Spindle apparatus uncoupling in endo-tetraploid asymmetric division of stem and non-stem cells

3234 Cell Cycle Volume 8 issue 19 cause is indicated in a three-dimensional study in which BrdU labeling (new DNA) of the structurally complex, heterochromatic centromere area occurred “at the periphery of the domain”. In Figure 1 the copying restriction is taken into account for a schematic presentation of diplochromosomal replication with co-segregation in presence/absence of TH3 (see legend for detailed explanation). This “outer” copying-pattern of the template strands with co-segregation has the consequence of non-random segregation of old and new DNA for endo-replicated cells (see original Results & Illustrations). For stem cells, with assumed diploid cell cycling, the ability to undergo a co-segregation process has neither been proven nor is it likely (see above). Co-segregation in non-randomness occurred by a split in two halves of the four chromatid complexes into two units which moved as telo-metaphases to opposite poles (i.e., 46 bichromatid chromosomes co-segregated in each unit-group of complements). Noteworthy, segregation of intact, bichromatid chromosomes to opposite poles is in itself a gross deviation from normal mitotic, segregation of single chromatids to the poles. Previous studies indicated that this abnormal event seemed to be associated with absence of morphological markers for spindle involvement and with a reduction in regular, ana-telophase figures. Normal mitosis implicitly shows polar oriented centromeres followed by trailing chromosomal arms as a result of kinetochore attachments to spindle fibers emanating from opposite poles. Thus, in the present study normal anaphase figures are numerically tabulated, and the co-segregation process is documented for different poles. Contrary to this view, cytogenetics demonstrated that the parental cell to non-randomness with whole chromosomal complement-segregations (i.e., co-segregation) underwent tetraploidization from two consecutive S periods (i.e., endoreplication). These latter cells displayed diplochromosomes, pairs of sister chromosomes, forming four-chromatid complexes. Such endopolyploid cells are known to occur following genomic damage and are part of the differentiation process to non-replicative high-level polyploid, differentiated cells. Unfortunately, the non-proliferative ability of these cells has evolved into a firm belief of no proliferation even for low-level endo-tetraploid cells. Strangely, cells with diplochromosomes in mitosis abound in the literature, but there are no reported cases from these occurrences of either ana-telophase segregation products (i.e., telo-metaphases) or the possibility of a next generation of endopolyploid cells. For example, deficiencies of Suv39 h1,2 genes for methyltransferases led to lymphomas with diplochromosomes (“butterfly chromosomes”) in mitosis, but there was no follow-up for possible proliferative ability. However, one early cytogenetic description of diplo-chromosomal reduplication and segregation exists from acites tumor cells. Diplochromosomal mitoses for the non-stem cells with non-random distribution of old and new DNA revealed that copying of DNA template strands was spatially restricted. Segregation patterns over two division cycles indicated that the new copying DNA strands were always positioned on the “outside” of the template strands and not between them (inside). As suggested chromosomal structure as a An early study showed non-random (i.e., asymmetric) distribution of old and new DNA for tritiated thymidine (TH3) labeled chromosomes over two cycles of mitosis in marsupial, Potorous tridactylis kidney cells in vitro. This discovery was confirmed a year later for human and other non-stem cells, also labeled with TH3, but no further reports have been forthcoming on these data which suggested an involvement of chromosomal structure. This unusual, bipolar division appears similar to reports on asymmetric cell division of old and new DNA in stem cell replication. Most of these studies involved pluripotent stem cells labeled with TH3 or BrdU (bromodeoxyuridine) that were also observed over two division cycles. All labeled (new copying strands) and all un-labeled (old, template strands) DNA strands segregated in a certain percentage of cells (average 5–10%) to separate interphase nuclei. Furthermore, since the old template strands were retained in the stem cell and could be copied again and again, they were considered “immortal”. But not all stem cells show asymmetric division (e.g., blood cells), and neither was the similar non-random segregation in non-stem cells mentioned in the stem cell studies. These data from nonand stem cells appear to involve similar preferential chromosomal segregational mechanisms during mitosis. This mechanism(s) for human diploid stem cells with 46 metaphase chromosomes assumes that kinetochore-halves (centromeres) for all labeled and for all unlabeled chromatids, respectively attaches to spindle fibers from opposite poles. This unlikely happening was solved by the suggestion of co-segregation of all labeled and all unlabeled chromatids to opposite