Embryo Polarity Research Articles

Mechanisms of Auxin Regulation of Structural and Physiological Polarity in Plants, Tissues, Cells and Embryos

New experiments and insights extend current concepts to yield four integrated hypotheses on polarization. (1) In tissues that lack polarity, as in meristems and proliferating callus, short-distance diffusive movement of auxin between spatially separate sources and sinks can gradually induce polar transport of auxin and associated proton currents in the direction of the initially diffusive movement. (2) The formation of auxin at the shoot tip, on which maintenance of the plant's polarity depends, occurs by hydrolysis of auxin precursors such as IAA-myo-inositol, which readily hydrolyses (even non-enzymically) at rates that increase with pH, so that higher pHs arising in leaf primordia through auxin-proton cotransport can promote local formation of auxin. (3) The long axes of cells and the secondary wall banding of tracheary elements tend respectively to develop parallel and perpendicular to the direction of polar transport of auxin through tissue, especially vascular tissue; these orientations are mediated by the bioelectric current associated with polar auxin transport, through orientation of cortical microtubules transverse to the current. (4) Initiation of polarity in zygotic embryos is controlled by the direction of auxin movement in the surrounding parental tissues; in lower plants the exoscopic embryo's polarity is similar to that of the surrounding tissues, but in seed plants the endoscopic embryo's polarity is inverted as a result of physiological isolation, localised auxin breakdown, and suspensor formation.

Effects of relaxation of mechanical tensions upon the early morphogenesis of Xenopus laevis embryos.

In Xenopus laevis embryos at the early gastrula stage, circumferential tensions of embryonic ectoderm were relaxed by making sagittal or transversal slits in the ventral parts of embryos and inserting into surgical cuts the sectors of homologous tissue from same-stage embryos. Changes in tensile patterns were controlled by measuring cell surface angles. Immediate decreases in surface cell wall tension as related to transversal wall tension were registered. Within minutes of the operation, the lobopodial activity of the inner ectodermal surface increased. The subsequent gastrulation movements were disturbed, germ layers partially mixed and archenteron reduced. The areas of extensive cell columnarization in the ectoderm of operated embryos were less regularly arranged and were extended much more ventrally than in intact embryos. Ventro-dorsal migration and latero-medial intercalation of mesodermal cells also were suppressed. As the operated embryos developed, we observed increases in the total amount of neural tissue, associated sometimes with duplication and even triplication of neural tubes, duplication of otic vesicles, partial fusion of axial rudiments, suppression of mesodermal segmentation and branching or bending of notochord. In the gravest cases the antero-posterior embryo polarity was disturbed. In some cases we observed the formation of axial rudiments in ventral implants. The role of tensions in determining the patterns of morphogenetic cell movements and in establishing the morphological order of normal development is discussed.

Cell-cell interactions determine the dorsoventral axis in embryos of an equally cleaving opisthobranch mollusc.

Dorsoventral polarity in molluscan embryos can arise by two distinct mechanisms, where the mechanism employed is strongly correlated with the cleavage pattern of the early embryo. In species with unequal cleavage, the dorsal lineage, or "D quadrant", is determined in a cell-autonomous manner by the inheritance of cytoplasmic determinants. However, in gastropod molluscs with equal cleavage, cell-cell interactions are required to specify the fate of the dorsal blastomere. During the fifth cleavage interval in equally cleaving embryos, one of the vegetal macromeres makes exclusive contacts with the animal micromeres, and this macromere will give rise to the mesodermal precursor cell at the next division, thereby identifying the dorsal quadrant. This study examines D-quadrant determination in an equally cleaving species from a group of previously uninvestigated gastropods, the subclass Opisthobranchia. Blastomere ablation experiments were performed on embryos of Haminoea callidegenita to (i) determine the developmental potential of macromeres before and after fifth cleavage, and (ii) examine the role of micromere-macromere interactions in the establishment of bilateral symmetry. The results suggest that the macromeres are developmentally equivalent prior to fifth cleavage, but become nonequivalent soon afterward. The dorsoventral axis corresponds to the displacement of the micromeres over one macromere early in the fifth cleavage interval. This unusual cellular topology is hypothesized to result from constraints imposed on micromere-macromere interactions in an embryo that develops from a large egg and forms a stereoblastula (no cleavage cavity). Ablation of the entire first quarter of micromeres results in embryos which remain radially symmetrical in the vegetal hemisphere, indicating that micromere-macromere interactions are required for the elaboration of bilateral symmetry properties. Therefore, inductive interactions between cells may represent a general strategy for dorsoventral axis determination in equally cleaving gastropods.

Interaction between peptide growth factors and homoeobox genes in the establishment of antero-posterior polarity in frog embryos

The expression of the Xenopus homoeobox gene xhox3 is an early response to mesoderm induction by peptide growth factors and the level of xhox3 expression marks the antero-posterior character of the induced mesoderm. Different peptide growth factors specify different antero-posterior mesodermal cell fates as seen by the level of xhox3 expression and the capacity to induce specific secondary neural/epidermal structures. These factors and homoeobox genes thus form part of the mechanism necessary for establishing antero-posterior polarity in the frog embryo.

Cell polarity in sea urchin embryos: Reorientation of cells occurs quickly in aggregates

Four apical components were used as markers for the apical end of the cell in studies centering on cell polarity in the early blastula stage of sea urchin embryos and in aggregates of cleavage stage cells. Cells were observed to maintain their polarity for several hours if dissociated and cultured in suspension. Orientation of cells in aggregates initially is random; however, within 3 hr the cells have reoriented so that their apical-basal axis corresponds to the correct inside-outside position in the aggregate. This reorientation occurs before formation of a basal lamina or a new hyalin layer in the aggregate, and appears to take place by a rotation or other movement of individual cells. The polarity within each cell is maintained during reorientation. An apical surface antigen is colocalized with concentrations of filamentous actin. Treatment of isolated cells with cytochalasin B causes the antigen to lose its apical position and eventually become distributed around the outside of the cell. Microtubules are visible radiating from two foci closely associated with the nucleus in untreated cells. Treatment of isolated cells with nocodazole leaves the apical cell surface marker and its associated actin undisturbed, but causes the nucleus to lose its apical position. Cytochalasin B and colchicine both prevent reorientation of cells in aggregates. Thus polarity appears to be a constant for the cells, and their reorientation in aggregates occurs prior to the polarized release of extraembryonic matrix and basal lamina.

Molecular analysis of a gene involved in the establishment of dorsal-ventral polarity in drosophila embryos: fs(1)K10: [formula omitted]. LGME du CNRS/U.184 de l'INSERM, Faculté de Médecine, 11 rue Humann, 67085 STRASBOURG Cédex, FRANCE

Molecular analysis of a gene involved in the establishment of dorsal-ventral polarity in drosophila embryos: fs(1)K10: [formula omitted]. LGME du CNRS/U.184 de l'INSERM, Faculté de Médecine, 11 rue Humann, 67085 STRASBOURG Cédex, FRANCE

Precipitating monoclonal antibodies to an antigenic determinant of group a streptococcal polysaccharide

Cytoplasm organella segregation accompanies virtually all the changes occuring in embryonic and differentiated cells. The general scheme of mapping spatial distribution of cytoplasm texture parameters, realized using computed microscope LEITZ-T.A.S., allowed the imaging of geometrical relationship between yolk granules in the Rana Temporaria fertilized egg. The development of dorsoventral polarity in amphibian embryos was shown to be related to shape and location asymmetry of cytoplasm area containing clusters of yolk granules.

Amphibian egg cytoplasm response to altered g-forces and gravity orientation

Elucidation of dorsal/ventral polarity and primary embryonic axis development in amphibian embryos requires an understanding of cytoplasmic rearrangements in fertile eggs at the biophysical, physiological, and biochemical levels. Evidence is presented that amphibian egg cytoplasmic components are compartmentalized. The effects of altered orientation to the gravitational vector (i.e., egg inversion) and alterations in gravity force ranging from hypergravity (centrifugation) to simulated microgravity (i.e., horizontal clinostat rotation) on cytoplasmic compartment rearrangements are reviewed. The behavior of yolk compartments as well as a newly defined (with monoclonal antibody) non-yolk cytoplasmic compartment, in inverted eggs and in eggs rotated on horizontal clinostats at their buoyant density, is discussed.

Microgravity simulation as a probe for understanding early Xenopus pattern specification

Pattern specification in early amphibians (Xenopus) was monitored in embryos subjected to gravity compensation (microgravity simulation) by constant low-speed rotation on a horizontal axis (clinostat). The useful range of clinostat speeds was determined empirically. The results were interpreted in terms of a set of models which account for the reorganization of the egg cytoplasm that follows fertilization and that correlates with the establishment of dorsal/ventral polarity. Large percentages of clinostated eggs displayed a positive result (normal axial structure morphogenesis). Consequently, normal development of amphibian eggs in the microgravity environment of space should be possible. Models which depend upon gravity-driven rearrangements for cytoplasmic organization (e.g. dorsal/ventral polarization) of the early embryo should, therefore, not be favoured. At several clinostat speeds symmetrization of the egg in accordance with the site of sperm penetration, a natural phenomenon, was altered. The results at those clinostat speeds indicate that models which employ sperm entrance as an obligatory feature of the cytoplasmic rearrangements that generate egg polarity are not applicable.

On the fine Structure of Paracentrotus Lividus Permanent Blastulae Obtained after Treatment with 5-Bromodeoxyuridine: (5-bromodeoxyuridine/sea urchin embryos/ultrastructure).

This work reports some observations on the fine structure of the different types of Paracentrotus lividus permanent blastulae obtained after treatment with BUdR. Some permanent blastulae appeared similar to normal early mesenchyme blastulae and at the ultrastructural level they did not show any particular differences compared with the normal embryos. The only remarkable fact was that nucleoli often displayed the fibrous portion still abundant. Other permanent blastulae showed the blastocoel partially or completely, invaded by cells and it was often very difficult to recognize the embryo polarity. In these embryos, at the ultrastructural level, the most striking effects were observed in the blastocoelic cells. On the basis of the results reported in this paper and of the data already existing in the literature about the effects of BUdR on sea urchin embryos, the possibility is discussed that the drug might affect selectively some steps essential for the realizing of the embryo differentiation.

Segmentations in microspores ofNicotiana sylvestris andNicotiana tabacum which lead to embryoid formation in anther cultures

It is shown that an equal division in uninucleate microspores, giving two cells with similar staining nuclei, can initiate embryogenesis in anther cultures ofNicotiana sylvestris andN. tabacum var. Wisconsin 38 and evidence is presented that this pathway is quantitatively important and may be the only one leading to successful pollen embryos. Neither an initial asymmetric division in the uninucleate microspore nor the formation of a “suspensor” are essential to the development of embryo polarity.

The relationship between changes in cell wall composition and the establishment of polarity in Fucus embryos

Changes in the appearance and location of fucoidin in the cell walls of Fucus embryos were related to embryo development. Fucoidin was not present in the cell wall until 10–14 hr after fertilization, when the embryos began to incorporate fucoidin preferentially into a localized area of the wall. At this time the site of rhizoid initiation was determined; that is, the embryos had undergone axis commitment. Germination of the single-celled embryo occurred between 12 and 16 hr, after fertilization, with all cell walls from germinated embryos showing fucoidin localization at the rhizoid end of the cell. The percentage of embryos with localized fucoidin at the time of axis fixation equaled the percentage of embryos that subsequently germinated. Culturing the embryos in sea water plus 0.8 M sucrose prevented the outgrowth of the rhizoid, but not the localization of fucoidin in the wall or axis commitment. Cycloheximide, nitroprusside, cytochalasin B, sulfate-free sea water, high levels of Ca 2+, and a breakdown product of TIBA all prevented rhizoid growth and the specific localization of fucoidin. In addition, axis commitment could not be demonstrated in the presence of these inhibitors. DTNB, PCMBS, TIBA, HgCl 2, Mg 2+ were ineffective as reversible inhibitors of rhizoid initiation. The authors propose that the fixation of axis commitment is accompanied by localized changes in the cell wall involving the incorporation of fucoidin as a structural component of the wall.

Ultrastructural changes during growth and embryogenesis in carrot cell cultures

Clumps in suspension cultures of wild carrot have two histological regions; an inner region of highly vacuolate, nondividing cells and a peripheral region of small meristematic cells. Significant ultrastructural features of the meristematic cells include the presence of many starch-filled plastids, a dense background of free ribosomes, rough ER bearing conspicuous polyribosomal aggregates, and numerous multivesicular bodies apparently derived from dictyosomes. Multivesicular bodies fuse with the plasmalemma, releasing their contents into the wall space. Cell enlargement is accompanied by an increase in rough ER, eventual decrease in free ribosomal content, and two types of structural alteration in the walls which eventually cause clump disintegration and release of smaller fragments. Ultrastructural changes accompanying formation of embryos in auxin-free media include a striking increase in free ribosomal content, decrease in ER, loss of polyribosomal aggregates, and appearance of microtubules. The mechanism of embryo polarization is discussed.

Untersuchungen und Betrachtungen zum Blattstellungsproblem

In nearly all flowering plants, the basic body plan is laid down during embryogenesis. In Arabidopsis, the crucial cell types are established extremely early as reflected in the stereotypic sequence of oriented cell divisions in the developing young embryo. Research into early embryogenesis was especially focused on the role of the infamous tryptophan derivative auxin in establishing embryo polarity and generating the main body axis. However, it is becoming obvious that the mere link to auxin does not provide any mechanistic understanding of early embryo patterning. Taking recent research into account, we discuss mechanisms underlying early embryonic patterning from an evolutionary perspective.