There is no information about the characteristics of early cleavage in the Patagonian blennie (Eleginops maclovinus), which can be used as a diagnostic tool for embryo quality. The purpose of this investigation, therefore, was to characterize the first blastomeres of E. maclovinus morphologically. Of a ‘pool’ of incubated eggs at 10.7 ± 0.5°C, 100 microphotographs of blastodiscs were extracted at different incubation periods from 0.25 to 5 h after fertilization and analyzed. Blastodiscs taken at 3.5, 4.0 and 5.0 h were characterized and classified into symmetric or asymmetric groups according to their morphology. The proportions of length (L) and width (W) of each blastomere were determined to establish its symmetry. Additionally, 20 microphotographs of blastodiscs of normal appearance were analyzed morphologically (control blastodisc: CB) and compared other blastodiscs (4.0 and 5.0 h). The results showed that before fertilization oocytes presented a somehow turgid aspect (maximum average diameter of 987 ± 41 µm) and after fertilization and hydration, their diameter increased to 1001.5 ± 11 µm (but not statistically significant) and presented a spherical shape. First cleavage ends after 3.5 h of development, forming two blastomeres 467 ± 45 μm length (L) and 328 ± 21 μm width (W) with a L/W ratio of 1.43 ± 0.19. The second cleavage ends after development at 4.5 h forming four blastomeres 238 ± 65 μm length and 227 ± 65 μm width with a ratio L/W of 1.06 ± 0.09. Five categories were identified during the blastomere characterization: 70% normal or symmetric; 8% with odd numbers of blastomeres; 6% unequal; 6% ‘pie shaped’ and 10% amorphous.

For Salmo salar, there is a lack of information on the morphology of the first blastomeres formed during embryonic development and which could be used as a diagnostic tool for the first stages of development. The purpose of this investigation, therefore, was to characterize morphometrically the first blastomeres of S. salar. From a pool of eggs incubated at 7.5°C, 100 microphotographs of blastodiscs were extracted and analyzed at different incubation periods: 12, 14, 16, 20 or 24 h. Blastodiscs were characterized morphologically after 16, 20 or 24 h incubation, and classified into symmetric or asymmetric groups according to their morphology. The ratio of length (L) versus width (W) of each blastomere was determined, to establish its symmetry. In addition, 20 microphotographs of blastodiscs of normal appearance were analysed morphologically (control blastodisc: CB) for comparison (20 or 24 h). Results show that the first cleavage ends after 16 h of development. Seven categories were established during blastomere characterization: 47% normal (G1); 27% with dispersed margins (G2); 10% unequal (G3); 9% ‘pie-shaped’ (G4); 3% amorphous (G5); 2% three equal blastomeres and one different one (G6); and 2% with eccentric cleavage (G7). Although the incidence of abnormal cleavage in S. salar is uncertain, there is a potential for some asymmetries to be corrected during embryogenesis to generate viable individuals. More studies are necessary to correlate these abnormal cleavage patterns with indicators of quality in the later stages of embryogenesis in this species, to establish a quality assessment tool for gametes and/or embryos in salmonid species.

There is a lack of information on the morphology of the first blastomeres that could be used as a diagnostic tool for the first stages of embryonic development for Coho salmon. The purpose of this investigation, therefore, was to characterize morphometrically the first blastomeres of Coho salmon (Oncorhynchus kisutch). In total, 660 embryonic discs from a pool of eggs that had been fertilized and incubated at 5°C and after 19 h of incubation were extracted and photographed. Of these, 20 microphotographs of blastodiscs of normal appearance were analyzed morphologically (control blastodiscs: CB) and 100 random microphotographs from the whole group were classified as either symmetrical or asymmetrical according to their morphology and then compared with the CB. The length and width of each blastomere and the proportions of length and width were measured to determine symmetry in the embryos at the 4-cell stage. Seven categories were created to characterize the blastomeres: 38% normal (G1); 26% unequal (G2); 10% ‘pie-shaped’ (G3); 10% amorphous (G4); 8% with three equal and one unequal blastomere (G5); 6% ‘clover-shaped’ blastomeres (G7), and 3% with inclusions. The mean of the proportions of lengths and widths of the groups of blastomeres that were measured was 0.87 ± 0.08 and 0.85 ± 0.07, respectively. The morphometric results that were obtained in this investigation are compared with the results observed by other authors for teleostei and are discussed.

There is increasing evidence to show that 2-cell stage mouse blastomeres have differing developmental properties. Additionally, it has been suggested that such a difference might be due to their distribution of mRNA and/or protein asymmetry. However, to date, the exact genes that are involved in the orientation and order of blastomere division are not known. In this study, some differentially expressed transcripts were identified. Axin1, cell division cycle 25 homolog C (Cdc25c) and cyclin-dependent inhibitor 2D (Cdkn2d) were selected for validation by real-time polymerase chain reaction (PCR) based on published data. Our real-time PCR results demonstrated that Axin1, Cdc25c and Cdkn2d genes had different levels of expression among blastomeres of the mouse 2-cell embryo i.e. the level of Axin1 mRNA was significantly higher in one blastomere when compared with the other blastomeres of the 2-cell embryo (p < 0.05). The variation in Cdc25c (p < 0.05) and Cdkn2d (p < 0.01) mRNA expression followed a similar trend to that of Axin1. In addition, the highest levels of expression of these three genes were detected in the same blastomere in the 2-cell embryo. We confirmed that there was an asymmetrical distribution pattern for Axin1, Cdc25c and Cdkn2d transcripts in 2-cell embryos. In conclusion, this study demonstrated clearly that there is embryonic asymmetry at the 2-cell stage and that these differentially expressed genes may result in differentiation in expression in embryo development.

The events resulting in compaction have an important influence on the processes related to blastocyst formation. To analyse the quality of the embryos obtained by somatic cell nuclear transfer (SCNT) in aspects different from previous studies, not only the number of blastomeres of cloned embryos during the initiation of compaction, but also the distribution of microvilli in cloned, normal, parthenogenetic, and tetraploid embryos before and after compaction was preliminarily investigated in mouse. Our results showed that during compaction the number of blastomeres in SCNT embryos was fewer than that in intracytoplasmic sperm injection (ICSI) embryos and, before compaction, there was a uniform distribution of microvilli over the blastomere surface, but microvilli became restricted to an apical region after compaction in the four types of embryos. We also reported here that the time course of compaction in SCNT embryos was about 3 h delayed compared with that in ICSI embryos, while there was no significant difference between SCNT and ICSI embryos when developed to the 4-cell stage. We concluded that: (i) the cleavage of blastomeres in cloned embryos was slow at least before compaction; (ii) the distribution of microvilli in cloned, normal, parthenogenetic, and tetraploid embryos was coherent before and after compaction; and (iii) the initiation of compaction in SCNT embryos was delayed compared with that of ICSI embryos.

In the present study, the development in vitro and in vivo of nuclear transfer (NT) embryos reconstructed with embryonic cells (blastomeres) at the 32- to 63-cell (sixth cell cycle) and 64- to 127-cell (seventh cell cycle) stages was investigated to determine the optimum range of embryonic cell cycles for yielding the highest number of identical calves in Japanese black cattle. Rates of development to the blastocyst stage (overall efficiency) were higher in the sixth cell-cycle stage (45%) than in the seventh cell-cycle stage (12%). After the transfer of the blastocysts reconstructed with blastomeres of the sixth and seventh cell cycle-stage embryos to recipient heifers, there were no differences in the pregnancy (14/35: 40% versus 3/13: 23%, respectively) or calving rates (11/39: 28% versus 3/13: 23%, respectively). These results indicate that the highest number of identical calves would be obtained by using sixth cell cycle (32- to 63-cell)-stage embryos as nuclear donors.

We have investigated the possibility that mitotic nuclei originating from preimplantation stage embryos and placed in the oocyte cytoplasm can undergo remodelling that allows them to undergo meiosis in the mouse. To address this question, we have used enucleated germinal vesicle (GV) ooplasts as recipients and blastomeres from the 2-, 4- or 8-cell stage as nuclear donors. We employed two methods to obtain ooplasts from GV oocytes: cutting and enucleation. Although efficiency of the reconstruction process was higher after enucleation than after cutting (90% and 70% respectively), the developmental potential of the oocytes was independent of how they had been produced. Nuclei from the 2-, 4-, or 8-cell stage embryos supported maturation in about 35%, 55% and 60% of cases, respectively. The time between nuclear envelope breakdown and the first meiotic division was shortened by up to 5 h in reconstructed oocytes, a period equivalent to the mitotic division of control blastomeres. About one-third of oocytes reconstituted with blastomere nuclei divided symmetrically instead of extruding a polar body; however, in the majority of them metaphase plates were found, suggesting that reconstructed oocytes (cybrids) underwent a meiotic rather than mitotic division. The highest percentage of asymmetric divisions accompanied by metaphase plates was found in cybrids with 8-cell-stage blastomere nuclei, suggesting that the nuclei from this stage appear to conform best to the cytoplasmic environment of GV ooplasts. Our results indicate that the oocyte cytoplasm is capable of remodelling blastomere nuclei, allowing them to follow the path of the meiotic cell cycle.

The technique already adopted by the authors for the cryopreservation of rainbow trout blastomeres was applied to carp. Three stages were utilised for cell freezing: the morula stage, the early blastula stage and the late blastula stage. Survival rates were 89 ± 1.6 % for morula cells, 94 ± 0.6 % for early blastula cells and 96 ± 0.4 % for late blastula cells. Early blastula and late blastula cells reaggregated into morula-like bodies within 2 h in culture medium. Results suggest that the freezing technique adopted for rainbow trout may be successfully applied to carp. The morula stage blastomeres were more sensitive to freezing than blastula stage blastomeres, as already found in rainbow trout.