Functional aspects of cell patterning in aerial epidermis

Plants have evolved epidermal cells that have specialized functions as adaptations to life on land. Many of the functions of these specialized cells are dependent, to a significant extent, on their arrangement within the aerial epidermis. Considerable progress has been made over the past two years in understanding the patterning mechanisms of trichomes and stomata in Arabidopsis leaves at the molecular level. How universal are these patterning programmes, and how are they adjusted to meet the changing functions of specialized epidermal cells in different plant organs? In this review, we compare the patterning of stomata and trichomes in different plant species, describe environmental and developmental factors that alter cell patterning, and discuss how changes in patterning might relate to cell function. Patterning is an important aspect to the functioning of aerial epidermal cells, and a greater understanding of the processes that are involved will significantly enhance our understanding of how cellular activities are integrated in multicellular plants.     

Determining cell shape: adaptive regulation of cyanobacterial cellular differentiation and morphology

Similar to other bacteria, cyanobacteria exist in a wide-ranging diversity of shapes and sizes. However, three general shapes are observed most frequently: spherical, rod and spiral. Bacteria can also grow as filaments of cells. Some filamentous cyanobacteria have differentiated cell types that exhibit distinct morphologies: motile hormogonia, nitrogen-fixing heterocysts, and spore-like akinetes. Cyanobacterial cell shapes, which are largely controlled by the cell wall, can be regulated by developmental and/or environmental cues, although the mechanisms of regulation and the selective advantage(s) of regulating cellular shape are still being elucidated. In this review, recent insights into developmental and environmental regulation of cell shape in cyanobacteria and the relationship(s) of cell shape and differentiation to organismal fitness are discussed.     

The selective value of bacterial shape.

Why do bacteria have shape? Is morphology valuable or just a trivial secondary characteristic? Why should bacteria have one shape instead of another? Three broad considerations suggest that bacterial shapes are not accidental but are biologically important: cells adopt uniform morphologies from among a wide variety of possibilities, some cells modify their shape as conditions demand, and morphology can be tracked through evolutionary lineages. All of these imply that shape is a selectable feature that aids survival. The aim of this review is to spell out the physical, environmental, and biological forces that favor different bacterial morphologies and which, therefore, contribute to natural selection. Specifically, cell shape is driven by eight general considerations: nutrient access, cell division and segregation, attachment to surfaces, passive dispersal, active motility, polar differentiation, the need to escape predators, and the advantages of cellular differentiation. Bacteria respond to these forces by performing a type of calculus, integrating over a number of environmental and behavioral factors to produce a size and shape that are optimal for the circumstances in which they live. Just as we are beginning to answer how bacteria create their shapes, it seems reasonable and essential that we expand our efforts to understand why they do so.     

The Selective Value of Bacterial Shape

Why do bacteria have shape? Is morphology valuable or just a trivial secondary characteristic? Why should bacteria have one shape instead of another? Three broad considerations suggest that bacterial shapes are not accidental but are biologically important: cells adopt uniform morphologies from among a wide variety of possibilities, some cells modify their shape as conditions demand, and morphology can be tracked through evolutionary lineages. All of these imply that shape is a selectable feature that aids survival. The aim of this review is to spell out the physical, environmental, and biological forces that favor different bacterial morphologies and which, therefore, contribute to natural selection. Specifically, cell shape is driven by eight general considerations: nutrient access, cell division and segregation, attachment to surfaces, passive dispersal, active motility, polar differentiation, the need to escape predators, and the advantages of cellular differentiation. Bacteria respond to these forces by performing a type of calculus, integrating over a number of environmental and behavioral factors to produce a size and shape that are optimal for the circumstances in which they live. Just as we are beginning to answer how bacteria create their shapes, it seems reasonable and essential that we expand our efforts to understand why they do so.     

Testing heterochrony: Connecting skull shape ontogeny and evolution of feeding adaptations in baleen whales

Ontogeny plays a key role in the evolution of organisms, as changes during the complex processes of development can allow for new traits to arise. Identifying changes in ontogenetic allometry—the relationship between skull shape and size during growth—can reveal the processes underlying major evolutionary transformations. Baleen whales (Mysticeti, Cetacea) underwent major morphological changes in transitioning from their ancestral raptorial feeding mode to the three specialized filter-feeding modes observed in extant taxa. Heterochronic processes have been implicated in the evolution of these feeding modes, and their associated specialized cranial morphologies, but their role has never been tested with quantitative data. Here, we quantified skull shapes ontogeny and reconstructed ancestral allometric trajectories using 3D geometric morphometrics and phylogenetic comparative methods on sample representing modern mysticetes diversity. Our results demonstrate that Mysticeti, while having a common developmental trajectory, present distinct cranial shapes from early in their ontogeny corresponding to their different feeding ecologies. Size is the main driver of shape disparity across mysticetes. Disparate heterochronic processes are evident in the evolution of the group: skim feeders present accelerated growth relative to the ancestral nodes, while Balaenopteridae have overall slower growth, or pedomorphosis. Gray whales are the only taxon with a relatively faster rate of growth in this group, which might be connected to its unique benthic feeding strategy. Reconstructed ancestral allometries and related skull shapes indicate that extinct taxa used less specialized filter-feeding modes, a finding broadly in line with the available fossil evidence.     

Local actin dynamics couple speed and persistence in a cellular Potts model of cell migration

Cell migration is astoundingly diverse. Molecular signatures, cell-cell interactions, and environmental structures each play their part in shaping cell motion, yielding numerous morphologies and migration modes. Nevertheless, in recent years, a simple unifying law was found to describe cell migration across many different cell types and contexts: faster cells turn less frequently. This universal coupling between speed and persistence (UCSP) was explained by retrograde actin flow from front to back, but it remains unclear how this mechanism generalizes to cells with complex shapes and cells migrating in structured environments, which may not have a well-defined front-to-back orientation. Here, we present an in-depth characterization of an existing cellular Potts model, in which cells polarize dynamically from a combination of local actin dynamics (stimulating protrusions) and global membrane tension along the perimeter (inhibiting protrusions). We first show that the UCSP emerges spontaneously in this model through a cross talk of intracellular mechanisms, cell shape, and environmental constraints, resembling the dynamic nature of cell migration in vivo. Importantly, we find that local protrusion dynamics suffice to reproduce the UCSP—even in cases in which no clear global, front-to-back polarity exists. We then harness the spatial nature of the cellular Potts model to show how cell shape dynamics limit both the speed and persistence a cell can reach and how a rigid environment such as the skin can restrict cell motility even further. Our results broaden the range of potential mechanisms underlying the speed-persistence coupling that has emerged as a fundamental property of migrating cells.     

三种国产阴地蕨科植物叶的表皮构造

运用光学显微镜和扫描电镜对三种国产阴地蕨科植物：劲直蕨萁(Botrypus strictus)、小阴地蕨(Botrychium lunaria)和薄叶阴地蕨(Sceptridium daucifolium)〖WTBZ〗叶的成熟表皮构造进行了详细观察和研究。它们具有共同的特征：气孔散生,气孔类型无规则型,气孔长轴方向多与叶脉延伸方向一致。但也存在明显区别,特别是劲直蕨萁与另外二个种的区别更为明显：前者表皮细胞垂周壁直,相邻气孔不接触,保卫细胞平周壁具细条纹。但后二者之间亦有一定区别：薄叶阴地蕨的气孔为下生式,不下陷,而小阴地蕨的气孔为两面气孔型,气孔下陷。本文是国内首次对阴地蕨科叶的表皮构造进行研究,其研究结果表明,表皮构造在阴地蕨科植物的鉴定上具有重要意义,并且在研究阴地蕨科的分类以及起源和演化上也有一定参考价值。     

Cuticle micromorphology of leaves of Pinus (Pinaceae) from Mexico and Central America

Cuticle micromorphology of 34 taxa of Pinus from Mexico and Central America was studied with scanning electron microscopy, and leaf morphology was described. In total, 29 characters, 22 from the inner cuticular surfaces and seven from the outer, were described in detail. These characters have value either for testing infragenerie classifications or for identifying individual taxa. Characters relating to the periclinal wall texture of the epidermal cells, the shape and degree of development of the anticlinal walls of the epidermal cells, the basal and apical shapes of anticlinal epidermal cell walls, the continuity of the epidermal cells, the size ratio of the polar to lateral subsidiary cells, the grooves on subsidiary cells, the cuticular flanges between guard and subsidiary cells, the groove near the bristles and the elevation of the Florin ring ridge and striations on the Florin ring are particularly useful for infrageneric classification. The agreement between these characters and infrageneric classifications is discussed. Characters relating to the end wall shapes of the epidermal cells, the relative length of epidermal cells, the shape of the stomatal apparatus, the texture of guard and lateral subsidiary cell surfaces, the polar extensions, the number of subsidiary cells and epidermal cell layers between stomatal rows, the integrity of stomatal rows, cell numbers between stomata in a row, cuticular flanges between guard cells, bristle flanges and surface textures, epicuticular waxes, striations on Florin rings and stomatal shapes, contain some important information for identifying Mexican pines. The distribution of the states of each character is compared with that of the Asian pines. Cuticular characters are used to help determine the affinities of taxonomically difficult taxa.     

The Changing Shape of Plant Cells: Transformations During Cell Proliferation

Shapes of ideal cells can be inspected for the dynamic, or gnomonic,feature of producing daughter cells of the same shape. Suchfeatures can be found for (a) elongating epidermal cells, (b)isdiametrically enlarging epidermal cells, (c) elongating parenchymatouscells and (d) parenchymatous cells enlarging in three dimensions.Since each cell passes through a series of changes to finallyassume the form of the parental cell, a gnomonic cell must passthrough a gnomonic sequence of shapes during the cell cycle.A model tissue composed of gnomonic cells has complete stabilityof form through subsequent generations. Each of six parameters of ideal cells can be inspected in realcells in order to evaluate the effects of deviations from theideal on the stability of tissue pattern. (1) Cell plates ofreal and ideal cells do not expand for one generation. (2) Theangles in vertices of real cells shift over three cell cyclesfrom 170.1° to 137.3° to 124.0°, values close tothe expected set of 163°, 133° and 120° (3) Cellplates of real cells are not perpendicular to the longitudinalaxis of the cell. (4) Real cells do not divide synchronouslyas do ideal cells. (5) Real cells do not divide equally in halfas do ideal cells. (6) Finally, ideal cells have the same durationof the cell cycle whereas real cells have cycle times inverselyrelated to the initial size of the cell. It appears that a population of meristematic cells do not adhereto the restrictions of ideal cells, and consequently a significantamount of variance of form is added at each generation. Thereare two compensating mechanisms, one to hold size variationin check and one to keep shape deviations under control. Becauseof the probabilistic nature of cell division, cells increasein volume at various rates while the cell edges of all cellsexpand at a constant rate, indicating that the latter is theprimary element of growth while facet area and cell volume increasein dimension only for accommodation. Cell shape, gnomonic cells, Aponogeton elongatus, Lupinus alba     

Glial epigenetics in neuroinflammation and neurodegeneration

Epigenetic regulation shapes the differentiation and response to stimuli of all tissues and cells beyond what genetics would dictate. Epigenetic regulation acts through covalent modifications of DNA and histones while leaving the nucleotide code intact. However, these chromatin modifications are known to be vital components of the regulation of cell fate and response. With regards to the central nervous system (CNS), little is known about how epigenetic regulation shapes the function of neural cell types. The focus of research so far has been on epigenetic regulation of neuronal function and the role of epigenetics in tumorigenesis. However, the glial cell compartment, which makes up 90 % of all CNS cells, has so far received scant attention as to how epigenetics shape their differentiation and function. Here, we highlight current knowledge about epigenetic changes in glial cells occurring during CNS injury, neuroinflammatory conditions and neurodegenerative disease. This review offers an overview of the current understanding of epigenetic regulation in glial cells in CNS disease.     

Testing effects of signal transduction pathways on cadherin junctional complex assembly using quantitative image analysis

Cadherin adhesion molecules function in numerous cell biological processes that influence embryo development, normal cell physiology, and pathophysiology of many disease processes. Cadherins nucleate the assembly of the adherens junction, a cell-to-cell adhesion plaque that is prominent in simple epithelial cells and found in many cell types. Numerous cell biological approaches have been used to study this interesting class of molecules. Here, we outline methodology used in our studies of junctional complexes to examine effects of signaling molecules on assembly mechanisms. This is a quantitative method that allows the investigator to test the combined effect of two different signaling processes to determine whether these two signals act in concert within the same pathway. We discuss how this method could be generalized to other studies to examine consequences of various experimental manipulations on the assembly of cellular structures.     

Effectors of tridimensional cell morphogenesis and their evolution

One of the most challenging problems in biology resides in unraveling the molecular mechanisms, hardwired in the genome, that define and regulate the multiscale tridimensional organization of organs, tissues and individual cells. While works in cultured cells have revealed the importance of cytoskeletal networks for cell architecture, in vivo models are now required to explore how such a variety in cell shape is produced during development, in interaction with neighboring cells and tissues. The genetic analysis of epidermis development in Drosophila has provided an unbiased way to identify mechanisms remodeling the shape of epidermal cells, to form apical trichomes during terminal differentiation. Since hearing in vertebrates relies on apical cell extensions in sensory cells of the cochlea, called stereocilia, the mapping of human genes causing hereditary deafness has independently identified several factors required for this peculiar tridimensional organization. In this review, we summarized recent results obtained toward the identification of genes involved in these localized changes in cell shape and discuss their evolution throughout developmental processes and species.     

不同地区青檀叶片的解剖及表皮特征的比较研究

采用石蜡切片技术和叶表皮离析法,对我国8个省共15个居群的青檀(Pteroceltis tatarinowii Maxim.)叶片特征进行了比较和分析.研究发现:各居群的栅栏组织细胞层数多数为2层,显示出一定的稳定性,但不同居群的叶片横切面解剖特征存在显著性差异.叶片上表皮细胞有多边形和不规则形两种类型,而下表皮细胞均为不规则形.上表皮细胞垂周壁为平直形、圆形、浅波形或深波形,差异较明显,下表皮细胞垂周壁则仅有深波形和浅波形两种.叶表皮毛的分布位置在各居群间不同,而表皮毛的类型在居群间稳定,且在多数居群中表皮毛与周围细胞形成花环结构.各居群的气孔器类型均为不规则型且均仅在下表皮随机分布,但气孔器的密度、大小等特征在不同居群间存在显著性差异.生境地的气候因子对青檀叶的特征产生一定的影响.结果表明:青檀叶的解剖特征在不同居群间差异性较大,青檀叶表皮的气孔密度等特征在居群间有一定差异,而气孔器类型和表皮细胞形状等特征较稳定,有一定的分类学价值.     

Capabilities and Limitations of Tissue Size Control through Passive Mechanical Forces

Embryogenesis is an extraordinarily robust process, exhibiting the ability to control tissue size and repair patterning defects in the face of environmental and genetic perturbations. The size and shape of a developing tissue is a function of the number and size of its constituent cells as well as their geometric packing. How these cellular properties are coordinated at the tissue level to ensure developmental robustness remains a mystery; understanding this process requires studying multiple concurrent processes that make up morphogenesis, including the spatial patterning of cell fates and apoptosis, as well as cell intercalations. In this work, we develop a computational model that aims to understand aspects of the robust pattern repair mechanisms of the Drosophila embryonic epidermal tissues. Size control in this system has previously been shown to rely on the regulation of apoptosis rather than proliferation; however, to date little work has been done to understand the role of cellular mechanics in this process. We employ a vertex model of an embryonic segment to test hypotheses about the emergence of this size control. Comparing the model to previously published data across wild type and genetic perturbations, we show that passive mechanical forces suffice to explain the observed size control in the posterior (P) compartment of a segment. However, observed asymmetries in cell death frequencies across the segment are demonstrated to require patterning of cellular properties in the model. Finally, we show that distinct forms of mechanical regulation in the model may be distinguished by differences in cell shapes in the P compartment, as quantified through experimentally accessible summary statistics, as well as by the tissue recoil after laser ablation experiments.     

Examination of Drosophila Larval Tracheal Terminal Cells by Light Microscopy

Cell shape is critical for cell function. However, despite the importance of cell morphology, little is known about how individual cells generate specific shapes. Drosophila tracheal terminal cells have become a powerful genetic model to identify and elucidate the roles of genes required for generating cellular morphologies. Terminal cells are a component of a branched tubular network, the tracheal system that functions to supply oxygen to internal tissues. Terminal cells are an excellent model for investigating questions of cell shape as they possess two distinct cellular architectures. First, terminal cells have an elaborate branched morphology, similar to complex neurons; second, terminal cell branches are formed as thin tubes and contain a membrane-bound intracellular lumen. Quantitative analysis of terminal cell branch number, branch organization and individual branch shape, can be used to provide information about the role of specific genetic mechanisms in the making of a branched cell. Analysis of tube formation in these cells can reveal conserved mechanisms of tubulogenesis common to other tubular networks, such as the vertebrate vasculature. Here we describe techniques that can be used to rapidly fix, image, and analyze both branching patterns and tube formation in terminal cells within Drosophila larvae. These techniques can be used to analyze terminal cells in wild-type and mutant animals, or genetic mosaics. Because of the high efficiency of this protocol, it is also well suited for genetic, RNAi-based, or drug screens in the Drosophila tracheal system.     

Raman spectroscopy of DNA packaging in individual human sperm cells distinguishes normal from abnormal cells

Healthy human males produce sperm cells of which about 25–40% have abnormal head shapes. Increases in the percentage of sperm exhibiting aberrant sperm head morphologies have been correlated with male infertility, and biochemical studies of pooled sperm have suggested that sperm with abnormal shape may contain DNA that has not been properly repackaged by protamine during spermatid development. We have used micro‐Raman spectroscopy to obtain Raman spectra from individual human sperm cells and examined how differences in the Raman spectra of sperm chromatin correlate with cell shape. We show that Raman spectra of individual sperm cells contain vibrational marker modes that can be used to assess the efficiency of DNA‐packaging for each cell. Raman spectra obtained from sperm cells with normal shape provide evidence that DNA in these sperm is very efficiently packaged. We find, however, that the relative protein content per cell and DNA packaging efficiencies are distributed over a relatively wide range for sperm cells with both normal and abnormal shape. These findings indicate that single cell Raman spectroscopy should be a valuable tool in assessing the quality of sperm cells for in‐vitro fertilization. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Clonal analysis of epidermal patterning during maize leaf development

In plants, specialized epidermal cells are arranged in semiordered patterns. In grasses such as maize, stomata and other specialized cell types differentiate in linear patterns within the leaf epidermis. A variety of mechanisms have been proposed to direct patterns of epidermal cell differentiation. One class of models proposes that patterns of cellular differentiation depend on the lineage relationships among epidermal cells. Another class of models proposes that epidermal patterning depends on positional information rather than lineage relationships. In the dicot epidermis, cell lineage is an important factor in the patterning of stomata, but not trichomes. In this study, the role of cell lineage in the linear patterning of stomata and bulliform cells in the maize leaf epidermis is investigated. Clones of epidermal cells in juvenile leaves were marked by excision of dSpm from gl15-m and in adult leaves by excision of Ds2 from bz2-m. These clones were analyzed in relation to patterns of stomata and bulliform cells, testing specific predictions of clonal origin hypotheses for the patterning of these cell types. We found that the great majority of clones analyzed failed to satisfy these predictions. Our results clearly show that lineage does not account for the linear patterning of stomata and bulliform cells, implying that positional information must direct the differentiation patterns of these cell types in maize.     

Morphology and ultrastructure of Interfilum and Klebsormidium (Klebsormidiales,Streptophyta) with special reference to cell division and thallus formation

Representatives of the closely related genera, Interfilum and Klebsormidium, are characterized by unicells, dyads or packets in Interfilum and contrasting uniseriate filaments in Klebsormidium. According to the literature, these distinct thallus forms originate by different types of cell division, sporulation (cytogony) versus vegetative cell division (cytotomy), but investigations of their morphology and ultrastructure show a high degree of similarity. Cell walls of both genera are characterized by triangular spaces between cell walls of neighbouring cells and the parental wall or central space among the walls of a cell packet, exfoliations and projections of the parental wall and cap-like and H-like fragments of the cell wall. In both genera, each cell has its individual cell wall and it also has part of the common parental wall or its remnants. Therefore, vegetative cells of Interfilum and Klebsormidium probably divide by the same type of cell division (sporulation-like). Various strains representing different species of the two genera are characterized by differences in cell wall ultrastructure, particularly the level of preservation, rupture or gelatinization of the parental wall surrounding the daughter cells. The differing morphologies of representatives of various lineages result from features of the parental wall during cell separation and detachment. Cell division in three planes (usual in Interfilum and a rare event in Klebsormidium) takes place in spherical or short cylindrical cells, with the chloroplast positioned perpendicularly or obliquely to the filament (dyad) axis. The morphological differences are mainly a consequence of differing fates of the parental wall after cell division and detachment. The development of different morphologies within the two genera mostly depends on characters such as the shape of cells, texture of cell walls, mechanical interactions between cells and the influence of environmental conditions.     

FtsZ collaborates with penicillin binding proteins to generate bacterial cell shape in Escherichia coli

The mechanisms by which bacteria adopt and maintain individual shapes remain enigmatic. Outstanding questions include why cells are a certain size, length, and width; why they are uniform or irregular; and why some branch while others do not. Previously, we showed that Escherichia coli mutants lacking multiple penicillin binding proteins (PBPs) display extensive morphological diversity. Because defective sites in these cells exhibit the structural and functional characteristics of improperly localized poles, we investigated the connection between cell division and shape. Here we show that under semipermissive conditions the temperature-sensitive FtsZ84 protein produces branched and aberrant cells at a high frequency in mutants lacking PBP 5, and this phenotype is exacerbated by the loss of additional peptidoglycan endopeptidases. Surprisingly, certain ftsZ84 strains lyse at the nonpermissive temperature instead of filamenting, and inhibition of wild-type FtsZ forces some mutants into tightly wound spirillum-like morphologies. The results demonstrate that significant aspects of bacterial shape are dictated by a previously unrecognized relationship between the septation machinery and ostensibly minor peptidoglycan-modifying enzymes and that under certain circumstances improper FtsZ function can destroy the structural integrity of the cell.