Endopolyploidization and the interstitial invasion of the supergiant trophoblast cells of the field vole Microtus rossiaemeridionalis

The supergiant trophoblast cells characteristic of vole placenta prove to be highly invasive being found at the boundary of the decidualized endometrium and myometrium. Their size (100 μm and higher) suggests them to be highly polyploid, though their ploidy was not determined by now. We performed determination of the ploidy level of the supergiant trophoblast cells (SuGT) in order to verify whether the highly polyploid trophoblast cells are capable of deep intrauterine invasion. Anti-Cytokeratin trophoblast immunolabelling were performed to estimate the ways of the SuGT migration. DNA content measurement with help of image analysis was performed at the series of Feulgen-stained sections of the SuGT nuclei. The SuGT were observed to migrate through the endometrial stroma reaching myometrium. Most of the cells corresponded to 2048c-8192c; the maximum level was 16384c comparable to the salivary glands of Drosophila. The nuclei contained bundles of non-classic polytene chromosomes. At the final steps of differentiation when SuGT reach myometrium, the bundles of polytene chromosomes disintegrate into multiple separate endochromosomes. The supergiant trophoblast cells in Microtus rossiaemeridionalis represent an example of highly polyploid cells capable of deep intrauterine invasion.     

Intranuclear and cytoplasmic annulate lamellae in the polyploid giant cells of the trophoblast

Intranuclear and cytoplasmic annulate lamellae in polyploid giant cells of the trophoblast have been studied in rat placenta on days 12--17 of development. The annulate lamellae are present in the cytoplasm within a limited time, being visible on day 12 only. These are arranged in bundles near the nucleus to be moving then to the cytoplasm. The end parts of annulate lamellae are broadened to make cisterns of rough endoplasmic reticulum. Unlike the cytoplasmic annulate lamellae, those found within the nucleus are seen in part of the nuclei investigated throughout the whole period examined to look as single structures (not gathered in bundles), they can be branching, separating closed spaces within the nucleus (making local swellings in the loci of branching; the latter having electron dense or transparent vesicles). Association with nuclear chromatin in some regions is a peculiar feature of the intranuclear annulate lamellae. This association is especially obvious at endoprophase in the cycle ofthe polytene nucleus during the somatic conjugation--chromonemes unite in a bundle and condense. Ultrastructural changes of the annulate lamellae is noted throughout the polytene nucleus cycle and during the cell differentiation. It is supposed that in the case of temporary labile chromosome polyteny in the nuclear cycle, which is characteristic of mammalian trophoblasts, annulate lamellae can well compare, in their function, with the synaptonemal complex--these prevent from too tight associations of homologues in the course of somatic conjugation of chromosomes.     

Localization of rRNA genes in the nuclear space of Calliphora erythrocephala Mg. nurse cells during polytenization

Multicolor 3D fluorescence in situ hybridization was used to study arrangement of rRNA genes in Calliphora erythrocephala nurse cell nuclei with different levels of polyteny. It has been shown that the rRNA genes are exclusively localized to chromosome 6, suggesting that chromosome 6 is the only C. erythrocephala chromosome responsible for nucleolar formation. We have also described changes in localization of ribosomal genes within the chromosome territory during polytenization, namely, that rDNA signals are detected in the peripheral region of chromosome territory starting from the stage of polytene chromosomes. In addition, it has emerged that large nucleolus associated with chromosome 6 starts to develop in the central nuclear region in the C. erythrocephala nurse cell nuclei at the stage of a primary reticular structure. The central position and nucleolar structure are retained at the stages when chromosome 6 occupies the central position, that is, at the stages of polytene and bloblike chromosomes. When the nucleus restores a reticular structure but at a higher polyteny level, the displacement of chromosome 6 to the nuclear periphery is accompanied by disruption of the large nucleolus into micronucleoli. The micronucleoli are distributed in the nuclear space retaining their association with the nucleolar-organizing regions of chromosome 6. Thus, our data suggest that the large-scale alterations in the organization of chromosome 6 and the nucleolus during polytenization are the correlated processes directly dependent on the rRNA gene activity. The earlier described dynamics of nucleolar-organizing chromosome territory and nucleolus in the nuclear space is likely to be associated with the change in the total expression activity of the nucleus, which complies with the hypothesis on the correlation between spatial nuclear organization and expression regulation of genetic material.     

Morphological and cytofluorometric study of the giant cells of the trophoblast of the common vole]

The primary and secondary giant cells of trophoblast in placenta Microtus arvalis were studied. The giant polyploid nuclei are formed in result of series of successively proceeding endomitotic polyploidization of chromosomes. Two stages of endomitosis are described: endointerphase with the uniform net of thin chromatin threads and the stage when small round or rod-shaped paired chromosomes gather mostly under the nuclear membrane. Great number of round, oval, and complex-shaped nucleoli may be seen in nuclei during both stages of endomitosis, the number growing during polyploidization. The morphology of the chromosome-nucleolar apparatus involves peculiarities of the polyploidization mechanism in placenta Microtus arvalis trophoblast. Endomitosis occurs both in low and high-polyploid nuclei. Cytofluorometric determination of the DNA amount in nuclei polyploid nature. The degree of polyploidy of the trophoblast giant cells nuclei during terminal differentiation of placenta corresponds to 128c-512c, and some nuclei contain the DNA amount corresponding to 1024 and 2048 chromosomal sets. The cause of origin of the polyploid cells in trophoblast of rodents placenta is discussed.     

Quantitative investigation of reproduction of condensed chromatin of sex chromosomes during trophoblast cell polyploidization and endoreduplication in the East European field vole Microtus rossiaemeridionalis

Simultaneous measurement of DNA content in cell nuclei and condensed chromatin bodies formed by heterochromatized regions of sex chromosomes (gonosomal chromatin bodies, GCB) has been performed in two trophoblast cell populations of the East-european field vole Microtus rossiaemeridionalis, namely in the proliferative population of trophoblast cells of the junctional zone of placenta and in the secondary giant trophoblast cells. One or two gonosomal chromatin bodies have been observed in trophoblast cell nuclei of all embryos studied (perhaps both male and female), In the proliferative trophoblast cell population, characterized by low ploidy levels (2c-16c), and in the highly polyploid population of secondary giant trophoblast cells (16c-256c), the total DNA content in GCB increased proportionally to the ploidy level. In separate bodies, the DNA content rose also in direct proportion with the ploidy level seen in the nuclei with both one and two GCBs in the two trophoblast cell populations. A certain increase in percentage of the nuclei with 2-3 GCBs was shown in the nuclei of the junctional zone of placenta; this may be accounted for by genome multiplication via uncompleted mitoses. In the secondary giant trophoblast cell nuclei (16c-256c), the number of GCBs did not exceed 2, and the share of nuclei with two GCBs did not increase, thus suggesting the polytene nature of sex chromosome in these cells. At different poloidy levels, the ratio of DNA content in the nucleus to the total DNA content in GCB did not change significantly giving evidence of a regular replication of sex chromosomes in each cycle of genome reproduction. In all classes of ploidy, the mean total DNA content in trophoblast cell nuclei with single heterochromatic body was less than in the nuclei with two and more GCBs. This may indicate that a single GCB in many cases does not derive from the fusion of two GCBs. To put it another way, in the nuclei with one GCB and in those with two or more GCBs, different chromosome regions may undergo heterochromatization. The regularities observed here are, most probably, associated with the peculiarities in the structure of X- and Y-chromosomes in a range of species of Microtus (M. agrestis, M. rossiaemeridionalis, M. transcaspicus). As a result, gonosomal chromatin bodies may include large blocks of both constitutive heterochromatin of X- and Y-chromosomes (in male and female embryos) and inactivated euchromatin of "lyonized" X-chromosome in female embryos. Therefore the presence of two or more GCBs in trophoblast cells of M. rossiaemeridionalis may be accounted for by both polyploidy and functional state of the nucleus, in which gonosomal constitutive heterochromatin and inactivated euchromatin form two large chromocenters rather than one. The differences in DNA content in GCBs in the nuclei with one and two GCBs seem to be an indirect indication that the two chromocenters may be formed by two different gonosomes, with the extent of their heterochromatization being higher than that in the nuclei with one GCB. GCBs in the trophoblast cells of M. rossiaemeridionalis are observed not only at the early developmental stages, as it was observed in rat at the first half of pregnancy (Zybina and Mosjan, 1967), but also at the later stages, up to the 17th day of gestation. At these stages, the nuclei with non-classical polytene chromosomes rearrange to those with a great number of endochromosomes, probably because of disintegration of chromosomes into oligotene fibrils. However, it does not seem unlikely that this process may involve heterochromatized gonosomal bodies, since only one or two large GCBs can be seen in the nuclei as before. The presence of prominent blocks of constitutive heterochromatin seems to favor a closer association of sister chromatids in polytene chromosomes, which prevents their dissociation into endochromosomes with the result that polyteny of sex chromosomes in the field vole trophoblast is probably retained during a longer period of embryonic development.     

Elementary structures in polytene chromosomes of Drosophila melanogaster

Whole-mounted polytene chromosomes were isolated from nuclei by microdissection in 60% acetic acid and analyzed by electron microscopy. Elementary chromosome fibers in the interchromomeric regions and individual chromomeres can be distinguished in polytene chromosomes at low levels of polyteny (2⁶–2⁷ chromatids). Elementary fibers in the interbands are oriented parallel to the axis of the polytene chromosome. Their number roughly corresponds to the expected level of polyteny. These fibers have an irregular beaded structure, 100–300 Å in diameter, and there is no apparent lateral association between them in the interchromomeric regions. Most bands, in contrast, form continuous structures crossing the entire width of the chromosome. Polytene chromosomes isolated in 2% or 10% acetic acid can be reversibly dispersed in a solution for chromatin spreading. The spread chromosomes consist of long uniform deoxyribonucleoprotein (DNP) fibers with a nucleosome structure. This supports the notion that continuous DNA molecules extend through the entire length of a polytene chromosome and that the nucleosome structure exists both in bands and interbands. Analysis of the band shape and of the fibrillar pattern in the interbands emphasizes that the polytene chromosome assumes a ribbonlike structure from which the more complex three-dimensional structure of the polytene chromosome at higher levels of polyteny develops.     

Increased number of nucleoli in the salivary gland cells of Drosophila melanogaster under conditions of rDNA dose compensation

A considerable increase in the number of nucleoli non-associted with the nucleolar organizer (NO) was shown in the salivary gland cells of Drosophila melanogaster mutants, heterozygous for a deficiency of NO. The frequency of formation of additional nucleoli increased with the raising of the chromosome polyteny level. By means of in situ hybridization we showed that in the mutant and the wildtype polytene cells the ribosomal DNA (rDNA) of these unlawful nucleoli included ribosomal gene repeats (18S+28S) with two types of insertions: ivs-I and ivs-II Such additional nucleoli can be attached to varying sites of the polytene chromosomes containing type I insertion sequences.     

Occurrence of amitotic division of trophoblast cell nuclei in blastocysts of the western spotted skunk (Spilogale putorius latifrons)

A cytogenetic examination of spreaded cells of diapausing and early activated blastocysts obtained from 7 female western spotted skunks was performed. Mitosis was not observed in 1626 cells obtained from 9 diapausing blastocysts; however, 12 (1.5%) figures of diploid mitosis were seen in 851 cells from 5 early activated embryos. Diameter of the cell nuclei varied from 4 to 29 microm during diapause, and from 5 to 40 microm in activated blastocyst, and the heterogeneity in nuclear size was significantly different between diapausing and activated embryos (P<0.01). About 80% of nuclei from diapausing blastocysts measured 9 to 16 microm, whereas a similar percentage of nuclei from activated blastocysts ranged from 15 to 27 microm. Many enlarged nuclei exhibited morphological features characteristic of mammalian polytene (i.e. endopolyploid with polytenic organization of chromosomes) trophoblast cells. The number of silver stained nucleoli in all the nuclei did not exceed 2, which corresponds to the number of nucleolus organizers in the diploid karyotype in this species of skunk and suggests the polytene organization of chromosomes in enlarged nuclei. About 10% of large interphase nuclei were observed to undergo amitosis, i.e. direct division by constriction. The resulting nuclear fragments in diapausing blastocysts usually had normal morphology and active nucleoli. In activated embryos, nearly 15% of amitotically divided nuclei appeared to be dividing into fragments of unequal size, one of which had normal cell nuclear morphology and extremely large silver positive nucleoli, and the other fragment exhibited signs of cell death. We interpret these data as indicating that 1) amitotic division of trophoblast endopolyploid cell nuclei in the skunk blastocysts may generate new trophoblast cells which contribute to increased cell number during both diapause and activation stages, and 2) activation of blastocysts after diapause is related to the production of trophoblast cells with enhanced synthetic capabilities.     

Multiple nucleoli and enhanced nucleolar activity in the nurse cells of the insect ovary

Origin and function of the nucleolar apparatus in nurse cell nuclei of Calliphora erythrocephala have been investigated by cytological and autoradiographic methods in some inbred lines of laboratory blowflies with well paired polytene chromosomes in the nurse cell nuclei. Besides the nucleolus at chromosome VI large numbers of multiple free nucleoli develop in the highly polyploidized nurse cells during oocyte growth. The nucleoli incorporate H³-uridine in a considerable amount producing a homogeneous and RNase-sensitive label even after short time incubation. Their capacity of RNA synthesis is independent of their spatial relationships to other nuclear components. DNA particles in the nucleoli could be identified by the Feulgen reaction and by fluorescence staining with N,N'-diethylpseudoisocya-ninchloride, which also demonstrates the existence of own templates for autonomous RNA synthesis. There are indications that the nucleolus' own DNA is produced by gene amplification beyond the level of endomitotic polyploidization in the nurse cell nuclei. A quantitative estimation of grain density in the autoradiograms shows a rigorous shift of rRNA synthesis: at least 72% of all newly synthesized macromolecular RNA in nurse cell nuclei as contrasted to 13 % of nucleolar RNA synthesis in bristle forming cells with a similar degree of polyploidy. It seems that the nurse cell nuclei of Calliphora in addition to polyploidization increase their template capacity for synthesizing rRNA in a similar way as has repeatedly been demonstrated for Amphibia. Cytological and physiological peculiarities of the nurse cells have been discussed from the viewpoint of their functional similarity to the oocyte nucleus.     

Conservation of nucleolar structure in polytene tissues of Ceratitis capitata (Diptera: Tephritidae)

Nucleolar structure was studied in mitotic and three polytene tissues of the Mediterranean fruit fly, Ceratitis capitata using in situ hybridization with a tritium-labelled rDNA probe and silver staining. In mitotic metaphase chromosomes nucleolar organiser regions were localised in the short arms of both sex chromosomes. In polytene nuclei of trichogen cells, salivary glands and fat body rDNA was detected within nucleoli. Nucleoli in these tissues have a similar structure with rDNA labelling concentrated in a central core. Silver staining resulted in very heavy staining of polytene nucleoli and interphase nucleoli in diploid cells. Silver staining of nucleolar organisers in metaphase chromosomes is weak or absent although the X chromosome has numerous dark silver bands in other locations. The results suggest that nucleolar structure is conserved in polytene tissues contrasting with the variability of autosomal banding patterns and sex chromosome structure. They also indicate that silver staining is not necessarily specific for nucleolar regions.     

Early chromosome condensation without cell fusion. I. Preliminary results with allogeneic and xenogeneic cells.

Early chromatin condensation in interphase cells (G1) of human peripheral blood lymphocytes has been induced without virus or cell fusion by exposure to allogeneic or xenogeneic mitotic cells. The event, although similar in some ways to the phenomenon described as "premature chromosome condensation," "chromosome pulverization," and "prophasing," differs in that it does not require the presence of viruses and cell fusion before mitosis proceeds in the G1 cell. Early chromatin condensation in interphase cells induced by mitotic cells only, consists of chromatids in the early or late G1 phase of the cell cycle that are not pulverized or fragmented at mitosis. Some of the chromosomes are twice as long as the metaphase chromosomes and exhibit natural bands. Almost twice as many of these bands are produced as by trypsin treatment of metaphase chromosomes. The nuclear membrane is intact and nucleoli are present, to which some chromosomes are attached. The DNA content of the precocious chromosomes in G1 is half the amount of the metaphase complement.     

Cytological structure of the native polytene salivary gland nucleus of Drosophila melanogaster: a microsurgical analysis

The polytene salivary gland nucleus of Drosophila melanogaster has been isolated and fractionated by microsurgery at physiological pH and ionic strength. This procedure permits observation of native components of the nucleus including polytene chromosomes, nucleoli, micronucleoli and the nuclear envelope. The three dimensional organisation of the chromosomes within the nucleus is seen to vary as a function of time and can be externally controlled. Evidence is adduced for previously unrecorded stage-related changes in the physical state of the polytene chromosomes. The first en face electron micrographs of the isolated D. melanogaster salivary gland nuclear envelope are presented. In addition, direct light microscopic observations are reported of a new sub-nuclear structure (organelle?) consisting of characteristic beaded fibres, some of which interconnect different chromosomal segments, while others connect the chromosomes to the nuclear envelope. These new structures are candidates for the physical basis of the “tracks in the cell nucleus” phenomenon seen indirectly by other approaches or could have some other, as yet undefined, role.     

Structural paradox of polytene chromosomes

The observation of thick chromatin fibers in interbands of Dipteran polytene chromosomes suggests that there should be 5 to 10 times more mass and DNA in interbands than is commonly thought to be present. To resolve this paradox, the chromatin content of interbands was estimated, using whole-mounted polytene chromosomes from Drosophila melanogster. Densitometry of high voltage electron microscopic negatives provides an estimate of less than 4:1 for the average ratio of cross-sectional dry mass (or mass per unit chromosome length) of bands relative to interbands. This ratio, combined with an estimate of the length of chromosome composed of interbands, indicates that at least 26% of chromosome mass is contributed by interband chromatin. Since DNA comprises a similar proportion of chromatin mass in bands and interbands (Laird et al., 1980b), these data imply that DNA sequences in interbands represent at least 26% of the euchromatic genome of D. melanogaster. This result calls for reinterpretation of some of the genetic and molecular data from Diptera. The discrepancy between this higher estimate of interband mass and DNA, and previous estimates of 3-5%, is discussed. One possibility is that previous measurements were made on prominent interbands, which are proposed here to be in regions that are delayed in DNA replication. Such interbands would be reduced in polyteny and DNA content compared with the average interband region. The concept of local variations in polyteny is also used here to explain major differences in the cross-sectional mass of bands. This leads to a revised model of polytene chromosomes in which at least three levels of polyteny, rather than one or two levels, can be present within one euchromatic region.     

Fragmentation of polyploid nuclei in the giant cells of the rat trophoblast. I. The ultrastructure of the nuclear fragments

Electron microscope study of the nuclear fragments in the rat trophoblast has demonstrated that the division of the trophoblast giant nucleus results first in the formation of a multinuclear cell. Each nuclear fragment is covered with its own nuclear envelope made of two membranes with numerous pore complexes. The chromatin in these nuclear fragments is condenced with various degrees of condensation, which depends on the step of placenta development, cell differentiation and the degree of nuclear fragmentation. The nuclear ultrastructure in nuclear fragments also depends on the degree of nuclear fragmentation and on the level of chromatin condensation. The nucleolus has no granular component. On large fragments, with lower chromatin condensation the nucleolus is not homogenous being made of fragments of more and of less electron dense fibrilles. Small light lacunae are seen in the nucleolus where chromatin threads and strands pass on. With a high chromatin condensation in the nucleus, round small nucleoli look homogenous being made of moderately electron dense fibrilles. Products of chromosome activity have been found in the nuclear fragments: accumulations of minute granules (d = 15--20 nm), perichromatinous granules (d = 35--40 nm), and fibrillar nucleolus-like bodies. In the multinuclear cell, made as the result of fragmentation of the initially giant nucleus, all the small nuclei are first arranged very close to each other, so that the contours of the neighbouring nuclei coincide.     

Polytene chromosomes in mouse trophoblast giant cells

Mouse trophoblast giant cells undergo successive rounds of DNA replication resulting in amplification of the genome. It has been difficult to determine whether giant cell chromosomes are polyploid as in liver cells or polytene as in Dipteran salivary glands because the chromosomes do not condense. We have examined the pattern of hybridization of mouse giant cells with a variety of in situ chromosome markers to address this question. Hemizygous markers displayed one hybridization signal per nucleus in both diploid and giant cells, while homozygous markers displayed two signals per nucleus in both cell types. These patterns are consistent with cytological evidence indicating that giant cell chromosomes are polytene rather than polyploid. However, in contrast to the situation in Dipteran salivary glands, the two homologues do not appear to be closely associated. We conclude that the mechanism of giant cell DNA amplification involves multiple rounds of DNA replication in the absence of both karyokinesis and cytokinesis, and that sister chromatids, but not homologous chromosomes, remain closely associated during this process.     

Dynamics of structural and functional association of nucleolar chromosomes in cells of the hexaploid wheat Triticum aestivum L. at different stages of the cell cycle and during genome polyploidization

Quantitative analysis of interphase association of the nucleolar chromosomes at different stages of the cell cycle and during genome polyploidization was carried out. Cells of various tissues of hexaploid wheat Triticum aestivum L. (Moskovskaya-35) were used, including diploid root meristematic cells, endopolyploid root cells, triploid endosperm cells and antipodal cells with polytene chromosomes. Interphase nucleoli impregnated with silver or stained with autoimmune antibodies to 53 kDa nucleolar protein served as markers of the nucleolar chromosome association. The following data were obtained: (1) silver-staining revealed two pairs of homologous chromosomes 1B and 6B with active nucleolus-organizing regions in the root meristematic cells; (2) maximal number of nucleoli in diploid meristematic cells reaches four, which corresponds to the number of chromosomes with active organizers; (3) analysis of cells at different stages of the cell cycle has shown that the tendency to the nucleoli association is observed as soon as cells pass individual stages of the cycle; (4) after DNA and chromosome reduplication, the nucleolus-organizing regions in sister chromatids function as a common structure-functional complex; (5) in endopolyploid root cells and antipodal cells with polytene chromosomes, the number of nucleoli does not correlate with ploidy level, and an additional nucleolus revealed in some cells is the result of activation of the latent organizer in one of the nucleolar chromosomes; (6) in the triploid endosperm nucleologenesis, the stage of prenucleolar bodies is missing. Our data suggest that "fusion" of nucleoli and reduction of their number due to the "satellite" association of the nucleolar chromosomes are two independent processes regulated by different mechanisms.     

Cytological studies on the organization of DNA in giant trophoblast nuclei of the mouse and the rat

Giant trophoblast nuclei of the mouse and the rat, known to contain hundreds, or even thousands, of times the haploid amount of DNA, have been studied by a number of cytological techniques. These nuclei appear in two morphological states:“reticulate,” in which large numbers of chromatin threads of uniform size intermingle throughout the nucleus, often radiating from clumps of heterochromatin adjacent to the nucleoli, and “homogeneous,” in which the chromatin is more evenly dispersed and individual threads are more difficult to distinguish. Intermediate morphologies are also observed. In neither case were structures resembling polytene chromosomes discernible. — Centromeric heterohromatin as revealed by the Giemsa BSG technique has been quantitatively analyzed in giant versus diploid trophoblast nuclei. Although the median number of chromocenters is slightly greater in giant than in diploid nuclei, the range is similar. In both cases, the chromocenter number is usually less than the diploid number of chromosome pairs, indicating the attraction between centromeres not only of homologous, but also of heterologous, chromosomes. By scanning microdensitometry, we have observed a constant ratio of chromocenter area: total nuclear area in giant cells. This ratio, which likely reflects the ratio of chromocenter volume: total nuclear volume, is in good agreement with that of satellite DNA: total DNA.     

Chromosomal organization of Drosophila tumours

In otu mutants of Drosophila melanogaster ovarian tumours develop because of the high mitotic activity of the mutant cystocytes; the latter are normally endopolyploid. In certain alleles of otu, however, a varying proportion of the mutant ovarian cystocytes undergo polyteny. Mutant cystocytes with polytene chromosomes are termed pseudonurse cells (PNC). Polytene chromosome morphology and banding patterns in PNC of otu ¹/otu³ flies were cytologically analysed. Extensive variability was noted in the quality of the banding pattern of the PNC chromosomes which ranged from highly condensed (condensed PNC chromosomes) to those with a banding pattern (banded PNC chromosomes) similar to that in larval salivary gland cells (SGC). Both the condensed and banded PNC chromosomes frequently enter into a diffuse state characterised by weakened synapsis of the polytene chromatids and alterations in their banding pattern (diffuse PNC chromosomes). Analysis of DNA synthesis patterns in the various morphological forms of PNC polytene chromosomes by ³H-thymidine autoradiography revealed a basic similarity to the pattern seen in polytene nuclei of larval SGC. Independently replicating sites, however, could be unambiguously identified only in banded PNC chromosomes. Comparison of late replicating sites in such PNC chromosomes with those of larval SGC showed a remarkable similarity in the two cell types. These results suggest a close correlation between the polytene chromosome banding pattern and its replicative organization.     

Distribution of chromosome material during division of giant nuclei by fragmentation in rodent trophoblast. Morphologic and cytophotometric study]

A study was made of a population of secondary giant cells (in the placenta of white rats and mice), of which a rather high polyploidy (128c--1024c) is characteristic, and which remains viable up to the end of pregnancy. At a certain stage of cell differentiation, some giant nuclei, looking as interphase nuclei, are divided into numerous smaller nuclear fragments bound with nuclear membranes. Two ways of division have been described: by a progressive budding of small nuclei into the cytoplasm, and the total division of the original nucleus into numerous tightly contracting nuclear fragments. Multinuclear cells originating from the nuclear fragmentation rather soon degenerate. The cytophotometrical measurement of the DNA amount in newly formed fragments has shown their ploidy extending from 1 to 32c, di-, three-, tetra-, and octoploid nuclei predominating. The distribution of chromosomal markers of the interphase nuclei (nucleoli, heterochromatinous blocks of nucleolus-forming chromosomes) confirms the photometrical evidence on the trends of chromosome fragmentation into genes. The fragmentation of the giant nucleus is preceded by a complex rearrangement of genetical material in the original nucleus, resulting in becoming polygenomal from polytene, with individual genomes separating to be segregated again, during division.     

Role of polyteny and chromosome-membrane interactions in plant genetic processes

Existing data in the literature on polyteny in plants and in eukaryotes in general, data on the structure of interphase chromosomes and their association with the nuclear membrane are analyzed in the paper. In light of these data, the results of our studies on the ratios of phenotypic classes of marker enzymes in agamospermy (apomictic) progeny of sugar beet are considered. The given data allow to state that chromosome polyteny and their association with nuclear membrane provide not only the realization of hereditary information, but also determine the ratio of genotypes and phenotypes in progeny.