Pectin and the role of the physical properties of the cell wall in pollen tube growth of<Emphasis Type="Italic"> Solanum chacoense</Emphasis>

The cell wall is one of the structural key players regulating pollen tube growth, since plant cell expansion depends on an interplay between intracellular driving forces and the controlled yielding of the cell wall. Pectin is the main cell wall component at the growing pollen tube apex. We therefore assessed its role in pollen tube growth and cytomechanics using the enzymes pectinase and pectin methyl esterase (PME). Pectinase activity was able to stimulate pollen germination and tube growth at moderate concentrations whereas higher concentrations caused apical swelling or bursting in Solanum chacoense Bitt. pollen tubes. This is consistent with a modification of the physical properties of the cell wall affecting its extensibility and thus the growth rate, as well as its capacity to withstand turgor. To prove that the enzyme-induced effects were due to the altered cell wall mechanics, we subjected pollen tubes to micro-indentation experiments. We observed that cellular stiffness was reduced and visco-elasticity increased in the presence of pectinase. These are the first mechanical data that confirm the influence of the amount of pectins in the pollen tube cell wall on the physical parameters characterizing overall cellular architecture. Cytomechanical data were also obtained to analyze the role of the degree of pectin methyl-esterification, which is known to exhibit a gradient along the pollen tube axis. This feature has frequently been suggested to result in a gradient of the physical properties characterizing the cell wall and our data provide, for the first time, mechanical support for this concept. The gradient in cell wall composition from apical esterified to distal de-esterified pectins seems to be correlated with an increase in the degree of cell wall rigidity and a decrease of visco-elasticity. Our mechanical approach provides new insights concerning the mechanics of pollen tube growth and the architecture of living plant cells.     

The mouse as a model of heart morphogenesis in mammals: origin and lineage of myocytes

In order to follow cardiac precursor cells, we have adopted a retrospective clonal approach, based on the nlaacZ genetic label. Random clones were generated and observed at different developmental stages in murine myocardium. The distribution of these clones in clusters suggest for the first time that cells fated to form myocardium proliferate in two steps. The first growth phase, before E8.5, is dispersive and polarised along the axis of the primitive cardiac tube, contributing to its elongation. The second growth phase is coherent and polarised differentially in different cardiac subregions. Interestingly, this can be correlated with production of geometrical forms (dilatation of a sphere, enlargement of a tube), showing the relation between heart morphogenesis and the controlled proliferation of myocardial cells. The restricted distribution of clones to the right or left ventricule was also investigated with the goal of establishing the time at which cardiac chamber identity emerges. Right and left ventricular lineages appear to segregate early, in agreement with the existence of two populations of cardiac precursors, the so-called primary (or posterior) and secondary (or anterior) heart fields.     

Development of mantle leaves inPlatycerium bifurcatum (Polypodiaceae)

InPlatycerium bifurcatum the leaf primordia emerge alternately right and left below the shoot apical cell on the dorsal surface of the rhizome. They arise from groups of small cells, a single large cell becoming the initial of the leaf apical cell. The longitudinal axis of the leaf apical cell is at a right angle to the rhizome axis and the leaf primordia are arranged longitudinally in two rows. The leaf apical cell gives rise to marginal initials which are responsible for leaf growth in one plain. Early marginal cells are crescent-shaped while the later ones are wedge-shaped. Hairy marginal cells appear in the very early stages of development. The interpretation of these cells as a promeristem and as initials are discussed.     

Signaling mechanisms in the establishment of plant and fucoid algal polarity

The establishment of polarity is a fundamental property of most cells. In tip‐growing plant and in fucoid algal cells, polarization specifies a growth pole, the center of localized secretion of new plasma membrane and cell wall material, generating a protrusion with a dome‐shaped apex. Although much progress has been made concerning the cellular machinery required to execute tip growth, less is known regarding the signaling mechanisms involved in selecting the growth site and regulating vectorial cell division and expansion. Fucoid algal zygotes use extrinsic cues to orient their growth axes and are thus well‐suited for studies of de novo selection of an axis. This process has been investigated largely by both pharmacological and immuno‐localization studies. In tip growing plant cells, polarity is often predetermined, as in the formation of root hairs or moss protonema branches. More focus has been on genomic and genetic studies to reveal the molecules involved in expressing a growth axis. Here we review the common roles of the cytoskeleton and signal transduction pathways in the formation of a developmental axis in fucoid algal cells and the control of tip growth in higher plant cells. Mol. Reprod. Dev. 77: 751–758, 2010. © 2010 Wiley‐Liss, Inc.     

Cellular recruitment and the development of the myocardium

The vertebrate embryo experiences very rapid growth following fertilization. This necessitates the establishment of blood circulation, which is initiated during the early somite stages of development when the embryo begins to exhibit three-dimensional tissue organization. Accordingly, the contractile heart is the first functional organ that develops in both the bird and mammalian embryo. The vertebrate heart is quickly assembled as a simple two-layer tube consisting of an outer myocardium and inner endocardium. During embryogenesis, the heart undergoes substantial growth and remodeling to meet the increased circulatory requirements of an adult organism. Until recently, it was thought that all the cells that comprise the muscle of the mature heart could trace their roots back to two bilaterally distributed mesodermal fields within the early gastrula. It is now known that the cellular components that give rise to the myocardium have multiple ancestries and that de novo addition of cardiac myocytes to the developing heart occurs at various points during embryogenesis. In this article, we review what is presently known about the source of the cells that contribute to the myocardium and explore reasons why multiple myocardial cell sources exist.     

Myocardial cell relationships during morphogenesis in normal and cardiac lethal mutant axolotls, Ambystoma mexicanum

Sarcomere formation has been shown to be deficient in the myocardium of axolotl embryos homozygous for the recessive cardiac lethal gene c. We examined the developing hearts of normal and cardiac mutant embryos from tailbud stage 33 to posthatching stage 43 by scanning electron microscopy in order to determine whether that deficiency has any effect on heart morphogenesis. Specifically, we investigated the relationships of myocardial cells during the formation of the heart tube (stage 33), the initiation of dextral looping (stages 34-36), and the subsequent flexure of the elongating heart (stages 38-43). In addition, we compared the morphogenetic events in the axolotl to the published accounts of comparable stages in the chick embryo. In the axolotl (stage 33), changes in cell shape and orientation accompany the closure of the myocardial trough to form the tubular heart. The ventral mesocardium persists longer in the axolotl embryo than in the chick and appears to contribute to the asymmetry of dextral looping (stages 34-36) in two ways. First, as a persisting structure it places constraints on the simple elongation of the heart tube and the ability of the heart to bend. Second, after it is resorbed, the ventral myocardial cells that contributed to it are identifiable by their orientation, which is orthogonal to adjacent cells: a potential source of shearing effects. Cardiac lethal mutant embryos behave identically during these events, indicating that functional sarcomeres are not necessary to these processes. The absence of dynamic apical myocardial membrane changes, characteristic of the chick embryo (Hamburger and Hamilton stages 9-11), suggests that sudden hydration of the cardiac jelly is less likely to be a major factor in axolotl cardiac morphogenesis. Subsequent flexure (stages 38-43) of the axolotl heart is the same in normal and cardiac lethal mutant embryos as the myocardial tube lengthens within the confines of a pericardial cavity of fixed length. However, the cardiac mutant begins to exhibit abnormalities at this time. The lack of trabeculation (normally beginning at stage 37) in the mutant ventricle is evident at the same time as an increase in myocardial surface area, manifest in extra bends of the heart tube at stage 39. Nonbeating mutant hearts (stage 41) have an abnormally large diameter in the atrioventricular region, possibly the result of the accumulation of ascites fluid. In addition, mutant myocardial cells have a larger apical surface area compared to normals.     

The cellular parameters of leaf development in tobacco: a clonal analysis

The cellular parameters of leaf development in tobacco (Nicotiana tabacum L.) have been characterized using clonal analysis, an approach that provides unequivocal evidence of cell lineage. Our results indicate that the tobacco leaf arises from a group of around 100 cells in the shoot apical meristem. Each of these cells contributes to a unique longitudinal section of the axis and transverse section of the lamina. This pattern of cell lincage indicates that primordial cells contribute more or less equally to the growth of the axis, in contrast to the more traditional view of leaf development in which the leaf is pictured as arising from a group of apical initials. Clones induced prior to the initiation of the lamina demonstrate that the subepidermal layer of the lamina arises from at least six files of cells. Submarginal cells usually divide with their spindles parallel to the margin, and therefore contribute relatively little to the transverse expansion of the lamina. During the expansion of the lamina the orientation and frequency of cell division are highly regulated, as is the duration of meristematic growth. Initially, cell division is polarized so as to produce lineages that are at an oblique angle to the midrib; later cell division is in alternating perpendicular planes. The distribution of clones generated by irradiation at various stages of development indicates that cell division ceases at the tip of the leaf when the leaf is about one tenth its final size, and then ceases in progressively more basal regions of the lamina. Variation in the mutation frequency within the lamina reflects variation in the frequency of mitosis. Prior to the mergence of the leaf the frequency of mutation is maximal near the tip of the leaf and extremely low at its base; after emergence, the frequency of mutation increases at the base of the leaf. In any given region of the lamina the frequency of mutation is highest in interveinal regions, and is relatively low near the margin. Thus, both the orientation and frequency of cell division at the leaf margin indicate that this region plays a minor role in the growth of the lamina.Abbreviation MF mutation frequency     

Genetic control of cell morphogenesis during Drosophila melanogaster cardiac tube formation

Tubulogenesis is an essential component of organ development, yet the underlying cellular mechanisms are poorly understood. We analyze here the formation of the Drosophila melanogaster cardiac lumen that arises from the migration and subsequent coalescence of bilateral rows of cardioblasts. Our study of cell behavior using three-dimensional and time-lapse imaging and the distribution of cell polarity markers reveals a new mechanism of tubulogenesis in which repulsion of prepatterned luminal domains with basal membrane properties and cell shape remodeling constitute the main driving forces. Furthermore, we identify a genetic pathway in which roundabout, slit, held out wings, and dystroglycan control cardiac lumen formation by establishing nonadherent luminal membranes and regulating cell shape changes. From these data we propose a model for D. melanogaster cardiac lumen formation, which differs, both at a cellular and molecular level, from current models of epithelial tubulogenesis. We suggest that this new example of tube formation may be helpful in studying vertebrate heart tube formation and primary vasculogenesis.     

Regular Array in the Cell Wall of Lactobacillus fermenti as Revealed by Freeze-Etching and Negative Staining1

Freeze-etching of Lactobacillus fermenti F-4 (NCTC 7230) revealed that the outer layer of the cell wall was composed of a regular array in which parallel lines ran obliquely to the longitudinal axis of the cell with an average distance between the centers of about 9.6 nm and were intersected by thinner lines with an average periodicity of approximately 6.2 nm at an angle of about 75°. Occasionally the direction of the striation was discontinuously shifted near one end of the cell. Beneath the regular array the middle cell wall layer packed with granules and the smooth inner cell wall layer were discernible and the mesosomes were also visible in the cytoplasm. When the ultrastructure of isolated outer cell wall fragments was examined by negative staining, the regular array appeared to be composed of subunits, about 3.6 nm in diameter, which were arranged in a tetragonal pattern. The tetragonal array consisted of the subunits in rows in two directions at an angle of about 75° to each other. The average spacing between the rows was about 9.3 nm in one direction and 5.5 nm in the other direction.     

Polar growth in pollen tubes is associated with spatially confined dynamic changes in cell mechanical properties

Cellular morphogenesis involves changes to cellular size and shape which in the case of walled cells implies the mechanical deformation of the extracellular matrix. So far, technical challenges have made quantitative mechanical measurements of this process at subcellular scale impossible. We used micro-indentation to investigate the dynamic changes in the cellular mechanical properties during the onset of spatially confined growth activities in plant cells. Pollen tubes are cellular protuberances that have a strictly unidirectional growth pattern. Micro-indentation of these cells revealed that the initial formation of a cylindrical protuberance is preceded by a local reduction in cellular stiffness. Similar cellular softening was observed before the onset of a rapid growth phase in cells with oscillating growth pattern. These findings provide the first quantitative cytomechanical data that confirm the important role of the mechanical properties of the cell wall for local cellular growth processes. They are consistent with a conceptual model that explains pollen tube oscillatory growth based on the relationship between turgor pressure and tensile resistance in the apical cell wall. To further confirm the significance of cell mechanics, we artificially manipulated the mechanical cell wall properties as well as the turgor pressure. We observed that these changes affected the oscillation profile and were able to induce oscillatory behavior in steadily growing tubes.     

Signal transduction in early heart development (II): ventricular chamber specification, trabeculation, and heart valve formation

The formation of a four-chambered heart with ventricular chambers aligned in a left-right orientation begins with the rightward looping of the linear heart tube in accordance with the left-right embryonic axis. The functional specification of the ventricular chambers in the looped heart occurs with the formation of a trabeculated myocardium along the outer curvature of the realigned heart tube. Two major signal transduction pathways are involved in this process, the retinoic acid and neuregulin signaling pathways, with the retinoic acid pathway also participating in rightward heart tube looping. With the establishment of the atrial and ventricular chambers, maintenance of a unidirectional flow of blood between the two chambers must be ensured. To achieve this, heart valves develop at the atrioventricular juncture. This process begins with formation of endocardial cushions, the primordia of heart valves, and ends with formation of heart valve leaflets. Underlying this process is a complex network of signal transduction pathways that mediate communication between the endocardial and myocardial cell layers to form the endocardial cushions and nascent heart valve. Some of the signaling molecules involved are vascular endothelial growth factor, Wnts, bone morphogenetic proteins, epidermal growth factor, hyaluronic acid, neurofibromin, and calcium.     

Effects of environmental factors on wood formation in larch (Larix sibirica Ldb.) stems

 Effects of temperature and precipitation on xylem cell production by the cambium, radial cell expansion and secondary wall thickening in larch stems have been studied. The observations were carried out over two seasons on ten 50- to 60-year-old trees, growing in central Siberia and chosen according to growth rate (the number of cells in radial rows of each of two of the preceding seasons was equal). The data on the number of cells in differentiation zones and mature xylem along radial rows of tracheids, radial and tangential sizes of tracheids and their lumina were used for calculating cambial activity, the rates and durations of cell development in the zones, and both the thickness and cross-sectional areas of tracheid walls. The mean day air temperature, mean maximum diurnal and mean minimum nocturnal temperatures as well as precipitation have been shown by correlation and regression analyses to affect differentially separate stages of tracheid differentiation. Throughout all the seasons it was temperature that had the main influence on the initial divisions in the xylem, radial cell expansion and biomass accumulation. However, the levels of such an effect on separate stages of cytogenesis were different, especially the influence of nocturnal temperature on xylem cell production by cambium and primary wall growth. The optimum values of temperature and precipitation for cell production by cambium, for radial cell expansion and secondary wall thickening have been calculated. These optimum values of the first and second processes proved to be practically equal, while the last differs considerably in response to temperature. The data are discussed in connection with formation of early and late tracheids. Received: 3 July 1996 / Accepted: 7 February 1997     

The anterior heart-forming field: voyage to the arterial pole of the heart

Studies of vertebrate heart development have identified key genes and signalling molecules involved in the formation of a myocardial tube from paired heart-forming fields in splanchnic mesoderm. The posterior region of the paired heart-forming fields subsequently contributes myocardial precursor cells to the inflow region or venous pole of the heart. Recently, a population of myocardial precursor cells in chick and mouse embryos has been identified in pharyngeal mesoderm anterior to the early heart tube. This anterior heart-forming field gives rise to myocardium of the outflow region or arterial pole of the heart. The amniote heart is therefore derived from two myocardial precursor cell populations, which appear to be regulated by distinct genetic programmes. Discovery of the anterior heart-forming field has important implications for the interpretation of cardiac defects in mouse mutants and for the study of human congenital heart disease.     

Properties of a clonal muscle cell line from rat heart

Two clonal cell lines have been derived from the thoracic aorta of embryonic rats. Both of these cell lines, at some stage of their development possess membranes capable of generating overshooting action potentials spontaneously. Contiguous cells of each of these lines are electrically coupled. Ultrastructural analysis consistently reveals the presence of rows of pinocytotic vesicles, a well-developed rough endoplasmic reticulum, massive tracts of thin filaments oriented parallel to the longitudinal axis of the cell and randomly dispersed intermediate sized filaments. The specific activities of the enzymes myokinase and creatine phosphokinase (CPK) increase 3- to 5-fold after growth has ceased. These two cell lines synthesize a muscle type CPK isoenzyme after the cessation of cell division. It is concluded that these cell lines proliferate as myoblasts and develop into cells which phenotypically resemble smooth muscle. A third clonal cell line, from fetal rat aorta, with properties of both smooth and skeletal muscle, is also described.     

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four‐chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart‐forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium‐secreted insulin‐like growth factor, the expression of which is regulated by hepatic mesoderm‐derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium‐derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.