Fig. 4.

Tet gene expression and 5mC and 5hmC levels during mouse preimplantation development. The expression levels of Tet1 and Tet2 are very low in oocytes and fertilized zygotes. By contrast, Tet3 is highly expressed in oocytes and fertilized zygotes but disappears rapidly during cleavage (Gu et al., 2011; Iqbal et al., 2011; Wossidlo et al., 2011). Tet3-mediated 5mC oxidation and subsequent dilution by DNA replication during embryo cleavage might contribute to the previous ‘active’ DNA demethylation observed in the developing preimplantation embryo (Gu et al., 2011; Inoue and Zhang, 2011). The expression levels of Tet1 and Tet2 increase during preimplantation development and both are highly expressed in the inner cell mass of the blastocyst, which also exhibits high 5hmC levels (Ruzov et al., 2011).