[Early embryonic mortality in water vole (Arvicola terrestris L.)].

A morphological study of forty-eight 3-4-day embryos from eight females of the water vole (Arvicola terrestris L.) was conducted. The data obtained demonstrated that early embryonic losses in the water vole can be 30-35%, and the main cause of these losses is anomalies of oogenesis.

[Cytogenetic study on the activity of the ferret embryonic genome before implantation].

A cytogenetic study of the activity of the embryonic genome in ferret (Mustelaputorius) blastocysts during 6 days after their transition from the oviduct to the uterus has been carried out. It has been found that the prolongation in the preimplantation period in the ferret is not accompanied by inhibition of mitosis or activity in nucleolus organizing regions of inner cell mass cells as occurs in species having an obligatory delay of implantation (obligate embryonic diapause). Amitosis of trophoblast cells starts at the periimplantation stage as in other species that do not have obligate diapause. The data obtained are consistent with the hypothesis that the obligatory stage of delayed implantation might occur in some mammals in different taxonomic groups as a result of chromosome mutations affecting the genetic control of the chronology of events (timing) of embryogenesis. Consequently the characteristics of delayed implantation should be different in different species.

[Three twinning types in mink (Mustela vison) embryogeny].

The frequency of emergence of monochorial twins in the postimplantational embryogeny of the American mink and their karyotypes were studied. Monochorial pairs were found in which embryos had different karyotypes: 2n, XX and 2n, XY or 2n and 3n. This fact contradicts the notion that monochorial twins should be monozygotic and genetically identical but confirms our earlier hypothesis that a third twinning type exists in mink: monozygotic but genetically different. The mechanism of the emergence of this twinning type in mammals is discussed. It is suggested that the high (up to 4.5%) frequency of its emergence in the American mink is related to obligate embryonic diapause, causing abnormal fertilization.

[Frequency ratio of two forms of amitotic division of trophoblast cell nuclei in the mink blastocysts during the period of delayed implantation]. / Sootnoshenie chastot dvukh form amitoticheskogo deleniia iader kletok trofoblasta v blastotsistakh norki v techenii perioda zaderzhannoi implantatsii.

A comparative study of amitotic division activity of trophoblast cells by constriction and by extrusion in blastocysts of American mink during the obligatory period of delayed implantation has been carried out. The frequency of occurrence of amitotic figures was found to be nearly 10% at the onset of renewal of blastocyst growth (the blastocyst size was 0.4 mm in diameter), and nearly 20% at the stage of active growth (0.9 mm), as well as at the stage of expansion prior to blastocyst attachment to the uterine wall (1.7 mm). The ratios between the frequencies of division by extrusion and by constriction were 2:1, 5:1, and 4:1 at the three stages, respectively. We suggest that the cells generated via different amitotic ways play different roles in trophoblast differentiation.

[Mechanisms and frequency of nuclear divisions in trophoblast and decidua cells during postimplantation embryogenesis in the mouse]. / Mekhanizmy i chastota deleniia iader kletok trofoblasta i detsidua v techenie postimplantatsionnogo émbriogeneza u myshi.

The finding of amitotic division of trophoblast cell nuclei in blastocysts of the American mink (Mustela vison), which has an obligatory period of delay in implantation (obligatory embryonic diapause) in its ontogenesis, led us to study the mechanisms and frequencies of division of trophoblast and decidua cell nuclei during the postimplantation embryogenesis of mouse (Mus musculus), which does not exhibit an obligatory diapause nor amitosis in blastocysts. It has been established that the main mechanism underlying the cell nuclei division in both tissues (trophoblast and decidua) forming the placenta is amitosis. These data suggest that the occurrence of an obligatory embryonic diapause in ontogenesis of certain animal species is related not only to the delay in implantation, but also to the alteration in the chronology of all processes of embryogenesis.

[Activity of embryonic mink genome during diapause (cytogenetic analysis): nucleolar and extranucleolar rna synthesis]. / Activnost' c'mbrional'nogo genoma norki vo vremia diapauzy (tsitogeneticheslii analiz): iadryshkovoi i vneiadryshkovoi sintez rnk.

The nucleolar and extranucleolar RNA synthesis was studied in the mink blastocysts at different stages of embryonic diapause and during the periimplantation period using cytoradioautography. The data obtained suggest a differential and stage specific activity of the embryonic mRNA and rRNA synthesis during the period of delayed implantation.

[Differential activity of chromosomal nucleolus-organizer regions during postimplantation embryogenesis in mink]. / Differentsial'naia aktivnost' iadryshkoobrazuiushchikh raionov khromosom v postimplantatsionnom émbriogeneze norki.

Using cytological preparations, we have conducted analysis of the activity of chromosomal nucleolus organizers during postimplantation embryogenesis in mink. The analysis was conducted by visual evaluation of their staining pattern with silver nitrate and incidence rate in satellite associations. The data obtained provide evidence that each nucleolus organizer during embryonic mink development shows a characteristic pattern of activity depending on cell type, tissue type, and the stage of development and also depending on changing location in the interphase nucleus. We discuss biological significance of the existence of "indispensable long-satellite" chromosome in the genome.

[Activity of the embryonal genome of mink during diapause (cytogenetic analysis): number of cells and cell nucleus size in blastocysts or various size and age]. / Aktivnost' émbrional'nogo genoma norki vo vremia diapauzy (tsitogeneticheski analiz): chislo kletok i razmer kletochnykh iader v blastotsistakh raznogo razmera i vozrasta.

The number of cells and the morphology and size of the nuclei of these cells in diapausal mink embryos were investigated. Growth of blastocysts from 0.25 to 1.2 mm in diameter in the complete absence of classical mitoses is accompanied by an increase in the average number of cells per embryo from between 100 and 150 to 2000, respectively; diameters of cell nuclei vary from 6 to 19 and from 6 to 53 micron, respectively. Some giant nuclei exhibit morphological signs of polyteny. In all nuclei, the number of active (silver-stained) nucleoli does not exceed that of nucleolar organizers in the mink diploid genome. The obtained results, combined with data from the literature, allow the inference that polytenization of trophoblast cells followed by cryptic segregation of genomes provides the key mechanism for rapid growth of diapausal blastocysts.

[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.

[Activity of nucleolus organizer regions (NORs) in embryonic livers from twin minks]. / Aktivnost' iadryshkoobrazuiushchikh raionov khromosom v kletkakh émbrional'noi pecheni bliznetsov norok.

Ag-NOR patterns were studied in hepatocytes from nine mink embryo siblings, including a pair of monochorionic (presumably monozygotic, MZ) twins. Both the number and the size of Ag-NORs per cell were found to be identical in MZ twins. All the other sibs had the patterns different from each other and from the MZ ones. The conclusion is that the NORs activity is a strongly inherited character and the Ag-NOR pattern can be used as a reliable genetic marker to distinguish the twin zygosity.

[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.

Mechanisms involved in the spontaneous occurrence of diploid-triploid chimerism in the mink (Mustela vison) and chicken (Gallus domesticus).

Diploid-triploid chimeras have been observed both in man and in a number of laboratory and livestock animals. The mechanism(s) of their origin remains enigmatic. One approach is to calculate for each proposed mechanism the expected frequencies of zygotes bearing different gonosomic complements in the two cell lines. Observed samples are then compared with the expectations. The mechanisms that have been considered include: (1) fertilization of a blastomere, (2) absorption of the second polar body into a blastomere, (3) fertilization of the first polar body, (4) independent fertilization of both nuclei in binucleated oocytes, (5) fertilization of the second polar body as well as the egg, and (6) fusion of two eggs. The sample of minks comprised three preimplantation embryos, nine postimplantation embryos, and three neonatal pups, with gonosomic complements of 7 XX/XXX, 3 XX/XXY, 4XY/XXY, and 1 XY/XYY; the chicks comprised 13 embryos at 1 day of incubation, 1 embryo at 4 days, and one adult bird, with gonosomic complements of 5 ZZ/ZZZ, 1 ZZ/ZZW, 1 ZW/ZZZ, 3 ZW/ZZW, and 5 ZW/ZWW. If it is assumed that within each species all, or most, of the 2n/3n chimeras arise from the same mechanism, then the occurrence of a type that has an expected frequency of zero for a given proposed mechanism effectively eliminates that mechanism as a source. All of the chicks could have resulted from only one mechanism, viz., independent fertilization of both nuclei in binucleated oocytes. The sample of minks could have resulted from the same mechanism or from fertilization of a blastomere of a two-cell, 2n embryo.

[Aneuploidy and polyploidy in the bone marrow cells of minks of different genotypes, ages and fertility]. / Aneuploidiia i poliploidiia v kletkakh kostnogo mozga norok raznykh genotipov, vozrasta i plodovitnosti.

An analysis of aneuploidy and polyploidy in bone marrow cells of female minks of different genotype and age has shown that in young minks with unsatisfactory reproductive characteristics, and in old females, the frequency of aneuploid and polyploid cells is significantly higher as compared to that in minks having a good reproductive capacity. The increase in aneuploidy level takes place mainly in the form of hypoploidy, and an increase of the frequency of polyploid cells is accompanied by a parellel increase of the proportion of cells with a low degree of ploidy. A correlation between frequencies of appearance of aneuploid and polyploid cells in minks is observed (r = 0.69 +/- 0.27). On the basis of original and bibliographic data, a hypothesis on a genetically determined instability of karyotype in mutant minks is argumented.