, in the majority of instances, be represented by a single, multiphasic curve or isotherm.

Structure and Function of Plant Cell Walls: Immunological Approaches

Abstract: Publisher Summary Plant cells are surrounded by a cell wall that gives them characteristics different from animal cells. The important functions of plant cells depend on the presence of cell walls and their characteristics. A plant cell cannot exist for long without its cell wall. Throughout its life, the cell wall provides the plant with the mechanical strength to construct and protect itself against the environment. The cell wall can be divided into primary and secondary cell walls. The primary cell wall is the wall of growing cells and the secondary cell wall is formed after growth has stopped and differentiation has started. Immunological methods offer potent probes for clarifying the location, metabolism, function, and structure of cell wall polymers. This chapter summarizes the advances in immunological approaches and its future prospects to study plant cell wall. Potent probes, such as antibodies, lectins, and enzymes are needed to specifically distinguish details of cell wall components. Antibodies are the most suitable, as they are more precise than other candidates. Antibodies can be generated against different types of target cell wall components. The occurrence and location of polysaccharides, structural glycoproteins, and enzymes constituting the cell wall have been significantly examined with antibodies specific for these compounds. The antibodies are also useful probes for clarifying the conformational structure and the regulation of metabolism or gene expression of different wall components in relation to their roles in diverse functions of the cell wall.

The cytoskeletal system of nucleated erythrocytes

Abstract: Publisher Summary This chapter focuses on cytoskeleton formation and function of the marginal band (MB) of microtubules (MTs). It highlights the cytological, ultrastructural, and molecular observations on nonmammalian erythrocytes. The chapter also discusses comparative work on the cytoskeleton of primitive nucleated erythrocytes in developing mammals. All of the mature nucleated erythrocytes of nonmammalian vertebrates contain marginal bands (MBs). The MB is principally a hooplike continuous microtubule (MT) bundle located close to the plasma membrane in the plane of flattening. For erythrocytes of nonmammalian vertebrates, MB thickness in the light microscope and MT number per electron microscopic (EM) cross section are positively correlated with cell size. The MB is flexible both in situ and after isolation—that is, it can bend and twist without breaking. Tubulin is the major molecular component of MBs. Tubulin was found to constitute 1% of total cell protein, with less than one-half in the MB. The assembly of long MTs typical of MBs might be regulated by a tubulin oligomer pool, limiting the nucleation rate and accounting for at least some non-MB tubulin in the cell.

Glycosylation in intestinal epithelium.

Abstract: Publisher Summary This chapter reviews the glycosylation reactions in the intestinal epithelium. The intestinal epithelium represents a good model system in which the glycosylation process can be studied. The intestinal epithelium is composed of two basic epithelial cell types: the absorptive enterocyte and the mucus-producing goblet cell. Gastrointestinal epithelial renewal ensues through the processes of cell proliferation, migration, and differentiation. This renewal occurs in discrete proliferative zones along the gastrointestinal tract. In the small intestine, this proliferative zone is restricted to the base of the crypts, whereas in the large intestine it is less restrictive, occurring in the basal two thirds of the crypt. A longitudinal section along the crypt-to-surface axis, cells in various degrees of differentiation is observed, providing a unique in vivo system in which to investigate differentiation-related glycosylation events. The glycoconjugate repertoire displayed by a given cell reflects its endogenous expression of glycosyltransferases. The role played by terminal oligosaccharide structures in cell–cell recognition phenomena and the expression of glycosyltransferases occupy a key position in the post-translational processing of glycoconjugates and thus influence cellular function.