Anaphase structure in mitotic cells typified by spindle elongation

Our simple instrumentation for generating a UV-microbeam is described. UVmicrobeam irradiations of the central spindle in the pennate diatom Hantzschia amphioxys have been examined through correlated birefringence light microscopy and TEM. A precise correlation between the region of reduced birefringence and the UV-induced lesion in the microtubules (MTs) of the central spindle is demonstrated. The UV beam appears to dissociate MTs, as MT fragments were rarely encountered.The forces associated with metaphase and anaphase spindles have been studied via localized UV-microbeam irradiation of the central spindle. These spindles were found to be subjected to compressional forces, presumably exerted by stretched or contracting chromosomes. Comparisons are made with the results of other writers. These compressional forces caused the poles of a severed anaphase spindle to move toward each other and the center of the cell. As these poles moved centrally, the larger of the two postirradiational central spindle remnants elongated with a concomitant decrease in the length of the overlap. Metaphase spindles, in contrast, did not elongate nor lose their overlap region. Our interpretation is that the force for anaphase spindle elongation in Hantzschia is generated between half-spindles in the region of MT overlap.Anaphase separation of chromosomes has long been known to be a two-stage process (specifically in diatoms: see references 20, 28). Inou6 and Ritter (16) have recently named the stages "anaphase A" (chromosome-to-pole movement) and "anaphase B" (spindle elongation). Early work in this area has been reviewed by Schrader (43) and Mazia (30). Schrader (43) identified Druner (7) as the first to propose a model of mitosis which includes these two stages. Ris (40) was able experimentally to distinguish between the two stages with chloral hydrate, which inhibits spindle elongation but not chromosome-to-pole movement.Most models of anaphase spindle elongation agree that interpolar fibers, composed of microtubules (MTs), form a stiff framework that grows and pushes the poles apart, but the models differ in how the elongation is accomplished. Manton et al. (28), working with the centric diatom Lithodesmiurn, suggested that central spindle elongated by growth of MTs at the pole, an interpretation echoed by Brinkley and Cartwright (6) for mitosis in cultured PtKI cells and Chinese hamster fibroblasts. Inou6 and Sato (14) envisaged a model in which MTs in a dynamic equilibrium with their subunits can move chromosomes via length changes. They proposed that MTs are disassembled primarily at the poles. It is known that MTs grow during anaphase spindle elongation in some cell types, for example in Diatoma (22), Ochromonas (50), and Barbulanympha (16). It is, however, not clear that this MT assembly is always involved in anaphase spindle elongation.McIntosh et al. (24) and Margolis et al. (29) proposed that sliding of antiparallel MTs in the interzone pushes half-spindles apart, while suggest that sliding of two half-spindles...

Section: Anaphasementioning

confidence: 68%

Ultraviolet microbeam irradiations of mitotic diatoms: investigation of spindle elongation.

Leslie¹,

Pickett‐Heaps²

1983

“…Centrioles have often been seen to give rise to flagella (i.e. to become basal bodies) (8,16,29,37,42,43), and basal bodies possibly act as mitotic division centers (i.e. as centrioles) in certain organisms (17,41).…”

Section: Basal Bodiesmentioning

confidence: 99%

“…Cleavage patterns of two general types have been described with the electron microscope. In the first type, groups of small vesicles are aligned along the cleavage plane and then appear to fuse (4,21,24,25,27,28,37,40). The second type involves the formation of a furrow, often associated with a dense region of underlying cytoplasm, which appears to constrict the cell in two (2,3,32,49).…”

Section: Cytokinesismentioning

confidence: 99%

FINE STRUCTURE OF CELL DIVISION IN CHLAMYDOMONAS REINHARDI

Johnson

Porter

1968

377

243

Cell division in log-phase cultures of the unicellular, biflagellate alga, Chlamydomonas reinhardi, has been studied with the electron microscope. The two basal bodies of the cell replicate prior to cytokinesis; stages in basal body formation are presented. At the time of cell division, the original basal bodies detach from the flagella, and the four basal bodies appear to be involved in the orientation of the plane of the cleavage furrow. Four sets of microtubules participate in cell division. Spindle microtubules are involved in a mitosis that is marked by the presence of an intact nuclear envelope. A band of microtubules arcs over the mitotic nucleus, indicating the future cleavage plane. A third set of microtubules appears between the daughter nuclei at telophase, and microtubules comprising the "cleavage apparatus" radiate from the basal bodies and extend along both sides of the cleavage furrow during cytokinesis. Features of cell division in C. reinhardi are discussed and related to cell division in other organisms. It is proposed that microtubules participate in the formation of the cleavage furrow in C. reinhardi.

“…This concept is an old one and can be substantiated from many studies that utilized isolation, micromanipulation, and centrifugation; Mazia (14) summarizes the data well and concludes that the MA must be considered as both "a region and a body." However, the concept is extended by this study to include interrelationships with such structures as nuclear envelope fragments and perhaps to others such as mitochondria, which are integrally related to the MA in another ameba (3), and Golgi bodies, which may be precisely positioned in relation to the MA (18). In these cases, the gelation seems to include structures peripheral and adjacent to the MA and probably aids in the equal distribution of some organelles to the daughter cells.…”

Section: The Gel Nature Of the Mitotic Apparatusmentioning

confidence: 92%

“…The observation that material adheres to microtubule surfaces has been directed to two hypotheses: (a) this material is the same as that composing the microtubules and has been, or will be, incorporated into the microtubules (16,18); (b) it is different material, perhaps that suggested for a two-component movement system (1, 5). However, attention should also be directed, on the basis of this study, to the role of this material in holding the MA together and connecting it to adjacent structures.…”

Section: The Gel Nature Of the Mitotic Apparatusmentioning

confidence: 99%

Electron Microscopy of Mitosis in Amebae

Roth

1967

100

The mitotic apparatus (MA) of the giant ameba, Chaos carolinensis, has characteristic sequences of microtubule arrays and deployment of nuclear envelope fragments. If mitotic organisms are subjected to 2C for 5 min, the MA microtubules are completely degraded, and the envelope fragments are released from the chromosomes which remain condensed but lose their metaphase-plate orientation. On warming, microtubules reform but show partial loss of their parallel alignment; displacement of the envelope fragments persists or is increased by microtubule reformation. This study demonstrates that cooling causes destruction of microtubules and intermicrotubular cross-bonds and further shows that such controlled dissolution and reformation can provide an in vivo test sequence for studies on the effects of inhibitor-compounds on microtubule subunit aggregation. Urea, at the comparatively low concentration of 0.8 M, inhibited reformation following cooling and rewarming but was ineffective in altering microtubules that had formed before treatment.