Numerous Cell Divisions Research Articles

Developmental stages and fine structure of surface callus formed after debarking of living lime trees (Tilia sp.).

Wounding of trees by debarking during the vegetative period sometimes results in the formation of callus tissue which develops over the entire wound surface or on parts of it. This light and transmission electron microscopy study of living lime trees found that the formation of such a surface callus is subdivided into three stages. During the first stage, numerous cell divisions take place in regions where differentiating xylem remains at the wound surface after debarking. This young callus tissue consists of isodiametric parenchymatous cells. Cambium cells, sometimes also remaining at the wound surface, collapse and do not contribute to callus formation. During the second stage, cells in the callus undergo differentiation by forming a wound periderm with phellem, phellogen and phelloderm. In the third stage, a cambial zone develops between the wound periderm and the xylem tissue laid down prior to wounding. This process is initiated by anticlinal and periclinal divisions of a few callus cells only. Later this process extends tangentially to form a continuous belt of wound cambium. Subsequently, this cambium produces both wound xylem and wound phloem and thus contributes to further thickening.

Fruit-developmental regulation of bell pepper knolle gene ( cakn) expression

A cDNA, cakn, orthologous to the Arabidopsis KN gene, which is involved in vesicle fusion during cell plate formation, was isolated from bell pepper fruit. cakn seems to be monogenic and its expression mainly transcriptionally regulated. During fruit development, transcript and protein levels increase significantly in the early stages in which numerous cell divisions occur, but in the stages corresponding to fruit growth by cell enlargement, whereas the messengers are undetectable, proteins are still faintly present, suggesting a different stability rate for the two types of macromolecules.

Marking of active genes on mitotic chromosomes.

During development and differentiation, cellular phenotypes are stably propagated through numerous cell divisions. This epigenetic 'cell memory' helps to maintain stable patterns of gene expression. DNA methylation and the propagation of specific chromatin structures may both contribute to cell memory. There are two impediments during the cell cycle that can hinder the inheritance of specific chromatin configurations: first, the pertinent structures must endure the passage of DNA-replication forks in S phase; second, the chromatin state must survive mitosis, when chromatin condenses, transcription is turned off, and almost all double-stranded DNA-binding proteins are displaced. After mitosis, the previous pattern of expressed and silent genes must be restored. This restoration might be governed by mass action, determined by the binding affinities and concentrations of individual components. Alternatively, a subset of factors might remain bound to mitotic chromosomes, providing a molecular bookmark to direct proper chromatin reassembly. Here we analyse DNA at transcription start sites during mitosis in vivo and find that it is conformationally distorted in genes scheduled for reactivation but is undistorted in repressed genes. These protein-dependent conformational perturbations could help to re-establish transcription after mitosis by 'marking' genes for re-expression.

Low incidence of tumourigenicity in agarose colonies from spontaneous murine mammary tumours

Abstract In these experiments individual colonies growing in agarose seeded with monocellular suspensions from freshly disaggregated naturally-occurring mouse mammary tumours, induced by the murine mammary tumour virus (MMTV), were reimplanted into mammary fat pads of virus-free mice. It was found that only a small proportion of these colonies generated tumours and that the implantation of multiple colonies in each site did not result in disproportionate, synergistic, increase in tumour takes. It was also observed that the proportion of colonies which were tumourigenic on reimplantation differed for each donor tumour and represented only a small fraction of the total cell population (0.001%-0.1%). However, this value was significantly higher in tumours which produced large numbers of deposits in lung colony assays following i.v. injections, than in tumours of low pulmonary colonisation potential. A point of particular interest was that tumours derived from agarose colonies of spontaneously metastatic donor tumours were substantially more spontaneously metastatic themselves than those from nonmetastatic donors, indicating that this property is heritable through numerous cell divisions, manipulations in vitro and transplantation procedures. From these results it is concluded that measurement of clonogenicity in agar is useful as an index of the cabability of a tumour to propagate itself and to colonise new sites, but that individual agarose colonies are not all the progeny of potentially immortal stem cells.

Induction of Macrostome Formation in Blepharisma americanum (Suzuki, 1954) by Alpha-Tocopheryl Succinate

LENNARTZ, D. & BOVEE, E. 1980. Induction of macrostome formation in Blepharisma americanum (Suzuki, 1954) by alpha-tocopheryl succinate. Trans. Amer. Micros. Soc., 99: 310-317. The transformation of the heterotrich ciliate, Blepharisma americanum (Suzuki, 1954), from a bactivore to a carnivore, or cannibal giant is described. This transformation process is induced by exogenous d-alpha-tocopheryl acid succinate present at 10-6 to 10-4 M. In the presence of such concentrations, B. americanum undergoes a sequential alteration of morphology, especially with respect to the oral ciliature and buccal cavity. The length and volume of the organism also are increased greatly. Over a 6-10-h period following the addition of the vitamin, Blepharisma americanum develops an enlarged undulating membrane and adoral zone of membranelles due to the elongation of the component cilia. At 10-4 M tocopheryl, some monsterism is observed in the form of L-shaped or greatly elongated organisms after 3.5 h. At lower concentrations, transformation takes 8-15 h, and fewer individuals attain giant size. When B. americanum is removed from the vitamin solution, washed, and placed in sterile Chalkley's solution at pH 6.8, the ciliates revert to a normal bactivorous form by means of numerous cell divisions over a 3-day period. The results obtained in this study are essentially the same as the results of similar research on forms found in declining cultures of B. americanum, and bear considerable resemblance to the stomatin-induced transformations of Tetrahymena vorax. However, in the present work, a defined substance was used to induce transformation.

Neoplastic transformation of human diploid fibroblast cells by chemical carcinogens

Cultured fibroblast cells derived from a skin biopsy sample taken from normal human adult were exposed to a potent carcinogen, 4-nitroquinoline 1-oxide. Alterations of cell growth pattern such as higher density and piling up of cells were noticed in some fractions of cultures that were successively subcultured after nitroquinoline oxide treatment. Morphologically altered cells retained this growth pattern and became established lines of transformed cells without showing the limited life-span characteristic of normal cells in culture. The transformed cells showed a higher saturation density and the ability to grow in soft agar, properties that are usually correlated with neoplastic transformation of cells in culture. Selection of preexisting transformed human cells as a mechanism of this observed transformation seemed unlikely because clones of these normal cells could also be used to assess the transforming effect of nitroquinoline oxide. Preliminary results suggest that numerous cell divisions were required for the development of the transformation after nitroquinoline oxide treatment of these human cells. When the transformed cell lines were injected subcutaneously into nude (athymic) mice, solid tumors were produced at the site of inoculation. Treatment with N-methyl-N'-nitro-N-nitrosoguanidine also induced cell transformation, in a manner similar to treatment with nitroquinoline oxide. However, transformation was not induced with (i) 4-aminoquinoline 1-oxide (a noncarcinogenic derivative of 4-nitroquinoline 1-oxide), (ii) 3-methylcholanthrene (a carcinogen that cannot be metabolically activated by the target cells employed), or (iii) the solvent dimethyl sulfoxide.

ACTIVITY OF MARGINAL AND PLATE MERISTEMS DURING LEAF DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

A B S T R A C T The mitotic and biosynthetic activities of the marginal and plate meristems were studied during the entire course of leaf development of Xanthium pennsylvanicum. In contrast to statements in the literature, marginal meristem activity is long in duration, as assayed by the mitotic counts and H3-thymidine incorporation. This me istem is active 23 days. The plate meristem is active for an additional 3 days after cessation of cell division in the marginal meristem, but the total duration of its mitotic activity is also approximately 23 days. Numerous periclinal cell divisions of the plate meristem form additional cell layers and contribute to the growth of the lamina in thickness. Incorporation of H3-thymidine increased during the course of leaf development. Cells between plastochronic ages 0 and 2.0 incorporated more of the radioisotopic precursor than those of younger leaf primordia. The uptake and incorporation of H3-thymidine into nuclear DNA was more sluggish during the early stages of development than in the more expanded leaves. No DNA synthesis was demonstrated after cessation of cell division in the leaf lamina. Metabolic or endomitotic DNA synthesis after leaf plastochron index (LPI) 3.0 seems improbable. No significant differences in the incorporation of H3-thymidine could be demonstrated between the marginal and plate meristems. This would indicate no distinct biosynthetic differences between the two meristems. The definitions of the marginal and plate meristems of Xanthium leaves were formulated in view of the above findings. Two MERISTEMS are active in the formation of the lamina during the early stages of leaf development. The marginal meristem, represented by a group of meristematic cells located on two flank sides of a young leaf primordium, is active in establishing usually five or six basic layers of the lamina (Esau, 1965). The plate meristem (Schiiepp, 1966), by its numerous anticlinal cell divisions, contributes to the extension of the leaf blade, in which the number of originally established cell layers remains unchanged for a considerable length of time. The flat blade of the leaf is a result of the activity of the plate meristem. Little information is available concerning the temporal aspects of the mitotic and biosynthetic activities of the marginal and plate meristems as related to the various stages of leaf development.

CHANGES IN INTERCELLULAR RELATIONSHIPS DURING THE GROWTH AND DIFFERENTIATION OF LIVING PLANT TISSUES

CHANGES IN INTERCELLULAR RELATIONSHIPS DURING THE GROWTH AND DIFFERENTIATION OF LIVING PLANT TISSUES