TEM of mitochondria after SCNT. TEM of reconstructed (somatic nuclear transfer; SCNT) oocyte at 30 min after fusion. The donor cell is still clearly separated from the enucleated oocyte (delineated by arrows) at this stage of SCNT. These cells contain somatic cell mitochondria with an elongated shape and cristae that co-exist with round oocyte mitochondria for at least up to the blastocyst stages and represent heteroplasmy in the developing pre-implantation embryo. From Zhong et al. .