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 microm 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.

Whole-genome chromosome distribution during nuclear fragmentation of giant trophoblast cells of Microtus rossiaemeridionalis studied with the use of gonosomal chromatin arrangement.

Gonosomal chromatin bodies (GCBs), i.e. blocks of condensed chromatin consisting of heterochromatized region of the sex chromosomes of the field vole M. rossiaemeridionalis, were used as a natural interphase chromosome marker in order to clarify the regularities of GCB rearrangement during nuclear fragmentation of secondary giant trophoblast cells (SGTCs) at the end of their differentiation. Cytophotometrical measurements of DNA content in the nuclei, nuclear fragments and simultaneously in the GCBs were made in the secondary giant SGTCs of field vole M. rossiaemeridionalis. In most cases 1 to 2 GCBs get into the nuclear fragments at different ploidy levels. In the nuclear fragments, GCB DNA content decreased mostly proportionally to DNA content in the whole fragments corresponding to 2c, 4c and 8c. The data obtained demonstrate a regular whole-genome chromosome distribution into nuclear fragments. A possible mechanism of nuclear fragmentation that largely ensures a balanced genome in nuclear fragments is discussed.

[A study of DNA depolyploidization and depolytenization of the heterochromatized gonosomal chromatin bodies in the secondary giant trophoblast cells of the field vole Microtus rossiaemeridionalis using cytophotometry].

A study was made of the distribution of the heterochromatized gonosomal chromatin bodies (GCB) material in the course of nuclear fragmentation of secondary giant trophoblast cells resulting in polykaryocyte formation at the late stage of their differentiation. A simultaneous DNA cytophotometry in GCBs and nuclear fragments showed a progressive GCB DNA content decrease proportional to that of DNA content in nuclear fragments. DNA contents in the nuclear fragments corresponded to 2c, 4c and 8c. In most cases 1-2 GCBs were found in the nuclear fragments of different ploidy levels. Both the total DNA content in GCBs and the DNA content in separate GCBs well correlated with the ploidy levels of fragments. The data obtained demonstrate a regular, whole-genome distribution of chromosomal materials into the nuclear fragments exemplified by sex chromosome distribution in compliance with the ploidy of nuclear fragments. We discuss a possible mechanism of nuclear fragmentation that may ensure substantially a balanced genome of nuclear fragments without leading to mitotic cycle renewal in the giant trophoblast cell population.

Genome multiplication of extravillous trophoblast cells in human placenta in the course of differentiation and invasion into endometrium and myometrium. II. Mechanisms of polyploidization.

Peculiarities of the structure of interphase nuclei, mitotic activity, and Ki-67 protein intranuclear immunolocalization were studied to elucidate mechanisms of genome multiplication in proliferative and differentiating invasive extravillous trophoblast cells in the human placenta. The presence of numerous chromocenters was shown to be a characteristic feature of proliferative cell nuclei of both villous and extravillous trophoblast. At the beginning of extravillous trophoblast cell differentiation, i.e. in the proximal part of cell columns, some amount of cells with large nuclei containing enlarged chromocenters were found. DNA content was measured simultaneously with counting the number of chromocenters in similarly looking nuclei of squash preparations of placental villi. The increase in the ploidy level up to 4c-8c, accompanied by a slight increase in the number of chromocenters being not proportional to the ploidy level and not exceeding the diploid number of chromosomes of the human genome, was demonstrated. This suggests that genome multiplication of extravillous trophoblast cells may be accomplished by endoreduplication. In addition, pictures of endomitosis were seen at early steps of differentiation of EVT cells. The lack of polyploid mitotic figures or any obvious polyploidizing or restitutional mitoses suggests that these are not of considerable importance in genome multiplication of human EVT cells. However, the prevalence of metaphases at the boundary of the distal part of cell columns suggests that restitutional mitoses may be involved, even partly, in human trophoblast cell polyploidization. At later steps of differentiation, i.e. in the distal part of cell columns, the nuclear structure obviously changes, with a uniform "network" chromatin arrangement prevailing, whereas numerous chromocenters and features of endomitosis are no longer seen. The pattern of Ki-67 protein immunolocalization is also changing along the invasive pathway. In the proliferating stem cells and trophoblast cells of the proximal part of cell columns, Ki-67 was localized in the karyoplasm, chromocenters and numerous small nucleoli, whereas in the distal part of cell columns this protein was detected predominantly in 1-2 large nucleoli. The comparative analysis of the literature data on Ki-67 localization at different stages of cell cycle provided another evidence that EVT cells in the course of invasion may switch to the endoreduplication cycle. In agreement with the relevant report on rodent placentation, our present data suggest that acquirement of an invasive phenotype of EVT cells is accompanied by switching from mitotic division to endoreduplication cycle.

[Quantitative investigation of reproduction of condensed chromatin of sex chromosomes during trophoblast cell polyploidization and endoreduplication in the East European field vole Microtus rossiaemeridionalis]. / Kolichestvennoe issledovanie reproduktsii kondensirovannogo khromatina polovykh khromosom pri poliploidizatsii i éndoreduplikatsii kletok trofoblasta vostochonoevropeiskoi polevki 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.

Genome multiplication of extravillous trophoblast cells in human placenta in the course of differentiation and invasion into endometrium and myometrium. I. Dynamics of polyploidization.

Polyploidization of the extravillous trophoblast (EVT) cells at different stages of differentiation and invasion into the uterine wall in human placenta has been studied. An increase in the ploidy level of EVT cells in the course of their differentiation within cell columns (CC) was shown. Stem cells were mainly diploid (86.2%); incidence of polyploid nuclei of highly proliferative cells of the proximal part of CC increased progressively. In the distal part of CC, where EVT cells did not divide mitotically, polyploid cells prevailed, with 58.0 and 3.5% nuclei being 4c and 8c, respectively. The highest percentage of polyploid cells was found in the population of EVT cells attached directly to the surface of the decidualized endometrium: percentage of tetraploid cells turned out to be 74.7% and the share of octaploid nuclei rose up to 4.9%; however, there appeared a few (0.3%) 16c cells. The majority of EVT cells invading the decidualized endometrium were polyploid, the share of octaploid and hexadecaploid cells rose up to 9.7 and 1.4%, respectively. On the other hand, the percentage of diploid cells also increased up to 29.2% as compared to EVT cells attached to decidua (20.0%). The same tendency proved to be even stronger in myometrium: the share of diploid EVT cells increased up to 46.0%, a prominent amount of tetraploid (45.1%) and highly polyploid (8c and 16c) cells retained in the EVT cell population (7.4 and 1.1%, respectively). Immunohistochemical staining of Ki-67 protein (MIB1), which labels cells held in the cell cycle, showed a high incidence of MIB1-positive stem cells (93.7%) and the EVT cells of the proximal part of CC (85.5%) characterized by high mitotic activity. A lower MIB1-positivity (43.2%) was found in the distal part of CC, whereas invasive EVT cells showed no MIB1-labeling. The presence of MIB1-positive nuclei in the distal part of CCs in the absence of mitoses, taken together with data on polyploidization of these cells, indicates their switch to the endoreduplication cycle. As a whole, the data obtained evidence that differentiation of EVT cells of the invasive pathway is accompanied by polyploidization. However, in a population of trophoblast cells capable of most profound invasion (up to myometrium), the proportion of diploid cells rose. These results suggest that the human cytotrophoblast invasion into the uterine wall requires an optimum, not the highest, ploidy level, whereas highly polyploid cells may form a subpopulation at the border between the maternal and fetal parts of placenta.

Polyploidization in the trophoblast and uterine glandular epithelium of the endotheliochorial placenta of silver fox (Vulpes fulvus Desm.), as revealed by the DNA content.

Dynamics of genome multiplication during establishment of interrelations between trophoblast and glandular epithelium of the endometrium has been studied in the course of formation of placenta in the silver fox. During formation of the placenta, penetration of the trophoblast into the zone of the endometrial glandular epithelium and of endometrial blood vessels into the zone of expanding trophoblast occurs. The trophoblast, which gradually replaces epithelium and a part of the stroma of the endometrium, closely adjoins endometrial vessels but does not disrupt them, thereby the endotheliochorial placenta is formed. Cytophotometric measurements of the DNA content in trophoblast nuclei have shown that most of them are polyploid: predominantly 4-64c, occasionally 128c and 256c. Polyploidy of the trophoblast may be a consequence of various types of polyploidizing mitoses. Cytophotometric measurements of the DNA content in mitotic figures have revealed the presence of mitoses of diploid cells, i.e. with the DNA amount of 4c (2n), and polyploid cells, i.e. 8c (4n), and 16c (8n), therefore trophoblast cells in the silver fox placenta are able to enter mitosis up to the octaploid level. Higher degrees of polyploidy in the trophoblast cells seem to be achieved by endoreduplication. Polyploidization of the uterine glandular epithelial cells during placentation in the silver fox occurs until the level of 8c. Thus, the tissue-specific response of the uterus to the implanting embryo consists of active proliferation and polyploidization of the glandular epithelium, which may compensate formation of prominent population of decidual cells (i.e., connective tissue cells). In the endotheliochorial placenta of the silver fox the regularity is confirmed that cells of both maternal and fetal origin are, as a rule, polyploid in sites of their contact in placenta, which may be of protective significance in the contact of allogenic organisms.

[Genome multiplication in the trophoblasts and glandular epithelium of endometrium during embryo implantation and placentation of silver fox]. / Umnozhenie genoma v kletkakh trofoblasta i zhelezistogo épiteliia éndometriia pri implantatsii zarodysha i platsentarii u serebristo- cherno'i lisitsy.

Dynamics of genome multiplication during establishment of interrelations between the trophoblast and the glandular epithelium of endometrium was studied in the course of placenta formation in the silver fox. Endometrium response on the embryo implantation exhibits some features of inflammation. In the course of placenta formation the trophoblast gains access to the endometrial glandular epithelium zone, while the endometrial blood vessels grow the other way into the expanding trophoblast zone. The trophoblast gradually replaces the whole epithelium and part of the stroma of the endometrium, closely adjoining the endometrial vessels but not disrupting them. Cytophometric DNA measurements in the trophoblast nuclei have shown that most of the nuclei are polyploid: predominantly 4c-64c, occasionally 128c and 256c. Polyploidy of the trophoblast may result from various types of polyploidizing mitoses. Cytophotometric DNA measurements in mitotic figures have revealed mitoses with DNA amounts equal to 4c (2n), 8c (4n), and 16c (8n), which indicates that trophoblast cells in the silver fox placenta are able to enter mitosis prior to the octaploid level. Higher degrees of polyploidy in the trophoblast cells may be achieved presumably by endoreduplication. In the silver fox polyploidization of uterine grandular epithelial cells during placentation occurs until the level of 8c. Thus, the tissue-specific response of the uterus to the implanting embryo is an active proliferation and polyploidization of the glandular epithelium, rather than formation of a population of polyploid decidual cells (i.e. connective tissue cells). Using the silver fox endotheliochorial placenta as an example, a regularity has been confirmed that cells of both maternal and fetal origin are polyploid in sites of their contact in placenta, which might be of protective significance in the contact of allogenic organisms.

Trophoblast cell invasiveness and capability for the cell and genome reproduction in rat placenta.

Using 3H-thymidine labeling and cytophotometric DNA content measurement in nuclei and mitotic figures, proliferative activity and genome reproduction peculiarities have been studied in highly invasive, primary and secondary giant trophoblast cells (pGTCs and sGTCs, respectively) as well as in the low invasive junctional zone and labyrinth trophoblast cells (JTCs and LTCs) of rat placenta. It has been shown that different extent and patterns of phagocytic activity of pGTCs and sGTCs correspond to different developmental stages. An inverse relationship has been observed between capability for mitoses and invasive and phagocytic activities. The pGTCs and sGTCs lose their mitotic activity from the start of their differentiation. Nevertheless, they continue reproduction of their genome and undergo a series of endoreduplication cycles to reach the ploidy degree of 256-1024c. In contrast, the JTCs and LTCs have, as a rule, no invasive and phagocytic activities, while preserving proliferative properties up to 15 day of gestation. They undergo initial polyploidization via uncompleted polyploidizing mitoses up to octaploid level and then pass to the endoreduplication cycle that excludes mitoses. Such a way of cell reproduction has been suggested to play a protective role, as it rules out contacts of the trophoblast cell genome with chromosomes of the phagocyted allogenic maternal tissue.

Genome multiplication in the tertiary giant trophoblast cells in the course of their endovascular and interstitial invasion into the rat placenta decidua basalis.

Processes of invasion of the tertiary giant trophoblast cells (tGTCs) into the uterine wall in the course of implantation of the rat embryo at 10-15 days post coitum (dpc) were studied. Reproduction of tGTCs was evaluated using 3H-thymidine incorporation and cytophotometric measurement of the nuclear DNA content. 1. Patterns of trophoblast invasion. Two patterns of invasion were observed: endovascular (from 10 to 15 dpc) and interstitial (from 12 to 15 dpc), via extracellular matrix of endometrial stroma towards the central arterial channel. The both types of the tGTC invasion seem to take part in modification of the arterial wall. The tGTCs were shown to differentiate from cambial trophoblast cells of ectoplacental cone (EC) or junctional zone of placenta. 2. Peculiarities of reproduction of tGTCs. By the start of the endovascular and interstitial migration, tGTCs lose completely their capability for mitotic divisions and are found autoradiographically to stop incorporation of 3H-thymidine soon after beginning of their migration. Cytophotometric measurements of the nuclear DNA content have shown tGTCs to be polyploid. Their degree of ploidy is mainly 8-32c, which is by 1-2 classes of ploidy higher than that of the initial cambial cell populations of ectoplacental cone (EC) and junctional zone trophoblast cells (JTCs). Since the replication processes in tGTCs are limited, their polyploidization seems to occur within cambial populations of the trophoblast cells. The final genome multiplication cycles in the beginning of the tGTC invasion can be accomplished only via endoreduplication, i.e., with the complete elimination of the mitotic mechanism. The not too high level of tGTC ploidy, 8-32 c, probably does not prevent the tGTC deep invasion against the blood flow and through the extracellular matrix and may be of a protective significance in their interrelations with allogenic maternal tissues.

Polytene chromosomes in mammalian cells.

This article deals with the structural and functional organization of polytene chromosomes in mammals. Based on cytophotometric, autoradiographic, and electron microscopic data, the authors put forward a concept of nonclassic polytene chromosomes, with special reference to polytene chromosomes in the mammalian placenta. In cells with nonclassic polytene chromosomes, two phases of the polytene nucleus cycle are described, such as the endointerphase (S phase) and endoprophase (G phase). The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes. The final step of the trophoblast giant cell differentiation is characterized by a transition from polyteny to polyploidy, with subsequent fragmentation of the highly polyploid nucleus into fragments of low ploidy. Similarities and dissimilarities of pathways of formation and rearrangement of nonclassic polytene chromosomes in mammals, insects, plants, and protozoans are compared. The authors discuss the significance of polyteny as one of the intrinsic conditions for performance of the fixed genetic program of trophoblast giant cell development, a program that provides for the possibility of a long coexistence between maternal and fetal allogenic organisms during pregnancy.

[Polyploid mitoses in mink trophoblast cells]. / Poliploidnye mitozy v kletkakh trofoblasta norki.

Mitotic figures in the mink placental trophoblasts have been observed under the light microscope using actions and air-dried preparations. The tetra- and octaploid metaphase chromosome spreads were found on Giemsa-stained air-dried preparations. A high percentage (up to 80%) of abnormal metaphases, including k-mitoses as well as a portion of restitution anaphases, was revealed on sections of the placental trophoblast suggesting a possible block of mitosis at the meta- and anaphase. Therefore, it is very likely that genome multiplication in a portion of placental trophoblast cells in mink involves block of mitoses at meta- and anaphase followed by restitution. The chromosomal arrangement on metaphase spreads in part of cells showed a degree of separation of the whole di- and tetraploid chromosome sets within tetraploid and octaploid chromosome plates. Several spreads exhibited some allocycly of diploid chromosome sets inside the polyploid metaphases. It is not inconceivable that such an arrangement may reflect some autonomy of low-ploidy chromosome sets within the polyploid trophoblast cells in mink.

[The quantitative study of the areas of active rDNA location and of the other parameters of the interphase nucleolus detectable by silver staining during the cell differentiation of the rat trophoblast]. / Kolichestvennoe issledovanie zon lokalizatsii aktivnosti rDNK i drugikh parametrov interfaznogo iadryshka, vyiavliaemykh metodom serebreniia v khode differentsirovki kletok trofoblasta krysy.

Different quantitative parameters of nucleolar silver staining have been studied in the cambial rat trophoblast cells on the 12th, 13th and 14th days of gestation. It has been shown that the number of Ag-positive granules in the nucleoli varied from 10 to 120. The number and the total area of silver stained granules in the nuclei increased progressively in the course of polyploidization, but was not doubled passing to the next ploidy level. Nevertheless, nucleolar area increased proportionally to the ploidy degree. The mean number and the total area of Ag-stained granules as well as the nucleolar area estimated for each ploidy level did not change significantly in the course of placenta development, suggesting an unchanged level of NOR activity at the studied stages of trophoblast cell differentiation. The data obtained on the interphase nucleoli differ from the data of the analysis of the metaphase Ag-NOR at the same period of placenta development, suggesting a diversity in the interphase and metaphase NOR organization. A proportion of cells with different number of nucleoli in the cambial rat trophoblast cells was maintained unchanged in the studied period of the placenta development, the majority (80-90%) of cells contained from 1 to 3 nucleoli. Such a proportion was similar in the cells of different levels of ploidy up to 16c. In this connection the association of NORs is suggested to be in relation with switching from the polyploid mitotic cycle to the endoreduplication leading to polyteny.

[Changes in the relative arrangement of the chromosomes and nucleolus in developing mammalian oocytes during meiotic prophase I in relation to functional changes in the oocytes]. / Izmenenie vzaimnogo raspolozheniia khromosom i iadryshka v razvivaiushchikhsia ootsitakh mlekopitaiushchikh v khode profazy I meioza v sviazi s izmeneniem funktsional'nogo sostoianiia ootsitov.

Data on chromosome transformation in meiotic prophase I during mammalian oogenesis are summarized. The main peculiarity of the female meiosis in mammals is an unusually long diplotene stage which may be subdivided into four periods: 1) the early diplotene (up to the beginning of follicle formation); 2) the dictyotene or "diffuse diplotene", implying primordial follicle oocytes; 3) the most pronounced lampbrush chromosome stage coinciding with the large growth period; 4) the stage of chromosome inactivation and karyosphere formation corresponding to the terminal stage of oocyte development before ovulation. These stages are associated with changes in the transcriptional chromosome activity. A correlation is revealed between the spatial chromosome arrangement in the oocyte nucleus and the transcriptional activity. Some regularities are followed in the transformation of the main nucleolar component arrangement during meiotic prophase I in mammalian oocytes. At the late pachytene and at the early diplotene, a segregation of the main nucleolar components has been observed. These components are disposed in the direction: chromatin--fibrillar center--dense fibrillar component--granulo-fibrillar component. At the dictyotene, signs of nucleolar segregation are still observed. At the lampbrush chromosome stage, when the nucleus is most highly transcriptionally active, an integration of nucleolar components occurs. At the late diplotene--prediakinesis stage, i.e. in the course of transcriptional activity lowering and karyosphere formation, the secondary segregation of the main nucleolar components occurs. These move to the nucleolar periphery to be disposed around a large fibrillar mass which is gradually displacing the rest of the nucleolar components. The fibrillar mass formation in the preovulatory oocyte nucleoli is one of the peculiarities of the diplotene and prediakinetic mammalian oocytes.

[Polyploidy and polyteny in the trophoblast cells of the mink]. / Poliploidiia i politeniia v kletkakh trofoblasta norki.

According to cytophotometry, trophoblast cells in the mink placenta are both diploid and polyploid, the ploidy level ranging from 2c to 64c. A great number of mink trophoblast cells were seen to divide mitotically. In addition to the ordinary mitotic figures, polyploid mitoses as well as abnormal mitotic figures were observed. Non-classic polytene chromosomes, peculiar to the mammalian trophoblast, appeared in the mink trophoblast cells to have the highest ploidy. A relatively low ploidy degree is due, probably, to a lesser invasive activity of the mink trophoblast cells as compared to the rodent giant trophoblast cells.

[The nucleolus organizer regions of the metaphase chromosomes in cambial cell populations of mouse and rat placenta]. / Iadryshkoobrazuiushchie raiony metafaznykh khromosom v kambial'nykh kletochnykh populiatsiiakh platsenty myshi i krysy.

The number of silver-stained nucleolus organizing regions (Ag-NORs) was counted in metaphase plates of the fetal part of placenta of mice and rats and in the tissues of their embryos. On day 9 of mouse gestation and on day 12 of rat gestation, up to 82% of metaphases in the fetal part of placenta have the highest possible number of chromosomes with Ag-NORs (9-10 for mice and 5-6 for rats). In the later embryogenesis (day 10 for mice and day 14 for rats), a great number of metaphases have either no Ag-NORs (34.9% for mice, 17.9% for rats), or only 1-2 Ag-NOR-chromosomes (17.4% for mice and 14.5% for rats). But in the tissues of embryos being on the same embryonic stages the changes in frequency of metaphases with different numbers of Ag-NOR-chromosomes have proved to be less obvious. In the differentiated polyploid placenta cells the number of chromosomes with Ag-NORs can be considerably lower than in diploid ones. It is proposed that the changes in frequency of metaphases with different numbers of Ag-NOR-chromosomes may reflect the regulation of ribosomal gene transcription in the embryogenesis, in particular, in differentiation of cambial cell populations of rodent placenta.

[A comparative study of the main components of the nucleolus in different populations of rat trophoblast cells during differentiation. I. The nucleoli of the trophoblast cells in the labyrinth section of the placenta]. / Sravnitel'noe izuchenie osnovnykh komponentov iadryshka v raznykh populiatsiiakh kletok trofoblasta krysy v khode differentsirovki. I. Iadryshki kletok trofoblasta labirintnogo otdela platsenty.

The nucleolus undergoes some steps of structural transformation during differentiation of the labyrinth trophoblast cells. Primarily (on day 13 of gestation) the nucleolar components become rather disjoined. The nucleolus is composed of a loose net of strands of granulofibrillar and dense fibrillar components bearing fibrillar centers (FCs). Strands are separated by large lacunae. This rare-occurring type of nucleoli is replaced on the next (14th) day by the nucleolonemal type and later--by the compact nucleolar type. FCs with dense fibrillar component strands become extended into the masses of granulofibrillar component. Such transformations of nucleolar structure seem to be an expression of a fast-proceeding differentiation of the labyrinth trophoblast cells.

[A comparative study of the main components of the nucleolus in different populations of rat trophoblast cells during differentiation. II. The nucleoli of the trophospongium cells, the glycogen cells and the secondary giant cells]. / Sravnitel'noe izuchenie osnovnykh komponentov iadryshka v raznykh populiatsiiakh kletok trofoblasta krysy v khode differentsirovki. II. Iadryshki kletok trofospongiuma, glikogenovykh kletok i vtorichnykh gigantskikh kletok.

A comparative study was performed of the arrangement of different nucleolar components during differentiation of trophoblast cell populations in the junctional zone of placenta (glycogen cells and trophospongium) and in the secondary giant cells. Each cell type is characterized by specific interrelation of nucleolar components. Some glycogen cells show signs of segregation of nucleolar components: strands of nucleolar components with fibrillar centers (FCs) are displaced to the periphery of the nucleolus and contact with the perinucleolar chromatin. Large reticular nucleoli in trophospongium cells contain many FCs which are gathered into several "chains" by strands of dense fibrillar component. Such a "chain" has also been found in nucleoli of secondary giant cells, with greater number of FCs in each "chain". Relationship between the arrangement of nucleolar components and the level of cell differentiation is discussed.