Showing papers in "International Review of Cytology-a Survey of Cell Biology in 1980"
TL;DR: It has proved feasible to categorize most if not all dying cells into one or the other of two discrete and distinctive patterns of morphological change, which have, generally, been found to occur under disparate but individually characteristic circumstances.
Abstract: Publisher Summary The classification of cell death can be based on morphological or biochemical criteria or on the circumstances of its occurrence. Currently, irreversible structural alteration provides the only unequivocal evidence of death; biochemical indicators of cell death that are universally applicable have to be precisely defined and studies of cell function or of reproductive capacity do not necessarily differentiate between death and dormant states from which recovery may be possible. It has also proved feasible to categorize most if not all dying cells into one or the other of two discrete and distinctive patterns of morphological change, which have, generally, been found to occur under disparate but individually characteristic circumstances. One of these patterns is the swelling proceeding to rupture of plasma and organelle membranes and dissolution of organized structure—termed “coagulative necrosis.” It results from injury by agents, such as toxins and ischemia, affects cells in groups rather than singly, and evokes exudative inflammation when it develops in vivo. The other morphological pattern is characterized by condensation of the cell with maintenance of organelle integrity and the formation of surface protuberances that separate as membrane-bounded globules; in tissues, these are phagocytosed and digested by resident cells, there being no associated inflammation.
7,417 citations
TL;DR: In insects, chemo-, thermo-, and hygroreceptive sensory cells are located exclusively in sensilla, which are organelles built up by a definite number of characteristic cells.
Abstract: Publisher Summary Chemo-, thermo-, and hygroreceptors are primary sensory cells, which bear one or several modified cilia whose membranes are specialized to take part in the transduction process. These membranes must be exposed to the stimuli but also be protected against desiccation, mechanical, and other injury. Hence, a variety of structural specializations is found that serves these functions. In insects, chemo-, thermo-, and hygroreceptive sensory cells are located exclusively in sensilla, which are organelles built up by a definite number of characteristic cells. During development, the innermost enveloping cell produces a cuticular sheath (scolopale), which envelops the dendritic processes for varying distances along their course through the receptor lymph cavity. This sheath producing cell is surrounded by the trichogen cell, which is itself surrounded by the trichogen cell. These cells build up shaft and socket, respectively, of the cuticular outer structures during development. The perikarya of all cells of the sensillum are enclosed within the epidermis.
651 citations
TL;DR: The stellate cells in the liver of various mammals are morphologically classified into two types: in the first type, branching of the cytoplasmic processes is well developed many medium-sized lipid droplets are apparent and one or two large lipid Droplets are located in the vicinity of the nuclei.
Abstract: Publisher Summary This chapter discusses the history of the stellate cells. These cells are the integral elements of the wall of the sinusoids and that notion has introduced deep-rooted misconceptions in liver histology. The chapter discusses the morphological characteristics of stellate cells. The stellate cells are distributed only in the lobules of the liver and constitute 1.4% of parenchymal volume in the rat liver. They are stellate-shaped, are located in the space of Disse, and adhere to the sinusoidal wall. Their dendritic processes spread over the outer surface of the sinusoids and encircle the sinusoids or run longitudinally along them. Along the surface of these processes, a number of micropinocytotic vesicles or caveolae are observed. No basal lamina is discernible. The stellate cells are separated from the parenchymal cells by intercellular spaces of varying dimensions in which collagen bundles are often seen. The stellate cells in the liver of various mammals are morphologically classified into two types: in the first type, branching of the cytoplasmic processes is well developed many medium-sized lipid droplets are apparent; in the second type, cytoplasmic processes are not so conspicuous and one or two large lipid droplets are located in the vicinity of the nuclei.
520 citations
TL;DR: The mitogenic effect of thyroxine on mammalian corneal epithelium in situ is mimicked by fibroblast growth factors (FGF) and epidermal growth factor (EFG), both of which stimulate stratification.
Abstract: Publisher Summary The development of the avian cornea involves a remarkable continuum of highly coordinated events. The corneal epithelium is the first of the component tissues to differentiate. This ectodermal derivative, with the help of the lens, secretes the highly ordered primary stroma composed of orthogonally arranged collagen fibrils embedded in a chondroitin sulfate (CS)-rich matrix. The fibroblasts of the avian cornea may produce the first fibronectin to appear within the stroma, although fibronectin is present earlier on the posterior stromal surface and lens. Several striking events occur together during the period of stromal condensation. An enzyme—hyaluronidase—appears, which may remove hyaluronic acid (HA), contributing to the dehydration that begins next to the endothelium and leads to the compaction of the posterior stroma. The dehydration of the stroma and acquisition of transparency are triggered by thyroxine. Thyroxine accelerates interdigitation of endothelial cells and conceivably tells this tissue to begin pumping water out of the cornea. The mitogenic effect of thyroxine on mammalian corneal epithelium in situ is mimicked by fibroblast growth factor (FGF) and epidermal growth factor (EFG), both of which stimulate stratification.
385 citations
TL;DR: It is concluded that the layer of chromatin decondensing on the periphery of dense chromatin areas represents the most active chromatin fraction in the nucleus.
Abstract: Publisher Summary This chapter summarizes current data concerning the ultrastructural localization of transcription sites and the subsequent distribution of newly synthesized ribonucleic acid (RNA) within the cell nucleus. It discusses the roles of ribonucleoprotein (RNP)-containing nuclear structures with respect to the synthesis and processing of nucleolar and extranucleolar RNA. RNA transcription in the nucleolar, as well as extranucleolar, areas takes place predominantly within the regions situated on the border of condensed chromatin. Perichromatin fibrils, considered as the first in situ detected structures containing newly transcribed extranucleolar RNA, originates in these perichromatin regions and the nucleolar fibrillar components are in intimate contact with intranucleolar chromatin areas. DNA replication sites also appear predominantly within this nuclear region. This particular nuclear area consequently plays a key role in the expression of chromatin functions and these findings lead to the conclusion that the layer of chromatin decondensing on the periphery of dense chromatin areas represents the most active chromatin fraction in the nucleus.
349 citations
TL;DR: This chapter discusses the characteristics and evolution of mitosis, a types of nuclear division that produce two, or rarely more, daughter nuclei, each containing a chromosome complement approximately similar to that of the original nucleus.
Abstract: Publisher Summary Mitosis is defined as all those types of nuclear division that produce two, or rarely more, daughter nuclei, each containing a chromosome complement approximately similar to that of the original nucleus. The greatest range of variations by which mitosis is accomplished, occurs in the protistan and fungal kingdoms, some members of which are probably most similar to the ancestors of higher plants and animals. The variations in the higher organisms are secondarily derived from the division patterns of typical plants and animals. This chapter discusses the characteristics and evolution of mitosis. The efficiency of mitosis consists of two basic components; the frequency with which each daughter nucleus receives the necessary complete genome complement (genetic efficiency) and the amount of energy and materials expended in the synthesis and operation of the mitotic apparatus. The chapter also discusses the use of mitosis as a phylogenetic marker. It is applicable to all eukaryotic cells and thus is valuable across boundaries where other structures are absent on one side and present in various forms on the other side.
318 citations
TL;DR: The synthesis, intracellular transport, storage, and discharge of exportable protein by exocrine-secreting cells are also discussed.
Abstract: Publisher Summary This chapter describes the cytology of salivary glands. The salivary glands are divided into two groups—major and minor. The parotid, submandibular (submaxillary), and sublingual glands are major salivary glands. The gross anatomical relationships of the salivary glands of mammals vary greatly between species. The nature of the secretory products of various salivary glands is widely investigated by histochemical staining methods. Staining methods for mucosubstances demonstrate that gross anatomically comparable glands from different species may secrete entirely different mucosubstances. The chapter discusses the secretory functions of the salivary glands. Salivary glands possess systems that allow these organs to secrete remarkable amounts of proteins, glycoproteins, and water. The secretory capacity of the salivary gland acinar cells has been well investigated, particularly the acinar cells of parotid glands. The synthesis, intracellular transport, storage, and discharge of exportable protein by exocrine-secreting cells are also discussed.
292 citations
TL;DR: Gap junction channels will be recognized as one of the most useful and exciting model systems to be exploited in studying the general properties of membranous channels in the years to come.
Abstract: Publisher Summary Gap junction channels are often regarded by membranologists as an unusual means of membrane permeability, having little in common with other membranous channels. Although gap junction channels establish hydrophilic paths between two cells' interiors and are a permissive type of channel, as they are bidirectionally and indiscriminately permeable to charged and neutral molecules, they have many structural and functional similarities with other membranous channels. Not only can they be easily localized in intact cells and studied by ultrastructural methods, such as thin sections, lanthanum infiltration, freeze-fracture, and negative staining, they can also be isolated in quite pure fractions and characterized chemically. Because of their crystalline configuration in isolated fractions, they can be studied by laser, X-ray, and electron diffraction techniques. Their permeability properties and the mechanisms of permeability modulation can be studied electrophysiologically, morphologically, and by tracer-transfer methods. Thus, in the years to come, gap junction channels will be recognized as one of the most useful and exciting model systems to be exploited in studying the general properties of membranous channels.
243 citations
TL;DR: This chapter focuses on the phycobilisome characteristics, their relationship to the photosystems in the thylakoid membrane, and major problems to be addressed in future investigations.
Abstract: Publisher Summary Phycobilisomes are specialized aggregated structures composed of phycobiliproteins, which are photosynthetic accessory pigments in red and blue-green algae. Phycobiliproteins, which can account for as much as 24% of the dry weight of blue-green algal cells and 40–60% of the total soluble protein, are the major light harvesters in these organisms. Chlorophyll a, which absorbs light primarily in the blue region and the red region of the visible spectrum, leaves a large absorption gap. This is filled in by the phycobiliproteins, which have an absorption range of 500–660 nm. These pigments work in conjunction with chlorophyll a to optimize light harvesting for photosynthesis, particularly under light limiting conditions. The only phycobiliprotein in higher plants is phytochrome, which occurs in very small amounts and serves as a photoregulatory receptor. In cryptophyte algae (flagellated unicells of indefinite taxonomic position), phycobiliproteins also serve as major photosynthetic accessory pigments, but phycobilisomes are absent. There are three major classes of phycobiliproteins—the red-colored phycoerythrins (PEs), the blue-colored phycocyanins (PCs), and allophycocyanins (APCs). This chapter focuses on the phycobilisome characteristics, their relationship to the photosystems in the thylakoid membrane, and major problems to be addressed in future investigations.
201 citations
TL;DR: Among invertebrates, the arthropods have the best-known physiology and a careful examination of the repartition of the junctions and their position and association with other types of junctions or with certain cytoplasmic organelles may throw some light on their functions.
Abstract: Publisher Summary The structure of invertebrate septate junctions is not exactly the same in the different phyla. It is a peculiarity of the arthropods to possess two types of septate junctions, the distribution of which is dependent on the organs, not on the systematic position. Additionally, the scalariform junctions have only been observed in the arthropods, but are restricted to some (but not all) transporting epithelia. This chapter discusses the functions of septate and scalariform junctions. Among invertebrates, the arthropods have the best-known physiology and a careful examination of the repartition of the junctions and their position and association with other types of junctions or with certain cytoplasmic organelles may throw some light on their functions. A septate junction is constituted by elements of two adjacent cells and appears as a bicellular junction. However, different and very elaborate structures are set up at the line of contact among three cells (abutment of three bicellular junctions); these special features are designated as “tricellular junctions.” Scalariform junctions are characterized by a regular extracellular space of 20 nm wide, crossed by very fine tubular pillars giving in cross section a ladder-like appearance.
195 citations
TL;DR: This chapter discusses coral skeletogenesis, focusing on the morphology of the skeletogenic tissues of the coral Pocillopora damicornis, and reviews the light and electron microscope studies carried out in this regard.
Abstract: Publisher Summary This chapter discusses coral skeletogenesis, focusing on the morphology of the skeletogenic tissues. It reviews the light and electron microscope studies carried out in this regard, illustrating the example of the morphology of the skeletogenic tissues of the coral Pocillopora damicornis. A structural organic matrix is present in the adult skeleton of Pocillopora damicornis. One component of this structural matrix is present transiently at the growth surface of the skeleton. It consists of individual sheaths enveloping each forming aragonite crystal. These crystal sheaths, plus some small extracellular vesicles that are presumed to represent a precursor for the sheaths, are together implicated in the hypotheses for coral calcification. The hypotheses fall into the following three broad categories: (1) algal removal of possible inhibitory substances; (2) a general stimulatory effect of algal metabolism; and (3) algal contribution to a skeletal organic matrix.
TL;DR: Cortical granules are small, spherical, membrane-limited organelles found mostly beneath the oolemma of the unfertilized ovum, and are absent in much of the egg hemisphere homolateral to the meiotic spindle.
Abstract: Publisher Summary Cortical granules are small, spherical, membrane-limited organelles found mostly beneath the oolemma of the unfertilized ovum. During folliculogenesis, the onset of cortical granule formation parallels the hypertrophy and hyperplasia of the Golgi complexes. As transformation of Golgi complexes occurs, they migrate centrifugally into the subcortical region of the oocyte. The Golgi complexes, in concert with the granular endoplasmic reticulum, are implicated in cortical granule formation. In this process, the contents of the cortical granules are packaged into membrane-limited granules. In most species, the formation of cortical granules ceases at ovulation, however, continued accumulation of cortical granules occurs in oocytes of a few species. The size of the cortical granules and the density of their matrix show considerable species variation. In the unpenetrated tubal eggs, cortical granules form an irregularly spaced monolayer beneath the plasma membrane. A polarity in the distribution of cortical granules is well exhibited in oocytes of some mammals; cortical granules are absent in much of the egg hemisphere homolateral to the meiotic spindle. Cortical granule exudate contains a trypsin-like protease; histochemical observations show both protein and polysaccharide components in cortical granules. At fertilization, the spermatozoon induces the cortical reaction.
TL;DR: This chapter describes the architecture of the contractile apparatus, which must have a design that not only conforms to requirements for flexibility in cell shape, but also tolerates the presence of the cell nucleus in a central position.
Abstract: Publisher Summary This chapter describes the architecture of the contractile apparatus. Smooth muscle cells, when isolated, are generally spindle-like in shape with tapered ends and possess a centrally placed, elongated nucleus. In vivo, where the cells occur in bundles and overlapping layers, they adapt themselves to the contours of the tissue; in some instances, they may be essentially straight, while in others, for example in the vascular wall, they may be markedly curved along their length. This adaptation extends further to their shape in cross section, which may vary within the same muscle layer from circular to polygonal to a flattened form. Thus, the contractile machinery must have a design that not only conforms to these requirements for flexibility in cell shape, but also tolerates the presence of the cell nucleus in a central position. The organization of the contractile material, such as fibrils and contractile units is discussed in the chapter.
TL;DR: One possible pathway across the cerebral endothelium for larger hydrophilic molecules appears to be vesicular transport comparable with that occurring in normal noncerebral tissue, indicative of the presence of a labile pore system in the cerebral microvasculature.
Abstract: Publisher Summary The endothelium of the cerebral microvasculature is, under normal conditions, tight to proteins and peptides, or conceivably, a very limited blood-to-brain (and brain-to-blood) transport by means of endothelial vesicles takes place in segments of the microvasculature. This barrier to lipid-insoluble macromolecules can be ascribed to the tight junctions among the endothelial cells. Uncertainties exist, however, with respect to the permeability properties of the cerebral endothelial junctions to ions and small lipid-insoluble molecules. Many experimental or pathological situations, for example, acute hypertension, cause an opening of the blood–brain barrier. One possible pathway across the cerebral endothelium for larger hydrophilic molecules appears to be vesicular transport comparable with that occurring in normal noncerebral tissue. This may be indicative of the presence of a labile pore system in the cerebral microvasculature, which can be reversibly opened, but normally is closed. Transendothelial channels may also be responsible for the extravasation. Additionally, intravenous administration of hypertonic solutions and long-term hypertension cause an opening of the tight junctions between the endothelial cells.
TL;DR: The procedure of indirect immunofluorescence, when applied with careful staining controls, provides one of the most effective means available for the in situ localization of proteins within the cytoskeleton.
Abstract: Publisher Summary The procedure of indirect immunofluorescence, when applied with careful staining controls, provides one of the most effective means available for the in situ localization of proteins within the cytoskeleton. Microtubules form an extensive network (CMTC) in the cytoplasm of most interphase cells. In well-spread cells, individual microtubules can be visualized by immunofluorescence and are very long structures that extend from one or more perinuclear initiation sites to the cell periphery. Microtubules are not straight, rigid organelles but bend gracefully to conform to the myriad shapes of cells and their processes. The centrosome appears to be a major microtubule organizing center but multiple organizing centers outside of the centrosome may exist in some cells. The organizing centers behave as autonomous sites that not only serve as assembly foci but also may influence microtubule pattern formation in cells. Two major microtubule complexes exist in proliferating cells, the mitotic spindle and the CMTC. The precision and regularity with which one complex is disassembled and the other is formed during the cell cycle indicates the existence of precise control mechanisms.
TL;DR: The consequences of changes in substrate concentration, in temperature, and in the effective kinetic parameters of transport and phosphorylation on rates of uptake demonstrate the complexity of this interdependence and the errors of interpretation that can accrue if the complexity is overlooked.
Abstract: Publisher Summary Three main classes of nutrients—nucleosides, nucleobases, and hexoses (and possibly a fourth, the water-soluble vitamins)—are taken up into cultured cells as a result of the tandem action of a nonconcentrative transport system and an intracellular enzyme that introduces into the transported substrate an anionic group thus confering impermeability. For several substrates representing the first three of these nutrient classes, both members of the uptake pathway are characterized kinetically: (1) the transport systems with cells and substrates that, by virtue of enzyme deficiency, adenosine triphosphate (ATP) depletion, or chemical design, are metabolically inert and (2) the enzymes, as purified proteins, in idealized milieu. The consequences of changes in substrate concentration, in temperature, and in the effective kinetic parameters of transport and phosphorylation on rates of uptake demonstrate the complexity of this interdependence and the errors of interpretation that can accrue if the complexity is overlooked. Flux in a tandem pathway follows a biphasic time course. The first phase corresponds to the attainment of steady-state levels of free intracellular substrate. The second phase represents steady-state flux through the pathway—that is, the rate of accumulation of impermeable product.
TL;DR: The majority of the glycosyltransferase activities require millimolar levels of divalent cations for optimal activity, and depending on the enzyme, Mn 2+, Mg 2+ , and Ca 2+ most often stimulate activity to the greatest extent.
Abstract: Publisher Summary Three basic components are involved in the reactions catalyzed by glycosyltransferases. These are the enzyme itself, a sugar donor, and a sugar acceptor. Often divalent cations are needed for catalytic activity. Another component, α-lactalbumin also participates in the reactions of some galactosyltransferases. The glycosyltransferases are named according to the sugars they transfer. A galactosyltransferase transfers galactose from UDP-galactose to an acceptor. In the more familiar reactions, sugar nucleotides serve as the sugar donor. These donors consist of a nucleoside di- or monophosphate linked through its terminal phosphate ester to the reducing terminus of the carbohydrate. The glycosyltransferases within each class may be defined according to the particular acceptors they utilize. The majority of the glycosyltransferase activities require millimolar levels of divalent cations for optimal activity. Depending on the enzyme, Mn 2+ , Mg 2+ , and Ca 2+ most often stimulate activity to the greatest extent. Some cell surface transferase activities require lower levels of cation for maximal stimulation than do the same activities of homogenates of cells.
TL;DR: This chapter discusses several important and unresolved issues concerning the origin, circulation, and fate of membranes in neurons, a special class of modified neurons—retinal photoreceptors—that are an attractive material for the study of membrane circulation.
Abstract: Publisher Summary This chapter discusses several important and unresolved issues concerning the origin, circulation, and fate of membranes in neurons. A special class of modified neurons—retinal photoreceptors—is described. These cells are an attractive material for the study of membrane circulation, especially as it relates to cell polarity. They are compact and highly polarized in morphological and functional aspects. In vertebrate photoreceptors, one end is concerned with light reception, the other with synaptic transmission. The membrane systems present at the two ends are quite different in arrangement and roles. The proteins and lipids that make up the stacked membranous discs of the light-sensitive outer segments of rods and cones are synthesized in the cells' inner segments. The mechanisms of transport from inner to outer segments and the behavior of the discs once formed are describes in the chapter. The features of macromolecular synthesis in the inner segment are also discussed. The participation of the endoplasmic reticulum (ER) and Golgi apparatus of the photoreceptor's inner segment in the synthesis of macromolecules slated for the outer segment is highlighted.
TL;DR: This chapter discusses the biochemical mechanisms of gamete interaction, which focuses on three principal subjects: the effect of the egg on the sperm; the contact between sperm and egg; and, theeffect of the sperm on the egg.
Abstract: Publisher Summary This chapter discusses the biochemical mechanisms of gamete interaction. The chapter focuses on three principal subjects: (1) the effect of the egg on the sperm; (2) the contact between sperm and egg; and, (3) the effect of the sperm on the egg. The most dramatic morphological response of the sperm to the egg is the acrosome reaction, in which a change that is a prerequisite for fertilization occurs in the head of sperm from many species. In mammals, sperms undergo a process known as capacitation before they respond to the egg. The moment of union of sperm and egg is the most dramatic of the events surrounding fertilization. The locus on the egg for interaction with sperm varies from species to species. Several enzymes have been implicated in the process of sperm–egg interaction. These enzymes play a role in sperm–egg membrane fusion, for it could lead to a transient state of membrane instability. The most dramatic effect of fertilization on the egg surface is the induction of the cortical reaction. The properties of the egg plasma membrane change enormously after cortical granule exocytosis.
TL;DR: A precursor model for the neurohypophyseal hormone biosynthesis is introduced, according to which the biosynthesis of the peptide bonds of these cyclic nonapeptide hormones would occur solely in the cell body, on ribosomes, via pathways common to the biosynthetic pathways of other peptide chains.
Abstract: Publisher Summary Staining of the hormones and neurophysins can be obtained with immunocytochemical methods. The function of the endocrine neurons of the hypothalamic magnocellular neurosecretory system is to synthesize, transport, and secrete neurohypophyseal hormones and neurophysins in response to a variety of physiological stimuli. This chapter introduces a precursor model for the neurohypophyseal hormone biosynthesis. According to this model, the biosynthesis of the peptide bonds of these cyclic nonapeptide hormones would occur solely in the cell body, on ribosomes, via pathways common to the biosynthesis of other peptide chains. Initially, the hormone would be constructed as part of a large precursor molecule, a protein, in which the hormone would be present in a bound, biologically inactive form. Neurophysin(s) and hormone(s) are made as part of the same precursor molecule(s). The activity of the hypothalamic magnocellular endocrine neurons is controlled by humoral and neurogenic stimuli. Neurohormones play an important role in the control of the release of the hormones of the pars distalis of the hypophysis. Like the neurohypophyseal hormones, the releasing and release-inhibiting hormones present in the neurohypophysis are contained in nerve fibers. These nerve fibers are terminals of axons belonging to different parvocellular peptide hormone-producing neuron systems the cell bodies of which are located in the hypothalamus, and possibly in other parts of the brain.
TL;DR: An impressive treatise on a unified theory of the control of actin and myosin in nonmuscle movements has gained additional weight by the recent findings that a Ca 2+ -dependent protein acts as the modulator of myosIn kinases, which regulate the interaction of act in and myOSin inNonmuscle cells.
Abstract: Publisher Summary The chapter discusses the immunological aspects of cytoplasmic contractile protein It describes the effect of the antibodies on contractile proteins in vitro and in vivo , as well as the localization of contractile elements in cells and tissues by immunohistochemical methods The immunofluorescent technique is used to describe antibodies to smooth muscle in patients with liver disease and with malignant disease Some of these antibodies react with microfilament bundles of cultured cells and with hepatocyte membranes Transmembrane linkages, either directly or via a mediator, bring up additional aspects about remote control interactions of external stimuli and the internal contractile system, such as in capping and phagocytosia Ca 2+ ions are important mediators of all these functions and impressive treatise on a unified theory of the control of actin and myosin in nonmuscle movements has gained additional weight by the recent findings that a Ca 2+ -dependent protein acts as the modulator of myosin kinases, which regulate the interaction of actin and myosin in nonmuscle cells
TL;DR: This work has shown that severe dehydration irreversibly switches off production of reserve materials and switches on a controlled senescence program that continues at variable rates until the seed reserve structure is destroyed.
Abstract: Publisher Summary Moisture content plays a role in seed physiology in two ways. First, it may convey information about the expected metabolic activity of the seed—that is, development, resting, or germination. The proportion of structured or vicinal water in the tissue compared to the bulk water, and the change in absolute water content, may play a role here. A dehydration to air dryness would be necessary for the unequivocal switching of the water gradient polarity. This may explain why it is necessary to dry immature seeds below 60% moisture for good viability, even though germination occurs at 60% moisture. Therefore, the absolute moisture content does not dictate a specific physiological activity. Second, the expression of the perceived information is also controlled by moisture status. Severe dehydration irreversibly switches off production of reserve materials and switches on a controlled senescence program that continues at variable rates until the seed reserve structure is destroyed.
TL;DR: This chapter examines the major applications of plant protoplasts to the study of plant cells and places emphasis on structural studies of higher plant protoplasts especially by electron microscopic methods.
Abstract: Publisher Summary Plant cells are surrounded by a complex cell wall composed primarily of polysaccharides. Techniques have been developed for the routine removal of this wall from a wide range of plant cells. The resulting naked cells or protoplasts provide a unique experimental system that is proving very useful for studying the structure and function of plant cells. Many manipulations that are not possible with intact plant cells are feasible with protoplasts. These include surface labeling of the plasma membrane, complete cell wall regeneration, and cell fusion. Protoplasts are particularly important for examining the plant plasma membrane, a component of the plant cell that is normally inaccessible because of the presence of a cell wall. This chapter examines the major applications of plant protoplasts to the study of plant cells. Emphasis is placed on structural studies of higher plant protoplasts especially by electron microscopic methods. Protoplasts are isolated from a wide variety of plant species and when cultured have produced callus and in some cases complete plants. This chapter presents several examples of plants that have been regenerated from protoplasts.
TL;DR: The centriolar complex undergoes a maturation process that is cell cycle-dependent and it often appears that it is the cloud that organizes the microtubules rather than the centriole.
Abstract: Publisher Summary The centriolar complex is centered on the mitotic phase of the cell cycle. The centriolar complex undergoes a maturation process that is cell cycle-dependent. Interphase complexes can only initiate a few microtubules but that prometaphase centriolar complexes can initiate large numbers of microtubules. The centriolar complex is divided into two general component regions: the centrioles and the cloud of electron-dense osmiophilic material surrounding the centrioles. The amount of cloud material appears to be quite variable from preparation to preparation and among animal species. In addition to varying amounts of cloud material, the organization of components within the pericentriolar material also varies considerably. Current laser microbeam experiments employing photosensitization with acridine orange and psoralens indicate the presence of nucleic acid in the pericentriolar cloud. The major component of the pericentriolar cloud is tubulin. Microtubules are usually abundant in the cloud material and it often appears that it is the cloud that organizes the microtubules rather than the centriole.
TL;DR: The utilization of an active sliding filament mechanism by the microtubule arrays in cilia, flagella, and spermtails as the basis for their motion is now firmly established and appears to be a likely mechanism for mitotic-based motion of chromosomes.
Abstract: Publisher Summary Biological motion dependent upon adenosine triphosphate (ATP) hydrolysis is often generated by two distinct groups of proteins: (1) actin and its mechanochemical coupling ATPase myosin and (2) tubulin and its ATPase dynein. In most instances, motion associated with either of these two systems is based upon a sliding filament mechanism. F-actin and microtubules are the filamentous components used by the ATP phosphohydrolases myosin and dynein to generate movement by causing linear displacement or sliding of the filaments. The motion of the filaments is coupled to otherwise stationary structures to produce, for example, contraction of the muscle sarcomere or separation of sister chromatids. Similarly, sliding filaments are used to transduce bending moments that propel surrounding viscous environments as occurs along ciliated epithelia or among free-swimming protozoa or spermatozoa. The utilization of an active sliding filament mechanism by the microtubule arrays in cilia, flagella, and spermtails as the basis for their motion is now firmly established. Active sliding also appears to be a likely mechanism for mitotic-based motion of chromosomes.
TL;DR: This chapter discusses calcification in plants and illustrates the examples of some individual phyla or groups of species and the only energy-requiring process may be concentration of the reactants above the critical supersaturation and this can be provided by thermodynamic favorable events, such as temperature fluctuations, evaporation, or loss of carbon dioxide to the atmosphere.
Abstract: Publisher Summary This chapter discusses calcification in plants and illustrates the examples of some individual phyla or groups of species. Direct photosynthetic involvement cannot be used as a criterion to distinguish between plant and animal calcification, as there are few well-substantiated examples of this. Light-stimulated deposition is, however, generally observed in the red, green, brown, and Chrysophyte algae and a 1:1 partition of carbon into organic compounds and CaCO 3 is often apparent, for example, in Emiliania , Coccolithus , and Corallina . Calcification is an energy-demanding process and energy is required in intracellular deposition for (1) transport of the reactants, Ca 2+ and HCO 3 - or Co 3 2- and the removal of inhibitors from the nucleation site, (2) transport and synthesis of an organic matrix, if present, (3) concentration of the reactants above the critical supersaturation, and (4) the transport of the solid phase. In cases of extracellular calcification, the only energy-requiring process may be concentration of the reactants above the critical supersaturation and this can be provided by thermodynamically favorable events, such as temperature fluctuations, evaporation, or loss of carbon dioxide to the atmosphere.
TL;DR: Several hypotheses are presented for the cytological mode of penetration of calcium carbonate by boring bacteria, fungi, and algae, suggesting that the mode of carbonate breakdown is via the organic matrix of skeletal carbonates.
Abstract: Publisher Summary Many organisms excavate calcified substrates by chemomechanical mechanisms. In marine environments, bacteria, fungi, algae, and invertebrates penetrate calcium carbonate substrates by secreting acids, chelators, or enzymes. These may function either singly or in combination. Boring bacteria, fungi, algae, and sponges penetrate calcium carbonate by chemical dissolution at the cellular level. These cellular mechanisms, particularly those utilized by sponges, are described in this chapter. Several hypotheses are presented for the cytological mode of penetration of calcium carbonate by boring bacteria, fungi, and algae. Bacteria isolated from coral skeletons are capable of digesting chitin in vitro, suggesting that the mode of carbonate breakdown is via the organic matrix of skeletal carbonates. Marine fungi penetrate calcium carbonate by first roughening and pitting the surface and then extending hyphae throughout the substrate. Soil fungi penetrate dead bone (calcium phosphate) by simultaneously dissolving both calcium phosphate and organic matrix. Resorption occurs at the site of contact of the fungal membrane with bone.
TL;DR: By using heteromorphic markers of chromosome 21, the origin of the extra chromosome in Down syndrome can be determined and this book helps clarify the chromosomal mechanisms involved in the production of mosaics, in studying chimeras, and in following the fate of transfused or transplanted cells.
Abstract: Publisher Summary The heteromorphic chromosomes of the human genome can be classified by size, position, staining intensity, or any combination of these types using various banding techniques, such as QFQ (Q-bands by fluorescence using quinacrine), CBG (C-bands by barium hydroxide using Giemsa), and RFA (R-bands by fluorescence using acridine organe). The classification of different types of heteromorphisms is based on estimation rather than actual measurements. Moreover, variation is continuous rather than discrete. However, several attempts are made to classify them on the basis of arbitrary scales and different codes (levels) are assigned. Chromosomal heteromorphisms found in human chromosomes provide a useful tool for several studies because they are inherited in a Mendelian fashion, are stable, and are presumed to have a low mutation rate. By using heteromorphic markers of chromosome 21, the origin of the extra chromosome in Down syndrome can be determined. Applications of heteromorphisms as markers is in elucidating the chromosomal mechanisms involved in the production of mosaics, in studying chimeras, and in following the fate of transfused or transplanted cells.
Journal Article•
TL;DR: The chapter presents a number of examples of spore germination to illustrate the hypothesis that there is probably but one control pattern, which may be variously triggered.
Abstract: Publisher Summary This chapter discusses the cytology, physiology, and biochemistry of germination of fern spores. Ferns (Filicales) represent a group of plants, which yield spores with considerable potential for physiological and biochemical analysis of germination. The majority of ferns produce spores of only one type, which are all similar in size and shape, a condition known as “homospory.” Fern spores are single-celled structures, which contain a quiescent protoplast closely invested by several layers of sporoderm constituting the exine and a thin inner layer or intine. Spores of many ferns germinate readily upon contact with water or a simple mineral salt medium in complete darkness or in response to a particular light quality or a specific growth hormone. All organic substances for germination are derived from the metabolism of stored reserves in the spores. The chapter presents a number of examples of spore germination to illustrate the hypothesis that there is probably but one control pattern, which may be variously triggered. The structure of the spore and morphological aspects of its germination are also discussed.