Side branch formation and orientation in the caulonema of the moss,Funaria hygrometrica: Normal development and fine structure

Summary The regular branching of theFunaria caulonema filaments is partly related to rhythms in nuclear and cell division. The formation and development of the branches were studied by light and electron microscopy with particular attention directed to the distribution of microtubules and the polar organization of the cytoplasm. The new side branch breaks through the wall of the mother cell. The site of branch development is determined by the position of the nucleus of the mother cell. In protonemata which grow in vertically placed Petri dishes gravity influences the position of nuclei and side branches, and also the direction of oblique cross walls in the caulonema filaments to a certain extent.     

Bud formation inFunaria: Organelle redistribution following cytokinin treatment

Summary Cytokinin stimulates caulonemata ofFunaria to undergo an asymmetric division leading to the gametophore. The earliest detectable event is a small protuberance at the distal portion of the cell accompanied by the reorganization of the underlying organelles into a polarized distribution reminiscent of a tip growing cell. Dictyosomes and associated vesicles accumulate in the protuberance directly beneath the plasma membrane with mitochondria subjacent to the vesicular layer. Endoplasmic reticulum lies beneath the mitochondrial zone directly above the large central vacuole, while chloroplasts are outside the bud. As development continues the bud elongates causing the outer cell wall to exfoliate. During the above events the nucleus migrates toward the bud site concomitant with an increase in the number of microtubules between the nucleus and the base of the outgrowth. Nucleoli, extruded from the nucleus during a previous division, persist as diffuse fragments within the protuberance. Upon reaching the bud site, division occurs with the developing phragmoplast being initiated distal to the caulonema tip cell. The former polarized distribution of the cytoplasm is altered as mitochondria, chloroplasts and small vacuoles become evenly dispersed throughout the cytoplasm; dicytosomes and endoplasmic reticulum occupy a cortical position. These events indicate a change from 2-D tip growth to 3-D diffuse growth. To quantify the ultrastructural changes associated with bud formation we performed a morphometric analysis of cells in various stages of budding. The relative volumes of dictyosomes and vesicles adjacent to the bud apex decrease during bud development coincident with an increase in these organelles in lower portions of the cytoplasm. Mitochondria and chloroplasts follow this same pattern although their highest relative volumes initially are 4 m from the bud apex and outside the bud site, respectively. These data, as well as density profile topographic maps for vesicle fractions, support the contention that cytokinin induces a change in morphological symmetry and polarity in the fine structure ofFunaria.     

Structural and functional compartmentalization in pollen tubes

Eukaryotic cellular functions are achieved by concerted activities in the cytosol and functions compartmentalized in the nucleus and other membrane-bound organelles. Moreover, the cytosol and nucleoplasm are populated with mega molecular ensembles that are specialized for different metabolic and biochemical processes. Pollen tubes are unique plant cells with a dramatic growth polarity. Tube growth is restricted to the tip and is supported by a polarized cytoplasmic organization. The apical region of elongating pollen tubes is a domain occupied exclusively by transport vesicles to support the secretion and endocytic activity needed for the rapid cell expansion at the apex. Larger organelles are predominantly segregated to the cytoplasm distal to the subapical region. Underlying the organelle compartmentalization is an elaborate actin cytoskeleton with distinct structural and dynamics properties at the tip, in the subapical region, and in the cytoplasm subtending it. Cytoplasmic domains with differential ionic conditions and spatially restricted localization of molecules in pollen tubes may also be important for regulating the polar cell growth process. The polarized cellular organization in pollen tubes drives an extremely efficient cell growth process that is responsive to extracellular signals, including directional cues. It may be an amplified framework of the cytoplasmic architecture that supports growth in other plant cell types that involves considerably more subtle and transient differential cell expansion.     

Development of protophloem in roots ofAegilops comosa var.thessalica. I. Differential divisions and pre-prophase bands of microtubules

Summary The formative divisions of protophloem mother cells in roots of the grassAegilops comosa var.thessalica have been investigated by means of light and electron microscopy. Two successive differential divisions create protophloem poles consisting of a protophloem sieve element and two companion cells. Pre-prophase bands of microtubules appear in premitotic cells anticipating the plane of orientation of the cell plate and indicating the site where the daughter wall will join the parent one. Evidence is accumulated that the site previously occupied by pre-prophase band is bisected by the new wall. Structural asymmetries that could express polarity, like organelle displacement, were not observed in premitotic cells. A working hypothesis is proposed integrating the conclusions of the present study in a diagram correlating pre-prophase bands of microtubules and differential divisions.     

Side branch formation and orientation in the caulonema of the moss,Funaria hygrometrica: Experiments with inhibitors and with centrifugation

Summary Colchicine treatment ofFunaria caulonemata, usually does not inhibit initiation of a side branch or its incipient elongation but does prevent movement of chloroplasts and the nucleus into the outgrowth. After colchicine and after cytochalasin B treatment side branches are formed about at the normal age of the cells; because of the inhibition of the apical cell they arise at an abnormal position,i.e., not in the third but in the second cell of a filament. After D₂O treatment the organelles are dislocated toward the basal cross wall. The site of side branch formation is then obviously determined by the position of the nucleus. Cells with an irreversibly reversed longitudinal polar axis can be found; by centrifugation in proximal direction the sites of side branch initiation likewise are displaced into the proximal region of the cell, especially if the remigration of the nucleus is inhibited by colchicine. High concentrations of Ca²⁺ ions induce the formation of side branch cells, without any outgrowth. The calcium ionophore A 23 187 influences the position of the nucleus and of the side branch only slightly. After these various treatments intercalary divisions frequently occur. The role and interrelationship of the nucleus and peripheral cytoplasm in establishing and maintaining the polar axes, and the role of microtubules are discussed.     

STRUCTURE AND DEVELOPMENT OF THE CARPOSPOROPHYTE OF NEMALION HELMINTHOIDES (NEMALIONALES,RHODOPHYTA)1

Following fertilization, the carposporophyte of Nemalion helminthoides (Velley in With.) Batters differentiates into four distinct regions: the fusion cell, the sterile gonimoblast cells, the carposporangial mother cells and the carposporangia. The gonimoblast is formed by apically dividing, monopodial filaments of limited growth which may later become pseudodichotomous. Upon differentiation of a terminal carposporangium, a gonimoblast filament may continue to grow sympodially. A single carposporangial mother cell may produce carposporangia in several different directions as well as proliferate successive carposporangia within the sporangial walls that remain after carpospore liberation. As the carposporophyte matures, the gonimoblast initial, the stalk cell, the hypogynous and subhypogynous cells fuse. Except for the fusion cell, all cells of the carposporophyte show organelle polarity and contain a distally located, lobed chloroplast and proximal nucleus.     

哺乳动物卵母细胞不对称分裂的研究进展

哺乳动物卵母细胞成熟过程需要进行两次连续的不对称分裂,最终形成体积差异巨大的子细胞：大体积的卵母细胞和两种体积较小的极体。不对称分裂现象是哺乳动物卵母细胞减数分裂的典型特征,不对称分裂后的卵母细胞是高度极化的细胞。精卵结合后,细胞重新恢复了对称分裂,但是在卵母细胞减数分裂过程中形成的极性特征却得以保留并影响早期胚胎的极性。本文对近年来在哺乳动物卵母细胞不对称分裂方面的相关研究展开综述,从细胞质不对称分裂和细胞核不对称分裂两个方面对染色体、细胞骨架在哺乳动物卵母细胞不对称分裂中的作用、细胞器在哺乳动物卵母细胞成熟过程中的重组分配、染色体非随机分离等过程进行介绍,旨在从细胞和分子水平阐述哺乳动物卵母细胞不对称分裂的主要机制。     

Involvement of colchicine-sensitive cytoplasmic element in premitotic nuclear positioning ofAdiantum protonemata

Summary When the red-light grown protonema ofAdiantum capillus-veneris was transferred to the dark, the nucleus ceased its migration ca. 5 hours before cell plate formation (Mineyuki andFuruya 1980). To see whether the nucleus was held by some cytoplasmic structure during nuclear positioning, protonemata were treated with various centrifugal forces at different stages of the cell cycle. Nuclei of G₁ phase were easily displaced by centrifugation at 360×g for 15 minutes, but those of G₂ or M phase were not displaced by it, suggesting that the nuclei were held by some cytoplasmic elements in G₂ or M phase. This nuclear anchoring was not detectable in protonemata that were treated with 5mM colchicine. With this treatment, the nucleus did not stop its migration at late G₂ and moved even in prophase. And the retardation of organelle movement which was observed in cytoplasm on the lateral side of the nucleus after the cessation of premitotic nuclear migration (Mineyuki andFuruya 1984) was not observed in the presence of colchicine. Thus the nuclei appear to be held by colchicine-sensitive structure in cytoplasm between the lateral surface of the nucleus and cell wall during the premitotic nuclear positioning. Electron micrographs showing cytoplasmic microtubules were consistent with the idea.Abbreviations PPN Premitotic positioning of the nucleus - L region Cytoplasm between the lateral surface of the nucleus and cell wall (seeMineyuki et al. 1984)     

The Actin Cytoskeleton and Signaling Network during Pollen Tube Tip Growth

The organization and dynamics of the actin cytoskeleton play key roles in many aspects of plant cell development. The actin cytoskeleton responds to internal developmental cues and environmental signals and is involved in cell division, subcellular organelle movement, cell polarity and polar cell growth. The tipgrowing pollen tubes provide an ideal model system to investigate fundamental mechanisms of underlying polarized cell growth. In this system, most signaling cascades required for tip growth...     

Cell polarity and the control of apical growth in Streptomyces

Streptomyces cells grow by building cell wall at one pole-the hyphal tip. Although analogous to hyphal growth in fungi, this is achieved in a prokaryote, without any of the well-known eukaryotic cell polarity proteins, and it is also unique among bacterial cases of cell polarity. Further, polar growth of Streptomyces and the related mycobacteria and corynebacteria is independent of the MreB cytoskeleton and involves a number of coiled-coil proteins, including the polarity determinant DivIVA. Recent progress sheds light on targeting of DivIVA to hyphal tips and highlight protein phosphorylation in the regulation of actinobacterial growth. Furthermore, cell polarity affects not only cell envelope biogenesis in Streptomyces, but apparently also assembly of fimbriae, conjugation and migration of nucleoids.     

The plasma membrane of the Funaria caulonema tip cell: morphology and distribution of particle rosettes,and the kinetics of cellulose synthesis

Freeze-fracturing of Funaria hygrometrica caulonema cells leads to a cleavage within the plasma membrane. The extraplasmatic and the plasmatic fracture faces differ in their particle density. The plasmatic fracture face in caulonema tip cells or in tip cells of side branches, but never in other caulonema cells, is further characterized by the occurrence of particle rosettes. The highest density of rosettes is found at the cell apex but decreases steeply toward the cell base. The shape of the rosettes varies remarkably; 20% of them are found in an incomplete, presumably disintegrating or aggregating state. The complete rosette has a diameter of about 25 nm and consists of five to six particles. The size of the single particles varies between 4 nm to 10 nm. The rosettes are thought to posses cellulose-synthase activity. It is assumed that one rosette produces one elementary fibril; rough calculations, considering the number of rosettes and the estimated amount of cellulose produced in the tip region, indicate that an elementary fibrillar length of 900 nm is formed in 1 min by one rosette. The consequence of the kinetics on the life-time of the rosettes and the cellulose-synthase activity are discussed.Abbreviations EF extraplasmatic fracture face - PF plasmatic fracture face     

Correlation of electrical current influx with nuclear position and division inFunaria caulonema tip cells

Summary Ionic currents around caulonema tip cells of the filamentous protonema of the mossFunaria hygrometrica were examined using a nonintrusive vibrating microelectrode to map electrical current before and during mitosis. Tip cells in interphase generate inward electrical currents that are maximal at the nuclear region. These currents remain concentrated over the nucleus as it migrates forward maintaining a constant distance from the growing tip. Just prior to mitosis this inward current increases twofold. During mitosis and cytokinesis current at the nuclear zone increases to four times the resting level and fluctuates, falling to zero after cell plate fusion with parental walls. The locus of outward current could not be dectected. These results suggest that plasma membrane ion currents may regulate both nuclear positioning and subsequent temporal and spatial control of cell division.     

Associations between membranes and microtubules during mitosis and cytokinesis in caulonema tip cells of the mossFunaria hygrometrica

Summary The interphase nucleus in theFunaria caulonema tip cells is associated with many non-cortical microtubules (Mts). In prophase, the cortical Mts disappear in the nuclear region; in contrast to moss leaflets, a preprophase band of Mts is not formed in the caulonema. The Mts of the early spindle are associated with the fragments of the nuclear envelope. Remnants of the nucleolus remain in the form of granular bodies till interphase. The metaphase chromosomes have distinct kinetochores; the kinetochore Mts are intermingled with non-kinetochore Mts running closely along the chromatin. Each kinetochore is associated with an ER cisterna. ER cisternae also accompany the spindle fibers in metaphase and anaphase. In telophase, Golgi vesicles accumulate in the periphery of the developing cell plate where no Mts are found. The reorientation of the cell plate into an oblique position can be inhibited by colchicine. It is concluded that the ER participates in controlling the Mt system, perhaps via calcium ions (membrane-bound calcium ions have been visualized by staining with chlorotetracycline) but that, on the other hand, the Mt system also influences the distribution of the ER. The occurrence and function of the preprophase band of Mts is discussed.     

Both chloronemal and caulonemal cells expand by tip growth in the moss Physcomitrella patens

Tip growth is a mode of cell expansion in which all growth is restricted to a small area that forms a tip in an elongating cell. In green plants, tip growth has been shown to occur in root hairs, pollen tubes, rhizoids, and caulonema. Each of these cell types has a longitudinally elongated shape, longitudinally oriented microtubules and actin microfilaments, and a characteristic cytoplasmic organization at the growing tip which is required for growth. Chloronema are elongated cylindrical shaped cells that form during the development of the moss protonema. Since there are no published reports on the precise mode of chloronema elongation and conflicting interpretations of its cytology, the mechanism of cell growth has remained unclear. To determine if chloronema elongate by tip or diffuse growth, time-lapse light microscopy was employed to follow the movement of fluorescent microspheres attached to the surface of growing cells. It is shown here that chloronemal cells elongate by a form of tip growth. However, the slower growth of chloronema compared with caulonema is probably the result of differences in cytological organization of the growing tip.     

Organelle growth control through limiting pools of cytoplasmic components

The critical importance of controlling the size and number of intracellular organelles has led to a variety of mechanisms for regulating the formation and growth of cellular structures. In this review, we explore a class of mechanisms for organelle growth control that rely primarily on the cytoplasm as a 'limiting pool' of available material. These mechanisms are based on the idea that, as organelles grow, they incorporate subunits from the cytoplasm. If this subunit pool is limited, organelle growth will lead to depletion of subunits from the cytoplasm. Free subunit concentration therefore provides a measure of the number of incorporated subunits and thus the current size of the organelle. Because organelle growth rates are typically a function of subunit concentration, cytoplasmic depletion links organelle size, free subunit concentration, and growth rates, ensuring that as the organelle grows, its rate of growth slows. Thus, a limiting cytoplasmic pool provides a powerful mechanism for size-dependent regulation of growth without recourse to active mechanisms to measure size or modulate growth rates. Variations of this general idea allow not only for size control, but also cell-size-dependent scaling of cellular structures, coordination of growth between similar structures within a cell, and the enforcement of singularity in structure formation, when only a single copy of a structure is desired. Here, we review several examples of such mechanisms in cellular processes as diverse as centriole duplication, centrosome and nuclear size control, cell polarity, and growth of flagella.     

Gradients in plastid and mitochondrial DNA synthesis inFunaria protonemata: Visualization by bromodeoxyuridine immunohistochemistry

Summary DNA synthesis was investigated by visualizing sites of bromodeoxyuridine incorporation with antibodies in protonemata of the mossFunaria hygrometrica. In apically elongating tip cells a pronounced gradient of organelle DNA synthesis from tip to base was visible, reflecting the distribution of proliferating organelles within the tip cell. Side branch development coincided with reinitiation of replication of plastid and mitochondrial DNA.Abbreviations BrdU 5-bromo-2-deoxyuridine - DABCO diazabicyclol (2,2,2) octan - FITC fluorescein-5-isothiocyanat - HSA human serum albumine - IgG immunoglobulin G - PBS saline phosphate buffer     

Nuclear behavior during branch formation in a centrifugedAdiantum protonema and the nuclear polarity

A new branch was induced on the side wall of fern protonema by cell centrifugation and subsequent polarized red light irradiation after the induction of cell division under white light. Nuclear behavior during the branch formation was analyzed. Immediately after cell division, the two daughter nuclei moved away from the division site in both red and dark conditions. Under continuous irradiation with polarized red light, cell swelling occurred as an early step of branching near the cell dividing wall, even though the nucleus was localized far from the branching site at the beginning of the swelling. After a new branch started to grow, the nucleus returned to the branching site and moved into the new branch from its basipetal end. When a protonema incubated in the dark was centrifuged again acropetally or basipetally just before the irradiation of polarized red light, the rate of apical growth or branch formation was increased, respectively. Moreover, growth of a branched protonema was altered from its former apex or from the branch again by dislocating the nucleus acropetally or basipetally by centrifugation, respectively. These facts suggest that the nucleus has no polarity physiologically, i.e. head and tail, namely either end of the spindle-shaped nucleus can be the nuclear front in a tip-growing protonema.     

Intrusive growth of flax phloem fibers is of intercalary type

Flax (Linum usitatissimum L.) phloem fibers elongate considerably during their development and intrude between existing cells. We questioned whether fiber elongation is caused by cell tip growth or intercalary growth. Cells with tip growth are characterized by having two specific zones of cytoplasm in the cell tip, one with vesicles and no large organelles at the very tip and one with various organelles amongst others longitudinally arranged cortical microtubules in the subapex. Such zones were not observed in elongating flax fibers. Instead, organelles moved into the very tip region, and cortical microtubules showed transversal and helical configurations as known for cells growing in intercalary way. In addition, pulse-chase experiments with Calcofluor White resulted in a spotted fluorescence in the cell wall all over the length of the fiber. Therefore, it is concluded that fiber elongation is not achieved by tip growth but by intercalary growth. The intrusively growing fiber is a coenocytic cell that has no plasmodesmata, making the fibers a symplastically isolated domain within the stem. Electronic Supplementary Material Supplementary material is available for this article at     

Sperm cells in pollen tubes ofNicotians tabacum L.: three-dimensional reconstruction,cytoplasmic diminution,and quantitative cytology

Summary The organization of the sperm cells and vegetative nucleus (male germ unit) ofNicotiana tabacum was examined 18 h after semivivo pollination using transmission electron microscopy, computerassisted serial section reconstruction and quantitative cytology. Based on a measurement of 11 cellular parameters in nine reconstructed sperm cell pairs, there are no statistically significant differences between the two cells. The S_vn is characterized by a strapshaped cytoplasmic extension that is physically associated with the surface of the vegetative nucleus. The nucleus is located adjacent to the sperm crosswall, with sperm organelles being distributed between the nucleus and the extension. The S_ua is a tapered cell with cytoplasmic areas at both poles and deep axial invaginations near the crosswall. This cell has a centrally-located nucleus and a largely polar distribution of organelles. Three mechanisms for cytoplasmic diminution were observed that appear to contribute actively to the loss of cytoplasmic volume and organelles: (1) enucleated cytoplasmic body production in the S_ua; (2) vesiculation at the tip of the cytoplasmic projection of the S_vn; and (3) vesicle-containing body accumulation in the periplasm of both the S_vn and S_ua.Abbreviations 3-D three-dimensional - ECB enucleated cytoplasmic body - MGU male germ unit - S_vn leading sperm cell - S_ua trailing sperm cell - TEM transmission electron microscopy - VCB vesicle-containing body     

Effects of microtubule-inhibitors on nuclear migration and rhizoid differentiation in germinating fern spores (Onoclea sensibilis)

Summary Germinating spores of the sensitive fern,Onoclea sensibilis L., undergo premitotic nuclear migration before a highly asymmetric cell division partitions each spore into a large protonemal cell and a small rhizoid initial. Nuclear movement and subsequent rhizoid formation were inhibited by the microtubule (MT) inhibitors, colchicine, isopropyl-N-3-chlorophenyl carbamate (CIPC) and griseofulvin. Colchicine prevented polar nuclear movement and cell division so that spores developed into enlarged, uninucleate single cells. CIPC and griseofulvin prevented nuclear migration, but not cell division, so that spores divided into daughter cells of approximately equal size. In colchicine-treated spores, MT were not observed at any time during germination. CIPC prevented MT formation at a time coincident with nuclear movement in the control and caused a disorientation of the spindle MT. Both colchicine and CIPC appeared to act at a time prior to the onset of normal nuclear movement. The effects of colchicine were reversible but those of CIPC were not. Cytochalasin b had no effect upon nuclear movement or rhizoid differentiation. These results suggests that MT mediate nuclear movement and that a highly asymmetric cell division is essential for rhizoid differentiation.