Abstract
ABSTRACTMammalian embryo cloning by nuclear transfer has a low success rate. This is hypothesized to correlate with a high variability of early developmental steps that segregate outer cells, which are fated to extra-embryonic tissues, from inner cells, which give rise to the embryo proper. Exploring the cell lineage of wild-type embryos and clones, imaged in toto until hatching, highlights the respective contributions of cell proliferation, death and asymmetric divisions to phenotypic variability. Preferential cell death of inner cells in clones, probably pertaining to the epigenetic plasticity of the transferred nucleus, is identified as a major difference with effects on the proportion of inner cell. In wild type and clones, similar patterns of outer cell asymmetric divisions are shown to be essential to the robust proportion of inner cells observed in wild type. Asymmetric inner cell division, which is not described in mice, is identified as a regulator of the proportion of inner cells and likely gives rise to resilient clones.
Highlights
Variability coincides with the possibility of adapting to changing environments (Darwin, 1859)
We have investigated the variability of cell dynamics in normal and cloned rabbit embryos from the entire cell lineage reconstructed from two-photon microscopy images throughout pre-implantation stages
Digital specimens were obtained from 3D+time imaging of three wild-type embryos and two clones that were nuclear stained by injection at the one-cell stage of synthetic mRNA encoding H2B-EGFP and developing from the 32-cell stage until hatching (Fig. 1, Movies 1 and 2)
Summary
Variability coincides with the possibility of adapting to changing environments (Darwin, 1859). Wild-type populations are intrinsically variable (Raj and van Oudenaarden, 2008). The production of inbred strains, as achieved in laboratory conditions in mice, aims to minimize genotypic and phenotypic variability (Beck et al, 2000). Animal cloning by somatic cell nuclear transfer (SCNT) has been developed to go a step further, by keeping desired traits and producing clones in different mammalian species (Hochedlinger and Jaenisch, 2002; Inoue et al, 2005; Wakayama et al, 1999). The cloning efficiency is low (Hochedlinger and Jaenisch, 2003; Yang et al, 2007) and much effort has been devoted to improving its success rate.
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