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  • All these results indicate that

    2021-05-14

    All these results indicate that cleaving embryos possess a regulative mechanism, sensing the number of inner ezh2 inhibitor and controlling the proportion of conservative and differentiative divisions of blastomeres. The current observations agree with the results from other laboratories and our previous studies, where we showed that the total number of inner cells is regulated by the frequency of differentiative divisions between 8- and 32-cell stage (Bischoff et al., 2008), (Fleming, 1987) and (Krupa et al., 2014). We also confirmed that at least in some 32-cell embryos certain outer blastomeres divide in a differentiative manner, giving rise to one outer (TE cell), and one inner cell localizing in ICM (Morris et al., 2010). As such divisions occur less often in ESCs-injected embryos, it seems that differentiative divisions of outer 32-cell stage blastomeres supplement the pool of inner cells in the embryos that have a low number of inner cells after the first two rounds of differentiative divisions. The exact mechanism, which is responsible for the effect of ESCs on the plane of blastomere divisions, and, in consequence, the number of inner, ICM precursor cells, remains elusive. Recent studies showed that the plane of blastomere division in the 8-cell embryos is partially controlled by the position of the blastomere nucleus. When blastomeres of 8-cell embryo progress through the cell cycle, their nuclei migrate from the apical to the more basal position. This basal position of the nucleus is required for the differentiative division of the blastomere (Ajduk et al., 2014). Therefore, it is possible that injection of ESCs leads to blastomere stretching, which shifts blastomere nucleus apically. This would in consequence facilitate conservative divisions. This is in agreement with data published previously for zebrafish (Castanon et al., 2013), sea urchin eggs (Minc et al., 2011) and a rat epithelial cell line (O'Connell and Wang, 2000). However, although we showed that blastomeres of 8-cell embryos with introduced ESCs are longer than blastomeres of the control embryos, other parameters are similar in both groups and thus we cannot say definitively, whether deformation of blastomeres is responsible for the effect of exogenous cells on the pattern of cleavage. Further studies utilizing apical-basal axis markers are needed to establish if and how this axis is distorted in blastomeres of the ESCs-injected embryos. On the other hand, numerous studies showed that spindle orientation during cleavage does not correlate with the fate of the resulting daughter cells and that the acquisition of final cell fates depends on their dynamic behavior (i.e. engulfment, bulging, displacement) (Dard et al., 2009), (MaƮtre et al., 2015), (McDole et al., 2011) and (Watanabe et al., 2014) rather than division plane as was suggested by Bischoff et al. (Bischoff et al., 2008). According to Samarage et al. (2015) divisions with spindle oriented exactly perpendicularly or in parallel to the embryo surface occur very rarely. The authors demonstrated that both daughter cells arising in a differentiative division locate initially outside but differ in tensile forces generated by the cortical cytoskeleton. Subsequently, inner cells exhibiting higher cortical tension (dependent on myosin II) than outer cells constrict and as a result are positioned inside the embryo (Samarage et al., 2015). Nevertheless, additional studies are necessary to characterize these biomechanical forces ezh2 inhibitor in blastomeres of embryos containing ESCs. It is an essential question whether these initial differences in tensile forces occur between inner and outer blastomeres but through the lack of space inside the embryo (occupied by ESCs) less inner cells can constrict and locate in the ICM. Hence, prolonged exposure of inner blastomeres to the outside environment may, in consequence, enforce their polarization and change of fate. An alternative but slightly less probable scenario could be that a regulative mechanism occurs in the mouse embryo, that enables blastomeres to sense the amount of space inside and adjusts the proportion of differentiative vs. conservative divisions accordingly.