Nucleolar and mitochondrial morphology in bovine embryos reconstructed by nuclear transfer

Abstract

The nucleolar and mitochondrial morphology of developing reconstructed bovine nuclear transfer (NT) embryos and stage‐matched in vivo‐produced control embryos were examined under the electron microscope. Each reconstructed embryo at the one‐cell (n = 12), two‐cell (n = 5), three‐cell (n = 3), four‐cell (n = 5), 5–8 cell (n = 5) and blastocyst (n = 3) stages was produced by fusion of a 16–32‐cell‐stage blatomere with an aged enucleated bovine oocyte. The normal and reconstructed embryos showed similar mitochondrial morphology. However, NT embryos produced several pleiomorphic forms not seen in controls, and were more heterogenous at early stages of development. Control embryos exhibited nucleolar features considered indicative of rRNA synthesis from the eight‐cell stage onwards. In contrast, the NT embryos presented nucleoli with morphology consistent with rRNA synthesis in all embryos examined, except in the three‐cell and in two of the five four‐cell embryos. From this nucleolar morphology, it was concluded that nuclear reprogramming does not occur immediately following nuclear transfer, but occurs gradually over the first two or three cell cycles. © 1996 Wiley‐Liss, Inc.

Role of mitochondrial DNA replication during differentiation of reprogrammed stem cells

“…However, mammalian oocytes generally contain spherical immature mitochondria, which are characterised by electron dense matrices with few or no cristae (Hillman and Tasca 1969;Hillman and Hillman 1975;King et al 1996). As development progresses mitochondrial differentiation is observed during progressive cleavages to an elongated shape, with reduced electron density and transverse cristae observed at the blastocyst stage (Van et al 1990;Plante and King 1994;King et al 1996;Crosier et al 2000;Crosier et al 2001;Tao et al 2008). These structural changes are associated with increased metabolic activity, oxygen consumption and CO 2 production observed in the blastocyst Barnett and Bavister 1996).…”

“…Nuclear transfer embryos display numerous ultrastructural changes compared to in vitro produced (IVP) and in vivo embryos (King et al 1996). Nuclear transfer embryos produced using embryonic (King et al 1996), foetal (Zhong et al 2007;Zhong et al 2008) or adult (Tao et al 2008;Han et al 2008) donor cells demonstrate heterogenous morphologies not seen in IVP or in vivo embryos that persist for several cleavages.…”

The role of mitochondrial genome (mtDNA) in somatic and embryo cloning of mammals. A review

“…King et al (1996) reported that, in bovine cloned embryos, mature mitochondria that were probably carried along with the donor cells were observed at the one-to four-cell stages. That is why it is speculated that mature autoreplicating mitochondria, transferred with nuclear donor embryonic or somatic cells into recipient ooplasts, may be broken down in the blastomere cytoplasm of reconstituted embryos during the above mentioned morphological transformation and genomic maturation of mitochondria during preimplantation embryogenesis (Takeda et al, 1999;Do et al, 2001;Meirelles et al, 2001).…”

Epigenetic reprogramming of somatic cells by nuclear transplant in zebrafish

“…Bovine cloned embryos display nucleoli with a transcriptionally active morphology, indicating that reprogramming of rRNA genes is slow (King et al, 1996;Lavoir et al, 1997). They also display heterogeneity in mitochondrial morphology, indicating possible alterations in mitochondrial gene expression (King et al, 1996).…”

“…They also display heterogeneity in mitochondrial morphology, indicating possible alterations in mitochondrial gene expression (King et al, 1996).…”