CELL DIVISION KINETICS IN THE SHOOT APICAL MERISTEM OF CERATOPTERIS THALICTROIDES BRONG. WITH SPECIAL REFERENCE TO THE APICAL CELL

The duration of mitosis and the cell cycle were determined for defined cell populations of the shoot apical meristem of Ceratopteris thalictroides Brong. by using the colchicine-induced metaphase accumulation technique. The results indicate that the apical cell is mitotically active and cycles at an apparently greater frequency than the cells of subjacent populations. Duration of mitosis was similar for all cells of the meristem. These results are correlated with mitotic indices of control apices, the geometry of the apex, and the mean number of cells in the meristem. Shoot apices from adult plants were examined to determine mitotic indices within the meristem; mitotic activity was again noted for the apical cell. These results contradict recent proposals that the pteridophyte apical cell serves as a unicellular quiescent center which lacks histogenic potential and offer experimental support for the classical concept of apical cell function in those fern shoot meristems which terminate in a single apical cell.     

MITOTIC ACTIVITY AT THE SHOOT APEX OF AZOLLA FILICULOIDES

The mosquito fern, Azolla filiculoides Lam., was grown in a growth chamber on a nitrogen-free culture solution at 24 C under the following photoperiod: 16 hr light/8 hr darkness. Shoot tips were fixed every 2 hr for 24 hr to determine the mitotic index for the apical cell, immediate derivatives, and remaining cells to the level of the first leaf or lateral shoot primordium. Mitotic indices were 6.9%, 6.5% and 6.3%, respectively. The colchicine method was employed to determine the cell-cycle durations and duration of mitosis for the same populations of cells. The cell-cycle duration and duration of mitosis of the apical cell were 28.2 hr and 2.8 hr, respectively; for the immediate derivatives, 26.7 hr and 2.5 hr; for the remaining cells, 23.6 hr and 2.1 hr. Conclusions: the apical cell is as mitotically active as its immediate derivatives, and there is no evidence of a quiescent apical cell.     

Heterogenic Final Cell Cycle by Chicken Retinal Lim1 Horizontal Progenitor Cells Leads to Heteroploid Cells with a Remaining Replicated Genome

Retinal progenitor cells undergo apical mitoses during the process of interkinetic nuclear migration and newly generated post-mitotic neurons migrate to their prospective retinal layer. Whereas this is valid for most types of retinal neurons, chicken horizontal cells are generated by delayed non-apical mitoses from dedicated progenitors. The regulation of such final cell cycle is not well understood and we have studied how Lim1 expressing horizontal progenitor cells (HPCs) exit the cell cycle. We have used markers for S- and G2/M-phase in combination with markers for cell cycle regulators Rb1, cyclin B1, cdc25C and p27Kip1 to characterise the final cell cycle of HPCs. The results show that Lim1+ HPCs are heterogenic with regards to when and during what phase they leave the final cell cycle. Not all horizontal cells were generated by a non-apical (basal) mitosis; instead, the HPCs exhibited three different behaviours during the final cell cycle. Thirty-five percent of the Lim1+ horizontal cells was estimated to be generated by non-apical mitoses. The other horizontal cells were either generated by an interkinetic nuclear migration with an apical mitosis or by a cell cycle with an S-phase that was not followed by any mitosis. Such cells remain with replicated DNA and may be regarded as somatic heteroploids. The observed heterogeneity of the final cell cycle was also seen in the expression of Rb1, cyclin B1, cdc25C and p27Kip1. Phosphorylated Rb1-Ser608 was restricted to the Lim1+ cells that entered S-phase while cyclin B1 and cdc25C were exclusively expressed in HPCs having a basal mitosis. Only HPCs that leave the cell cycle after an apical mitosis expressed p27Kip1. We speculate that the cell cycle heterogeneity with formation of heteroploid cells may present a cellular context that contributes to the suggested propensity of these cells to generate cancer when the retinoblastoma gene is mutated.     

QUANTITATIVE STUDIES OF THE ROOT APICAL MERISTEM OF EQUISETUM SCIRPOIDES

An investigation was made of the meristematic activity of the apical cell, its immediate derivatives (merophytes), and of other selected cell populations of the root of Equisetum scirpoides Michx. The plane of the first division of a derivative of the apical cell is radiallongitudinal, which provides evidence that merophytes immediately adjacent to the apical cell cannot be the ultimate root initials. The apical cell is as active mitotically in roots 20–40 mm long as it is in roots that are 0.25–1 mm in length. The mitotic activity of the apical cell and of other cell populations was determined from the mitotic index, and from determination of the durations of the cell cycle and of mitosis of the apical cell by using the colchicine method of metaphase accumulation. Microspectrophotometric measurements of DNA content indicated that there was no consistent increase in DNA (endopolyploidy) in the apical cell or in the other meristematic cells as roots increased in length. Conclusion: there is no evidence that the apical cell becomes quiescent or undergoes endopolyploidy as a root increases in length.     

Cell cycle-dependent changes in Golgi stacks, vacuoles, clathrin-coated vesicles and multivesicular bodies in meristematic cells of Arabidopsis thaliana: A quantitative and spatial analysis

Cytokinesis in plants involves both the formation of a new wall and the partitioning of organelles between the daughter cells. To characterize the cellular changes that accompany the latter process, we have quantitatively analyzed the cell cycle-dependent changes in cell architecture of shoot apical meristem cells of Arabidopsis thaliana. For this analysis, the cells were preserved by high-pressure freezing and freeze-substitution techniques, and their Golgi stacks, multivesicular bodies, vacuoles and clathrin-coated vesicles (CCVs) characterized by means of serial thin section reconstructions, stereology and electron tomography techniques. Interphase cells possess ∼35 Golgi stacks, and this number doubles during G2 immediately prior to mitosis. At the onset of cytokinesis, the stacks concentrate around the periphery of the growing cell plate, but do not orient towards the cell plate. Interphase cells contain ∼18 multivesicular bodies, most of which are located close to a Golgi stack. During late cytokinesis, the appearance of a second group of cell plate-associated multivesicular bodies coincides with the onset of CCV formation at the cell plate. During this period a 4× increase in CCVs is paralleled by a doubling in number and a 4× increase in multivesicular bodies volume. The vacuole system also undergoes major changes in organization, size, and volume, with the most notable change seen during early telophase cytokinesis. In particular, the vacuoles form sausage-like tubular compartments with a 50% reduced surface area and an 80% reduced volume compared to prometaphase cells. We postulate that this transient reduction in vacuole volume during early telophase provides a means for increasing the volume of the cytosol to accommodate the forming phragmoplast microtubule array and associated cell plate-forming structures.     

EXPERIMENTAL MODIFICATION OF THE MORPHOGENETIC BEHAVIOR OF THE ISOLATED SUB-APICAL CELL OF THE APEX OF SPHACELARIA CIRROSA (PHAEOPHYCEAE)1

Experimental isolation of the terminal cells of the apex of Sphacelaria cirrosa (Roth) C. Agardh shows that the isolated apical cell retains a self-maintained mode of functioning. The isolated sub-apical cell, however, gives rise to a newly-formed axis after undergoing a different type of developmental sequence. An experimental scheme allowing one to defer isolation of the sub-apical cell after killing the apical cell shows that the maintenance, for a few hours only, of connections with subjacent cells is sufficient to induce an apical type mode of functioning in the sub-apical cell. The sequential analysis of the first cytological events in this phenomenon allows one to relate this modification of the morphogenetic program to a functional and polarized arrangement of organelles within the cell reconstituting the apical. The morphogenetic program is characterized by a migration of the nucleus to a distal position prior to an asymmetrical mitosis characteristic of the apical mode of functioning.     

Autoradiographic Examination of Meristems of Intact Boron-deficient Squash Roots Treated with Tritiated Thymidine

Intact roots of boron-sufficient squash (Cucurbita pepo L.) plants, plants entering boron deficiency, and plants recovering from boron deficiency were exposed to tritiated thymidine at the end of the treatment period to label the replicating DNA of root tip cells. Using histological sections, autoradiographs of intact root meristems were prepared. The labeling pattern in +B root tips revealed the presence of a well defined quiescent center. The ability of root tip cells to incorporate label is correlated with the total root elongation during the −B treatment period. A greater amount of total root elongation during boron deficiency and recovery reflects the fact that root tip cells have retained their ability to synthesize DNA and enter mitosis for a longer time. In roots recovering from boron deficiency, cells of the quiescent center were seen to play no part in the recovery process in roots treated for as long as 20 hours in a −B nutrient solution. They were inactive before, during, and after the −B treatment. Cessation of mitosis occurs as early as 6.5 hours after boron is withheld from the nutrient solution while DNA synthesis can occur for as long as 20 hours after withholding boron. It was concluded that boron is essential for continued DNA synthesis and mitotic activity. The absence of boron results in the cessation of mitosis and DNA synthesis within 20 hours from the time boron is withheld.     

Secretagogue response of goblet cells and columnar cells in human colonic crypts1

Crypts of Lieberkühn were isolatedfrom human colon, and differential interference contrast microscopydistinguished goblet and columnar cells. Activation with carbachol(CCh, 100 µM) or histamine (10 µM) released contents from gobletgranules. Stimulation with prostaglandinE₂(PGE₂, 5 µM) or adenosine (10 µM) did not release goblet granules but caused the apical margin ofcolumnar cells to recede. Goblet volume was lost during stimulationwith CCh or histamine (~160 fl/cell), but not withPGE₂ or adenosine. Three-quartersof goblet cells were responsive to CCh but released only 30% of gobletvolume. Half-time for goblet volume release was 3.7 min.PGE₂ stimulated a prolonged fluidsecretion that attained a rate of ~350 pl/min. Columnar cells lost~50% of apical volume during maximalPGE₂ stimulation, with a half-timeof 3.3 min. In crypts from individuals with ulcerative colitis, goblet cells were hypersensitive to CCh for release of goblet volume. Theseresults support separate regulation for mucus secretions from gobletcells and from columnar cells, with control mechanisms restrictingtotal release of mucus stores.

     

Secretagogue response of goblet cells and columnar cells in human colonic crypts

Crypts of Lieberkühn were isolated fromhuman colon, and differential interference contrast microscopydistinguished goblet and columnar cells. Activation with carbachol(CCh, 100 µM) or histamine (10 µM) released contents from gobletgranules. Stimulation with prostaglandinE₂(PGE₂, 5 µM) or adenosine (10 µM) did not release goblet granules but caused the apical margin ofcolumnar cells to recede. Goblet volume was lost during stimulationwith CCh or histamine (~160 fl/cell), but not withPGE₂ or adenosine. Three-quartersof goblet cells were responsive to CCh but released only 30% of gobletvolume. Half-time for goblet volume release was 3.7 min.PGE₂ stimulated a prolonged fluidsecretion that attained a rate of ~350 pl/min. Columnar cells lost~50% of apical volume during maximalPGE₂ stimulation, with a half-timeof 3.3 min. In crypts from individuals with ulcerative colitis, goblet cells were hypersensitive to CCh for release of goblet volume. Theseresults support separate regulation for mucus secretions from gobletcells and from columnar cells, with control mechanisms restrictingtotal release of mucus stores.

     

A NOVEL PATTERN OF APICAL CELL POLYPLOIDY,SEQUENTIAL POLYPLOIDY REDUCTION AND INTERCELLULAR NUCLEAR TRANSFER IN THE RED ALGA POLYSIPHONIA

Nearly a century ago, Rosenvinge published a now-classic paper reporting nuclear transfer between cells of Polysiphonia during secondary pit connection (SPC) formation. While reinvestigating this phenomenon, we discovered that the uninucleate apical cell, which is the progenitor of all cells in the plant, has many times (ca. 64–128 ×) the level of nuclear DNA characteristic of nuclei of gametes or mature pericentral cells. Via a regular sequence of cell divisions, the polyploid apical cell gives rise to tiers of cells, each composed of a number of pericentral cells which surround a single central cell. A large proportion of the nuclear divisions are not accompanied by DNA replication. Thus, as the number of nuclei within elongating pericentral cells increases, the DNA level of nuclei in these cells “cascades” down to the DNA level expected for the particular life history generation (i.e., gametophyte or tetrasporophyte). In mature pericentral cells, the number of nuclei is proportional to the volume of the cell. The pattern of nuclear division, reduction in ploidy level and the timing of intercellular nuclear transfer via SPC formation is regular and characteristic of a species. Nuclei transferred from one cell to an adjacent cell participate in the further nuclear divisions of the recipient cell. The degree of polyploidy in apical cells may determine the number of cells in a “determinant” branch or even the number of cells in “indeterminant” axes. In addition, the highly polyploid state of the germinating spore and its pattern of development may provide for the rapid initial growth so characteristic of this taxon.     

Kinetic studies of septum synthesis,erosion and nuclear migration in a growing B-mutant of Schizophyllum commune

Summary Microscopic measurements of apical growth and primary branch elongation were compared with nuclear movements, septum synthesis and erosion in a growing B-mutant of Schizophyllum commune. Apical growth, mitosis, septum formation, and intercalary cell division were similar to wild-type hyphae. Nuclear replication and new cross-wall formation also occurred in either apical cells bounded by eroding septa or in subterminal cells adjoined by eroded septa. An anucleate subterminal unit of the B-mutant hypha was invaded by a migrant nucleus which subsequently replicated and laid down a new septum in this region. Septum erosion occurred as early as 1 h following synthesis. Cellular granules and filaments were implicated in both septum synthesis and erosion.     

Apical growth and mitosis are independent processes in Aspergillus nidulans

Summary. It is well established that cytoplasmic microtubules are depolymerized during nuclear division and reassembled as mitotic microtubules. Mounting evidence showing that cytoplasmic microtubules were also involved in apical growth of fungal hyphae posed the question of whether apical growth became disrupted during nuclear division. We conducted simultaneous observations of mitosis (fluorescence microscopy) and apical growth (phase-contrast microscopy) in single hyphae of Aspergillus nidulans to determine if the key parameters of apical growth (elongation rate and Spitzenkörper behavior) were affected during mitosis. To visualize nuclei during mitosis, we used a strain of A. nidulans, SRS27, in which nuclei are labeled with the green-fluorescent protein. To reveal the Spitzenkörper and measure growth with utmost precision, we used computer-enhanced videomicroscopy. Our analysis showed that there is no disruption of apical growth during mitosis. There was no decrease in the rate of hyphal elongation or any alteration in Spitzenkörper presence before, during, or after mitosis. Our findings suggest that apical growth and mitosis do not compete for internal cellular resources. Presumably, the population of cytoplasmic microtubules involved in apical growth operates independently of that involved in mitosis.Present address: Department of Plant Sciences, University of Oxford, Oxford, United Kingdom.     

FINE STRUCTURE STUDIES OF PHLEUM ROOT MERISTEM CELLS. II. MITOTIC ASYMMETRY AND CELLULAR DIFFERENTIATION

Avers , Charlotte J. (Douglass Coll., Rutgers—The State U., New Brunswick, N.J.) Fine structure studies of Phleum root meristem cells. II. Mitotic asymmetry and cellular differentiation. Amer. Jour. Bot. 50(2): 140–148. Illus. 1963.—An electron microscopical study showed that ultrastructural differences distinguished the cell dividing symmetrically from that undergoing asymmetrical division in timothy grass epidermis. The spindle orientation led to cytokinesis which produced either equal- or unequal-sized sister cells, but the mitotic apparatus itself varied in the mitoses. In asymmetrical cells, the basal pole showed more extensive endoplasmic reticulum (ER) polarizations, which intruded into the spindle area during metaphase and anaphase. Such ER polarity was not obvious in symmetrical mitosis or in the apical end of asymmetrically dividing cells. The mitotic sequence is described photographically. Foci of ER were observed as early as prophase in the polar region, and it is suggested that there is a resemblance to astral ray foci seen in prophase of animal cell mitosis. Cell plate formation could be detected in anaphase by accumulations of vesicles and ER fragments along the spindle equator. Phragmosomes apparently were not involved in cell plate formation in Phleum, unlike Allium, cytokinesis. The mitotic asymmetry is discussed as a consequence of an intracellular gradient separate from the intercellular gradient of differentiation along the entire length of growing root tip epidermis.     

hyp loci control cell pattern formation in the vegetative mycelium of Aspergillus nidulans.

Aspergillus nidulans grows by apical extension of multinucleate cells called hyphae that are subdivided by the insertion of crosswalls called septa. Apical cells vary in length and number of nuclei, whereas subapical cells are typically 40 microm long with three to four nuclei. Apical cells have active mitotic cycles, whereas subapical cells are arrested for growth and mitosis until branch formation reinitiates tip growth and nuclear divisions. This multicellular growth pattern requires coordination between localized growth, nuclear division, and septation. We searched a temperature-sensitive mutant collection for strains with conditional defects in growth patterning and identified six mutants (designated hyp for hypercellular). The identified hyp mutations are nonlethal, recessive defects in five unlinked genes (hypA-hypE). Phenotypic analyses showed that these hyp mutants have aberrant patterns of septation and show defects in polarity establishment and tip growth, but they have normal nuclear division cycles and can complete the asexual growth cycle at restrictive temperature. Temperature shift analysis revealed that hypD and hypE play general roles in hyphal morphogenesis, since inactivation of these genes resulted in a general widening of apical and subapical cells. Interestingly, loss of hypA or hypB function lead to a cessation of apical cell growth but activated isotropic growth and mitosis in subapical cells. The inferred functions of hypA and hypB suggest a mechanism for coordinating apical growth, subapical cell arrest, and mitosis in A. nidulans.     

Meristems under Continuous Irradiation

Root tips of Vicia faba and Zea mays have been subjected tocontinuous irradiation from radium or cobalt-60 for long periodsat various dose rates. The rates of mitosis and the damage tothe chromosomes (assessed as percentages of cells with micronulei)have been measured in four regions of the meristems. In Zearates of mitosis are reduced under chronic irradiation, exceptin the quiescent centre, and the cap initials are particularlysensitive. In Vicia the main change in mitosis is that the quiescentcentre increases its rate, but at 12°C there is a slightstimulation of division all over the meristem. In Vicia increasingthe dose rate or lowering the temperature increases the nucleardamage. At 19° C there is an increase in damage with accumulateddose percell cycle, but the data at 12°C do not fall onthe 19°C curve, suggesting that there may be a temperatureeffect on damage other than that caused by changing the durationof the cell cycle. The differences in radiosensitivity betweenthe different regions of the meristem are due solely to differencesin the rates of mitosis of the cells.     

The mitochondrial cycle of Arabidopsis shoot apical meristem and leaf primordium meristematic cells is defined by a perinuclear tentaculate/cage-like mitochondrion

Plant cells exhibit a high rate of mitochondrial DNA (mtDNA) recombination. This implies that before cytokinesis, the different mitochondrial compartments must fuse to allow for mtDNA intermixing. When and how the conditions for mtDNA intermixing are established are largely unknown. We have investigated the cell cycle-dependent changes in mitochondrial architecture in different Arabidopsis (Arabidopsis thaliana) cell types using confocal microscopy, conventional, and three-dimensional electron microscopy techniques. Whereas mitochondria of cells from most plant organs are always small and dispersed, shoot apical and leaf primordial meristematic cells contain small, discrete mitochondria in the cell periphery and one large, mitochondrial mass in the perinuclear region. Serial thin-section reconstructions of high-pressure-frozen shoot apical meristem cells demonstrate that during G1 through S phase, the large, central mitochondrion has a tentaculate morphology and wraps around one nuclear pole. In G2, both types of mitochondria double their volume, and the large mitochondrion extends around the nucleus to establish a second sheet-like domain at the opposite nuclear pole. During mitosis, approximately 60% of the smaller mitochondria fuse with the large mitochondrion, whose volume increases to 80% of the total mitochondrial volume, and reorganizes into a cage-like structure encompassing first the mitotic spindle and then the entire cytokinetic apparatus. During cytokinesis, the cage-like mitochondrion divides into two independent tentacular mitochondria from which new, small mitochondria arise by fission. These cell cycle-dependent changes in mitochondrial architecture explain how these meristematic cells can achieve a high rate of mtDNA recombination and ensure the even partitioning of mitochondria between daughter cells.     

Hydrodynamics and cell volume oscillations in the pollen tube apical region are integral components of the biomechanics of Nicotiana tabacum pollen tube growth

Pollen tube growth is localized at the apex and displays oscillatory dynamics. It is thought that a balance between intracellular turgor pressure (hydrostatic pressure, reflected by the cell volume) and cell wall loosening is a critical factor driving pollen tube growth. We previously demonstrated that water flows freely into and out of the pollen tube apical region dependent on the extracellular osmotic potential, that cell volume changes reflect changes in the intracellular pressure, and that cell volume changes differentially induce, increases or decreases in specific phospholipid signals. This article shows that manipulation of the extracellular osmotic potential rapidly induces modulations in pollen tube growth rate frequencies, demonstrating that changes in the intracellular pressure are sufficient to reset the pollen tube growth oscillator. This indicates a direct link between intracellular hydrostatic pressure and pollen tube growth. Altering hydrodynamic flow through the pollen tube by replacing extracellular H₂O with ²H₂O adversely affects both cell volume and growth rate oscillations and induces aberrant morphologies. Normal growth and cell morphology are rescued by replacing ²H₂O with H₂O. Further studies revealed that the cell volume oscillates in the pollen tube apical region. These cell volume oscillations were not from changes in cell shape at the tip and were detectable up to 30 μm distal to the tip (the longest length measured). Cell volume in the apical region oscillates with the same frequency as growth rate oscillations but surprisingly the cycles are phase-shifted by 180°. Raman microscopy yields evidence that hydrodynamic flow out of the apex may be part of the biomechanics that drive cellular expansion. The combined results suggest that hydrodynamic loading/unloading in the apical region induces cell volume oscillations and has a role in driving cell elongation and pollen tube growth.     

Orientation of spindle axis and distribution of plasma membrane proteins during cell division in polarized MDCKII cells

MDCKII cells differentiate into a simple columnar epithelium when grown on a permeable support; the monolayer is polarized for transport and secretion. Individual cells within the monolayer continue to divide at a low rate without disturbing the function of the epithelium as a barrier to solutes. This presents an interesting model for the study of mitosis in a differentiated epithelium which we have investigated by confocal immunofluorescence microscopy. We monitored the distribution of microtubules, centrioles, nucleus, tight junctions, and plasma membrane proteins that are specifically targeted to the apical and basolateral domains. The stable interphase microtubule cytoskeleton was rapidly disassembled at prophase onset and reassembled at cytokinesis. As the interphase microtubules disassembled at prophase, the centrioles moved from their interphase position at the apical membrane to the nucleus and acquired the ability to organize microtubule asters. Orientation of the spindle parallel to the plane of the monolayer occurred between late prophase and metaphase and persisted through cytokinesis. The cleavage furrow formed asymmetrically perpendicular to the plane of the monolayer initiating at the basolateral side and proceeding to the apical domain. The interphase microtubule network reformed after the centrioles migrated from the spindle poles to resume their interphase apical position. Tight junctions (ZO-1), which separate the apical from the basolateral domains, remained assembled throughout all phases of mitosis. E-cadherin and a 58-kD antigen maintained their basolateral plasma membrane distributions, and a 114- kD antigen remained polarized to the apical domain. These proteins were useful for monitoring the changes in shape of the mitotic cells relative to neighboring cells, especially during telophase when the cell shape changes dramatically. We discuss the changes in centriole position during the cell cycle, mechanisms of spindle orientation, and how the maintenance of polarized plasma membrane domains through mitosis may facilitate the rapid reformation of the polarized interphase cytoplasm.     

A Highlights from MBoC Selection: Cdc42 Regulates Apical Junction Formation in Human Bronchial Epithelial Cells through PAK4 and Par6B

Cdc42 has been implicated in numerous biochemical pathways during epithelial morphogenesis, including the control of spindle orientation during mitosis, the establishment of apical-basal polarity, the formation of apical cell–cell junctions, and polarized secretion. To investigate the signaling pathways through which Cdc42 mediates these diverse effects, we have screened an siRNA library corresponding to the 36 known Cdc42 target proteins, in a human bronchial epithelial cell line. Two targets, PAK4 and Par6B, were identified as necessary for the formation of apical junctions. PAK4 is recruited to nascent cell–cell contacts in a Cdc42-dependent manner, where it is required for the maturation of primordial junctions into apical junctions. PAK4 kinase activity is essential for junction maturation, but overexpression of an activated PAK4 mutant disrupts this process. Par6B, together with its binding partner aPKC, is necessary both for junction maturation and for the retention of PAK4 at sites of cell–cell contact. This study demonstrates that controlled regulation of PAK4 is required for apical junction formation in lung epithelial cells and highlights potential cross-talk between two Cdc42 targets, PAK4 and Par6B.     

Immunofluorescence Study of Microtubule Organization in Some Polarized Cell Types of Selected Brown Algae

The microtubule (Mt) organization in apical cells of Sphacelaria rigidula. as well as in branch initials of S. rigidula and Ectocarpus siliculosus, was studied by immunofluorescence. The apical interphase cells of S. rigidula show an impressive cytoskeleton of Mts, converging on the centrosome(s). A number of Mt bundles are perinuclear, but most of them run in axial orientation from the centrosomes to the cell cortex. The anterior Mt system consists of numerous thin Mt bundles, whereas the posterior system contains fewer and thicker bundles. In cells entering prophase, the cytoplasmic Mts gradually disappear. This process is somewhat faster at the posterior than at the anterior pole of the premitotic nucleus. After mitosis, the cytoplasmic Mts of the apical region appear to be re-organized more rapidly than those of the basal part of the cell. The apical daughter nucleus retains a lobed shape and condensed chromatin for a longer time, and increases considerably in size between telophase and cytokinesis, compared to the basal one. Duplication of the centrosomes proceeds more rapidly in the anterior region of apical cells than in the basal part. During branch formation in S. rigidula and E. siliculosus, a new polarity axis is established, and the Mts extend towards the protrusion into which the nucleus migrates before mitosis. After nuclear division, one of the daughter nuclei is positioned at the tip of the branch, where the apical Mt focussing point is localized.