Mitotic stages

The influences of herbicides on cell division fall into two classes, ie, dismption of the mitotic sequence and inhibition of mitotic entry from interphase (G, S, G2). If ceU-cycle analyses indicate increases in abnormal mitotic figures, combined with decreases in one or more of the normal mitotic stages, the effect is upon mitosis. Mitotic effects usually involve the microtubules of the spindle apparatus in the form of spindle depolymerization, blocked tubulin synthesis, or inhibited microtubule polymerization (163). Alkaloids such as colchicine [64-86-8J,viahla.stiae [865-21-4] and vincristine [57-22-7] dismpt microtubule function (164). Colchicine prevents microtubule formation and promotes disassembly of those already present. Vinblastine and vincristine also bind to free tubulin molecules, precipitating crystalline tubulin in the cytoplasm. The capacities of these dmgs to interfere with mitotic spindles, blocking cell division, makes them useful in cancer treatment. 

Evidence for the involvement of a Cdc2/Cyc B complex in the mitotic stages of germ cell development comes from studies in Drosophila. Cyc B transcripts are abundant and uniformly distributed in the early Drosophila embryo (Whitfield et al., 1989, 1990 Lehner and O Farrell, 1990b). Maternally derived Cyc B transcripts are also concentrated at the posterior pole of the oocyte. Later, during embryogenesis, Cyc B tran-... 

Seed mitotic stages, abnormal, 73 355-356 Seed mucilages, 20 454 Seed oils, 70 807... 

Phases in the life cycle of a typical eukaryotic cell. The cell cycle is divided into the resting stage (Go), the prereplication stage (G,), the synthesis or replication stage (S), the postreplication stage (G2), and the mitotic stage (M). 

Fig. 10.2. Mitotic stages. Mouse L929 cells, pulse labelled with tritiated thymidine, were processed for autoradiography. Among the S-phase cells (covered with grains) are cells in other stages of interphase and several mitotic cells. A metaphase cell, two anaphase cells and a late telophase cell are distinguishable.

The principle techniques of the micronucleus assay can be applied at the end of a subacute/sub chronic study in the bone marrow of rodents (princliples are described in the section on genotoxicity elsewhere in this book). Usually, only bone marrow from one femur is used for histopathology, thus, the marrow from the second femur can be used for the micronucleus assay. It is not recommended to perform chromosomal analysis in animals from short-term or sub-chronic studies because this technique requires the in vivo treatment with antimitotic agents to arrest the mitotic stage of cell division. 

The standard karyotype is performed by photomicroscopy of cells at which mitotic stage ... 

A EXPERIMENTAL FIGURE 20-30 Fluorescence microscopy reveals changes in the organization of chromosomes and microtubules at four mitotic stages. 

Types of Cell Division Effects. Classification of herbicidal effects on cell division is not uniform. This has lead to confusion about the action of herbicides on cell division. Terms such as "mitotic poisons", "meristem active", and "mitotic inhibitors" have been used to describe the same effect of a herbicide on cell division. A more useful classification of herbicidal effects would be to divide herbicides into 2 classes those inhibiting cell division and those disrupting cell division (Figure 1). Inhibition of cell division will result in treated meristems that only contain interphase cells. If cell division is disrupted, one or more mitotic stages normally present in the meristem tissue will not be found. These two effects on cell division result from different mechanisms. 

Cell Division Disruption. If a cell cycle kinetic analysis reveals the herbicide effect is not caused by inhibition, the effect may be on mitosis. In this instance, the kinetic analysis will identify mitotic stages not recognizable as prophase, metaphase, anaphase, or telophase. 

Podophyllotoxin 87, isolated from Podophyllum species shows antitumor properties it inhibits microtubule assembly, in vivo, the result of which is the destruction of the cytoskeleton in the cytoplasm. As a consequence, the cell division is stopped at the mitotic stage of the cell cycle (3), Semi-synthetic derivatives of podophyllotoxin, i.e., Etoposide 88 and Teniposide 89 have been developed. Tliey do not possess the toxicity of podophyllotoxin 87 and are now being used (alone or in conjunction with odier drugs) in treatment for germinal testicular cancer, small cell lung cancer, and certain form of leukemia (3), They have been shown to induce, both, in... 

FIG. 3. HistoneHl kinase activity and schematic representation of the morphology of one-cell mouse embryos (3A) and two-cell stage blastomeres (3B) bisected at the respective G2 phases. N ote that histone H1 kinase activity rises autonomously in anucleate halves of both embryos and blastomeres. However, the degree of the autonomous activation is lower than in theit nucleate counterparts. Activity detected in nucleate halves obtained during respective M phases was taken as 100%. Note that the nucleate halves obtained at theit respective G2 stages do not activate histone HI kinase to the levels observed in the halves obtained in the M phase, and that the mitotic disassembly of microtubules was observed only when the level of histone HI kinase was between 35% and 46% in anucleate halves. 

However, in interphase delaminating neuroblasts, which are known to have completed S-phase and are at the G2 stage of the cell cycle, this codependence of Baz/Insc/Pins seen in mitotic neuroblasts does not apply. Delaminating neuroblasts possess an apical membrane stalk which retains contact with the epithelial surface and this is where apical cortical localization of Insc is initially seen (see Fig. 2). This initial localization of Insc to the apical stalk occurs... 

Peripheral blood cultures are stimulated to divide by the addition of a T cell mitogen such as phytohaemagglutinin (PHA) to the culture medium. Mitotic activity is at a maximum at about 3 days but begins at about 40 h after PHA stimulation and the chromosome constitution remains diploid during short-term culture (Evans and O Riordan, 1975). Treatments should commence at about 44 h after culture initiation. This is when cells are actively proliferating and cells are in all stages of the cell cycle. They should be sampled about 20 h later. In a repeat study the second sample time should be about 92 h after culture initiation. Morimoto et al. (1983) report that the cycle time for lymphocytes averages about 12-14 h except for the first cycle. 

Lu W, Peterson R, Dasgupta A, Scovell WM (2000) Influence of HMG-1 and adenovirus oncoprotein ElA on early stages of transcriptional preinitiation complex assembly. J Biol Chem 275 35006-35012 Luger K, Mader AW, Richmond RK, Sargent DF, Richmond TJ (1997) Crystal structure of the nucle-osome core particle at 2.8 A resolution. Nature 389 251—260 Lusser A, Kadonaga JT (2004) Strategies for the reconstitution of chromatin. Nat Methods 1 19-26 Maeshima K, Laemmli UK (2003) A two-step scaffolding model for mitotic chromosome assembly. Dev Cell 4 467-480... 

Ota T, Suto S, Katayama H, Han ZB, Suzuki F, Maeda M, Tanino M, Terada Y, Tatsuka M (2002) Increased mitotic phosphorylation of histone H3 attributable to AIM-l/Aurora-B overexpression contributes to chromosome number instability. Cancer Res 62(18) 5168-5177 Paulson JR, Taylor SS (1982) Phosphorylation of histones 1 and 3 and nonhistone high mobihty group 14 by an endogenous kinase in HeLa metaphase chromosomes. J Biol Chem 257(11) 6064—6072 Petersen J, Paris J, Wilier M, Philippe M, Hagan IM (2001) The S. pombe aurora-related kinase Arkl associates with mitotic structures in a stage dependent manner and is required for chromosome segregation. J Cell Sci 114(Pt 24) 4371 384... 

Vincristine and vinblastine are generally considered to act specifically on the metaphase portion of the mitotic (M) stage of the cell cycle as a consequence of perturbations of the structure and function of tubulin. A characteristic action of the drugs is production of mitotic arrest in which the tJercentage of cells in mitosis in a given population of cells will rise from a few percent to 50% and more after treatment with a drug such as vinblastine. There are reports, however, that these drugs can interfere with other phases of the cell cycle in ways not clearly related to interference with tubulin function (5). 

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