Onset Of Cytokinesis Research Articles

Cell cycle-related fluctuations in transcellular ionic currents and plasma membrane Ca2+/Mg2+ ATPase activity during early cleavages of Lymnaea stagnalis embryos.

During the first four mitotic division cycles of Lymnaea stagnalis embryos, we have detected cell cycle-dependent changes in the pattern of transcellular ionic currents and membrane-bound Ca2+-stimulated ATPase activity. Ionic currents ranging from 0.05 to 2.50 μA/cm2 have been measured using the vibrating probe technique. Enzyme activity was detected using Ando's cytochemical method (Ando et al. 1981) which reveals Ca2+/Mg2+ ATPase localization at the ultrastructural level, and under high-stringency conditions with respect to calcium availability, it reveals Ca2+-stimulated ATPase. The ionic currents and Ca2+-stimulated ATPase localization have in common that important changes occur during the M-phase of the cell cycles. Minimal outward current at the vegetal pole coincides with metaphase/anaphase. Maximal inward current at the animal pole coincides with the onset of cytokinesis at that pole. Ca2+-stimulated ATPase is absent from one half of the embryo at metaphase/anaphase of the two- and four-cell stage, whereas it is present in all cells during the remaining part of the cell cycle. Since fluctuations of cytosolic free calcium concentrations appear to correlate with both karyokinesis and cytokinesis, we speculate that part of the cyclic pattern of Ca2+-stimulated ATPase localization and of the transcellular ionic currents reflects the elevation of cytosolic free calcium concentration during the M-phase.

Cleavage and membrane formation in the blastoderm of the dipteran Ceratitis capitata wied.

In the Ceratitis capitata embryo, furrow formation follows the last mitosis divisions and leads to cellular blastoderm formation. This process displays several interesting features and requires the participation of bundles of microfilaments which are located at the furrow base at the onset of cytokinesis and contract synchronously, determining furrow growth. The new membranes for furrow growth seem to be largely provided by the fusion of many vesicles. Surface projections do not appear to contribute significantly to this phenomenon. At the end of cytokinesis the microfilaments are sandwiched between the plasma membrane and cisternae of endoplasmic reticulum. Subsequently the microfilaments disappear from the cytoplasmic side of the membrane but remain beneath the membranes of the connections and at the periphery of the yolk sack until gastrulation. On the basis of these observations some ultrastructural aspects of furrow formation and the role that the microfilaments may play during this process are discussed.

A SEM study of general architecture and cortical patterning in the ciliate Bursaria truncatella O. F. M. during the cell cycle

Summary The general architecture and cortical surface structures have been studied in the giant freshwater ciliate Bursaria truncatella at various stages of its cell cycle: (1) prior to division, (2) during division, (3) after division in the course of differentiation of the new oral apparatus, (4) after completion of differentiation and growth. Changes in the structure of both the somatic and oral ciliatures have been traced. The somatic cilia become sparse during the preparation to division, while before the onset of cytokinesis new cilia arise and grow in the daughter cells after the division. The adorai membranelles and the old perisomal cavity dedifferentiate before the onset of cytokinesis, while new ciliary zones of the peristome and the new peristomal cavities begin to form in early cytokinesis. In the opisthe, the primordium of the new oral apparatus arises later than in the proter. At initial stages of formation of the new oral apparatuses, the primordia of the peristomal cavities and those of the new peristomal ciliary zones are spatially separated in either of the daughter cells. The development of the ventral cut of the peristome margin, typical of B. truncatella , has been followed. In both daughter cells it results from fusion of two apertures, the subapical and the ventral ones. Stages of formation of the superficial cortical structures inside the peristome, leading to the appearance of cirromembranelles, have been demonstrated. Differences in the structural organization of the dorsal surface of the peristome exist between fed and unfed cells. Our SEM study of morphogenesis in B. truncatella lends support to the transfer of this ciliate from the class Polyhymenophorea to the lower ciliate class Kinetofragminophorea and to the creation of a special order Bursariida in it. Some of the structures observed may be used as morphological markers when studying the regulatory mechanisms of morphogenesis in ciliates.

Variability in individual cell cycles of Saccharomyces cerevisiae

The kinetics of cell proliferation of Saccharomyces cerevisiae were studied at 4 growth rates using time-lapse cinephotomicrography. Cells were grown on media with a high refractive index to reveal greater intracellular detail under the phase-contrast microscope. The morphological cell-cycle events scored were: bud emergence, nuclear migration, nuclear division, onset of cytokinesis and cell separation. Cell size was measured at cell separation and at bud emergence. The daughter-cycle time was always longer than the parent-cycle time mainly due to the large difference in the lengths of the unbudded phases. Parent cells had a shorter budded period than daughter cells. The large variance in daughter-cycle times was accounted for by the large variance in the lengths of the unbudded phase of daughter cells. The duration and variability of the periods in the cyclc from nuclear migration onwards were equivalent for parent and daughter cells. Daughter cells were always smaller than parent cells at division. There was wide variation in cell size at both division and bud emergence. The results indicated that a modified deterministic model could best explain cell proliferation kinetics in yeast. The data were used to evaluate 2 different models. The 'sloppy size control' model of Wheals (1981 a) was more consistent with the data than the 'tandem' model of Shilo, Shilo & Simchen (1976). The distribution of unbudded periods of daughter cells suggested that there was an additional incompressible period not present in parent cells.

Studies of membrane formation in Tetrahymena pyriformis: IX. Variations in intracellular phospholipid and protein deployment during the cell division cycle

Tetrahymena pyriformis, strain GL, synchronized by the 1 heat shock/generation method, was pulse-labeled at various times during the division cycle with either 32P t or 3H-leucine. Significant differences were noted in the amounts of 32P-phospholipids and 3H-proteins incorporated into individual cell fractions. In the case of lipids, mitochondria and microsomes were most actively labeled before the onset of cytokinesis whereas cilia and ciliary supernatant received most radioactivity during division. Comparison of labeling data for proteins and lipids of a particular organelle indicated that an enhanced incorporation of proteins generally preceded the peak of lipid incorporation and is apparently the triggering step leading to selective membrane expansion during the cell division cycle. The observed periodic increases in lipid labeling were therefore the result not the cause, of the amplified incorporation of specific proteins.