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M Kirschner

Publications and source records attributed to M Kirschner.

72 records · Page 4Linked to original sources
PubMed1984

Role of the centrosome in organizing the interphase microtubule array: properties of cytoplasts containing or lacking centrosomes.

To study the role of the centrosome in microtubule organization in interphase cells, we developed a method for obtaining cytoplasts (cells lacking a nucleus) that did or did not contain centrosomes. After drug-induced microtubule depolymerization, cytoplasts with centrosomes made from sparsely plated cells reconstituted a microtubule array typical of normal cells. Under these conditions cytoplasts without centrosomes formed only a few scattered microtubules. This difference in degree of polymerization suggests that centrosomes affect not only the distribution but the amount of microtubules in cells. To our surprise, the extent of microtubules assembled increased with the cell density of the original culture. At confluent density, cytoplasts without centrosomes had many microtubules, equivalent to cytoplasts with centrosomes. The additional microtubules were arranged peripherally and differed from the centrosomal microtubules in their sensitivity to nocodazole. These and other results suggest that the centrosome stabilizes microtubules in the cell, perhaps by capping one end. Microtubules with greater sensitivity to nocodazole arise by virtue of change in the growth state of the cell and may represent free or uncapped polymers. These experiments suggest that the spatial arrangement of microtubules may change by shifting the total tubulin concentration or the critical concentration for assembly.

Animals↗
PubMed1984

Respective roles of centrosomes and chromatin in the conversion of microtubule arrays from interphase to metaphase.

We report the results of studies in which partially purified centrosomes, nuclei, and DNA were injected into frog's eggs, which are naturally arrested in metaphase or interphase. These results have led to an independent assessment of the contributions of the centrosome and the chromatin to the formation of the mitotic spindle and suggest a simple explanation for the transition from interphase to metaphase microtubule arrays.

Animals↗
PubMed1983

Centrosome development in early mouse embryos as defined by an autoantibody against pericentriolar material.

A human autoantibody from a schleroderma patient was found to immunostain interphase and mitotic centrosomes in a variety of vertebrate cells. Electron microscopic immunocytochemistry localized this antigen in dense pericentriolar material (PCM) surrounding the centrioles. The meiotic spindle of the mouse egg has no centriole but it exhibited a broad PCM band at each pole. This pattern was also found from the first through fourth mitotic divisions. During this time PCM was found assembled at a single locus in the cell and exclusively in mitotic cells; it was not observable in interphase cells. In the blastocyst, only polar trophoblast cells had characteristic centrosomes throughout the cell cycle. Results suggest PCM can exist, disperse, and reorganize during the cell cycle independently of the centriole, and its distribution in the embryo differs in cells having different fates.

Animals↗
PubMed1983

Maturation-promoting factor induces nuclear envelope breakdown in cycloheximide-arrested embryos of Xenopus laevis.

We have studied the effect of maturation-promoting factor (MPF) on embryonic nuclei during the early cleavage stage of Xenopus laevis development. When protein synthesis is inhibited by cycloheximide during this stage, the embryonic cell cycle arrests in an artificially produced G2 phase-like state, after completion of one additional round of DNA synthesis. Approximately 100 nuclei can be arrested in a common cytoplasm if cytokinesis is first inhibited by cytochalasin B. Within 5 min after injection of MPF into such embryos, the nuclear envelope surrounding each nucleus disperses, as determined histologically or by immunofluorescent staining of the nuclear lamina with antilamin antiserum. The breakdown of the nuclear envelope occurs at levels of MPF comparable to or slightly lower than those required for oocyte maturation. Amplification of MPF activity, however, does not occur in the arrested egg as it does in the oocyte. These results suggest that MPF can act to advance interphase nuclei into the first events of mitosis and show that the nuclear lamina responds rapidly to MPF.

Animals↗
PubMed1983

Anticentromere and anticentriole antibodies in the scleroderma spectrum.

We studied serum samples from 106 patients, including 80 in the scleroderma spectrum, by indirect immunofluorescent microscopy, using PtK1 rat kangaroo tissue culture cells as substrate. Anticentromere (Kinetochore) antibodies were present in 28 patients, and anticentriole antibodies were present in four patients. Anticentromere antibodies were usually present in patients with a benign, chronic form of systemic scleroderma, which has been termed the CREST (Calcinosis, Raynaud's phenomenon, esophageal involvement, sclerodactyly, and telangiectasia) syndrome. The four patients with a previously undescribed anticentriole antibody were all in the scleroderma spectrum. Possibly, these antibodies may have diagnostic and prognostic importance. Further, they will be useful in studying the structure and function of these cellular organelles.

Antibodies↗
PubMed1982

A major developmental transition in early Xenopus embryos: I. characterization and timing of cellular changes at the midblastula stage.

The Xenopus embryo undergoes 12 rapid synchronous cleavages followed by a period of slower asynchronous divisions more typical of somatic cells. This change in cell cleavage has been termed the midblastula transition (MBT). We show that at the MBT the blastomeres become motile and transcriptionally active for the first time. We have investigated the timing of the MBT and found that it does not depend on cell division, on time since fertilization or on a counting mechanism involving the sequential modification of DNA. Rather, the timing of the MBT depends on reaching a critical ratio of nucleus to cytoplasm. We view the MBT as a consequence of the titration of some substance, originally present in the egg, by the exponentially increasing nuclear material. When this substance is exhausted a new cell program is engaged, leading to the acquisition of several new cell properties.

Age Factors↗
PubMed1982

A major developmental transition in early Xenopus embryos: II. Control of the onset of transcription.

We have shown in the accompanying paper that a developmental transition occurs at the midblastula stage (cleavage 12) in Xenopus embryos, and that this midblastula transition (MBT) is apparently initiated when the ratio of nucleus to cytoplasm reaches a critical value. One manifestation of this transition is the onset of transcription. We show here that a plasmid containing a cloned gene coding for a yeast leucine tRNA comes under developmental control when injected into cleaving eggs. In pre-MBT eggs this plasmid is transiently transcribed and then becomes inactive; however, it becomes transcriptionally active again at the MBT. This pre-MBT suppression of transcription can be reversed by addition of competing DNA. The amount of DNA needed to induce premature transcription is equal to the amount of nuclear DNA present after 12 cleavages (24 ng), suggesting that the MBT is triggered by the DNA through titration of suppressor components present in the egg.

Animals↗
PubMed1982

Mitosis in a cell with multiple centrioles.

N115 mouse neuroblastoma cells possess a large number of microtubule organizing centers (MTOCs) which can be identified ultrastructurally as single centrioles. The distribution and activity of these organizing centers can be followed through all stages of the cell cycle by labeling microtubules with anti-tubulin and chromatin with the Hoechst dye, Bisbenzimid. We have found that multiple MTOCs persist and continue to organize microtubules during mitosis. They exhibit a well-defined sequence of movements, starting from a loose cluster during interphase, proceeding to a widely and evenly dispersed arrangement in prophase, gathering into small clusters and chains during prometaphase, and residing in two ring-shaped groups at the mitotic poles during metaphase and anaphase. Despite their large number of centrioles, virtually all N115 cells show a normal bipolar mitosis, but often with unequal numbers of centrioles at the two poles. Such observations bring into question the importance of the centriole in establishing bipolar division in this cell type.

Animals↗
PubMed1981

A reinvestigation of the role of the grey crescent in axis formation in xenopus laevis.

Gravitationally induced displacements of the contents of the frog egg can predictably determine the orientation of the subsequent dorsal-ventral axis of the embryo, regardless of the original position of sperm entry or of the grey crescent. In certain conditions, these displacements in the egg can also lead to the formation of a second axis, that is, to twinning. The previously reported ability of grafts of grey crescent cortex to induce secondary axes in recipient eggs is interpreted here as an unrecognized twinning effect of gravity. Our results lead to question the classic interpretation of the grey crescent as a dorsal determinant in amphibian development.

Animals↗
PubMed1980

A cytoplasmic clock with the same period as the division cycle in Xenopus eggs.

In most species the cell cycle is arrested in the unfertilized egg. After fertilization the cell cycle is reestablished and a rapid series of cleavages ensues. Preceding the first cleavage in Xenopus the egg undergoes a contraction of its cortex, called the "surface contraction wave," which can be visualized by time-lapse cinematography. This wave of contraction is propagated in a circular manner from the animal pole to the equator. We have found that eggs prevented from cleaving by treatment with antimitotic drugs undergo a sequence of periodic surface contraction waves timed with the cleavage cycle in untreated eggs. In addition, artificially activated eggs, which fail to cleave presumably for lack of a functioning centriole, undergo the same periodic contractions. No nuclear material is required for the periodic waves because a separated egg fragment, produced by constricting a fertilized egg, still undergoes contraction waves with the same period as the cleaving nucleated fragment. These results demonstrate that some expression of the cell cycle persists in the absence of any nuclear material or centrioles, suggesting to us that a biological clock exists in the cytoplasm or cortex of vertebrate eggs, which may be involved in timing the cell cycle.

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PubMedDate not supplied

Microtubule assembly nucleated by isolated centrosomes.

Microtubules are involved in the morphogenesis of most cells and are the structural basis of the mitotic spindle. We report here that purified centrosomes nucleate the assembly of microtubules with unusual dynamic properties. This may have important implications for the mechanism by which microtubule arrays are organized and stabilized in cells.

Animals↗
PubMedDate not supplied

Dynamic instability of microtubule growth.

We report here that microtubules in vitro coexist in growing and shrinking populations which interconvert rather infrequently. This dynamic instability is a general property of microtubules and may be fundamental in explaining cellular microtubule organization.

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