Summary
Observations on the formation of nuclear membranes and of the changing ultrastructural appearance during mitosis in developing eggs ofLocusta migratoria are reported.
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1.
Nuclear membranes are formed while the mitotic chromosomes are still in a condensed state. How exactly the membranes are formed, is not yet understood. Vesicles which occur in the interchromosomal space might participate in the membrane formation but they might just as well appear simultaneously with the membranes.
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Depending on the developmental stage of the egg, differences in the way in which the interphase nucleus is built up, are found. In early cleavage, karyomeres are formed from single chromosomes which are individually covered by a double membrane. These karyomeres expand by uncoiling their chromosomal mass and subsequently fuse to form one nucleus. In embryonic cells of the germinal rudiment the double envelope is formed only after the chromosomes have clustered and have formed the telophase figure.
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3.
Nuclear membranes are formed in close contact with the condensed chromosomes. Only after the membranes have been built up, the chromosomes lose their electron density. Chromosomes in early developmental stages (cleavage) uncoil in karyomeres, although the mitotic cycle may only have reached anaphase. In cells from later stages (germinal rudiment), where the chromosomes cluster before membrane formation occurs, the membrane formation as well as the uncoiling are postponed to telophase. This behavior may be related to changes in the duration of the mitotic cycle, indicating progressive differentiation.
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During the growth of karyomeres, chromosomes lose most of their electron density. Dense cores of 0.2 Μ in diameter, however, persist in the karyomeres of cleavage energides and may prove the existence of a chromosomal axis in certain mitotic stages.
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5.
Nucleoli are not found in karyomeres during early cleavage stages. Prenucleoli are observed to form at telophase in the later stages of development. More prenucleoli seem to occur in telophase than do nucleoli in interphase nuclei.
Zusammenfassung
Die Formierung der Kernmembran und das wechselnde Erscheinungs-bild der Feinstrukturen von Chromosomen während einiger Phasen des Mitosezyklus wurden an verschiedenen Entwicklungsstadien des Eies vonLocusta migratoria beobachtet.
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1.
Kernmembranen werden gebildet, solange die Chromosomen noch in kondensiertem Zustand vorliegen. Bisher bleibt ungeklärt, auf welche Weise sie zustande kommen. Die zahlreichen Vesikel, die in der Umgebung der Chromosomen gefunden werden, könnten daran beteiligt sein, könnten aber ebenso gut gleichzeitig mit den Kernmembranen in diesem Bereich auftreten.
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2.
Die Rekonstruktion des Kerns findet, je nach Entwicklungsstadium, in unterschiedlicher Weise statt. In frühen Furchungsstadien werden, durch Ausbildung von Doppelmembranen um Einzelchromosomen, Karyomeren gebildet. Diese nehmen an Ausdehnung zu, während sich die Chromosomen in ihrem Inneren entspiralisieren, bis schlie\lich die Karyomeren miteinander zum Interphasekern verschmelzen. In Keimanlagenzellen wird dagegen die Doppelmembran erst ausgebildet, nachdem die Chromosomen sich zur Telophasefigur zusammengeschart haben. In diesem einheitlichen Kern entspiralisieren sich die Chromosomen.
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Erst nach der Ausbildung der Kernmembran verlieren die Chromosomen ihre Elektronendichte. In frühen Entwicklungsstadien erfolgt die Entspiralisation in den Karyomeren, unabhängig davon, da\ sich die Energide erst im Anaphasestadium befindet. In Zellen der Keimanlage wird sowohl die Ausbildung der Kernmembran als auch die Entpiralisation der Chromosomen bis zur Telophase verzögert. Diese Beobachtung mag mit der Verlängerung des Mitosezyklus in Zusammenhang stehen und eine fortgeschrittene Differenzierung andeuten.
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4.
Während der Ausbildung der Karyomeren verlieren die Chromosomen fast ihre gesamte Elektronendichte. Nur ein Strang von etwa 0,2 Μ Durchmesser, anscheinend spiralförmig in der Längsachse des Karyomers angeordnet, behält seine verdichtete Konsistenz während der Stadien der Karyomerenbildung und -verschmelzung bei. Dieser Strang könnte die Existenz einer häufig geforderten Chromosomenachse beweisen. Möglicherweise liegt die Achse in dieser Form nur in bestimmten Mitosestadien vor.
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In den Karyomeren der frühen Furchungsstadien sind Nucleoli nicht beobachtet worden. An den noch verdichteten Chromosomen der späteren Entwicklungsstadien fallen dichtere Bezirke auf, die als Praenucleoli gedeutet werden. Allerdings erscheinen davon mehr als zwei, während im Interphasekern nur zwei Nucleoli auftreten.
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These studies were mainly carried out during the tenure of an Advanced Research Fellowship from the Anglo American Corporation of South Africa. They were continued under N.S.F. Grant GB 4847 X. I thank the director of the Zoology Department at the University of the Witwatersrand, Prof. Dr. B. I.Balinsky and his staff for their hospitality and assistance during my stay in South Africa. Thanks are also due to the Professors Dr.Bodenstein and Dr.Kretsinger, for reading the manuscript.
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Schwalm, F.E. Changes in the formation of nuclear membranes and the ultrastructure of chromosomes during the early development of locust eggs (Locusta migratoria). W. Roux' Archiv f. Entwicklungsmechanik 162, 41–55 (1969). https://doi.org/10.1007/BF00581545
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DOI: https://doi.org/10.1007/BF00581545