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PubMed2005

Can artificial techniques supply morally neutral human embryos for research? Part I. Creating novel categories of human embryos.

Manipulations of the molecular composition and formation of human embryos are posing vital new challenges to traditional concepts of human identity and procreation. Current trends in embryology in particular are reshaping the ethical question of how scientific research should treat experimentally derived embryos. Some investigators have argued that embryos created through artificial means are technologically novel entities that should be exempt from ethical restraints placed on research involving human embryos that come into being through natural processes. These include uniparental embryos derived through cloning or parthenogenesis, as well as multiparental, hybrid-parental, and xenohybrid-parental embryos. If confined to natural means many of these genetic unions could not occur, but through the intervention of technology, it is becoming possible to design and grow strange and unusual forms of embryos, in some cases using human gametes. Regardless of the genetic contributors or the processes used to fertilize and stimulate egg activation, in each case the new embryo represents an individual organism that begins a process of development. We conclude that the prospect of creating or redesigning new human life should be held to a stringent ethical standard of precaution, even higher than that of deciding to destroy existing embryonic life. Accordingly, we urge cautious ethical reflection and broad public discussion prior to deciding whether to permit embryologic research into novel forms of procreative means in nonhuman animals, to be further extended to humans.

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PubMed2007

Centrosome inheritance after fertilization and nuclear transfer in mammals.

Centrosomes, the main microrubule organizing centers in a cell, are nonmembrane-bound semi-conservative organelles consisting of numerous centrosome proteins that typically surround a pair of perpendicularly oriented cylindrical centrioles. Centrosome matrix is therefore oftentimes referred to as pericentriolar material (PCM). Through their microtubule organizing functions centrosomes are also crucial for transport and distribution of cell organelles such as mitochondria and macromolecular complexes. Centrosomes undergo cell cycle-specific reorganizations and dynamics. Many of the centrosome-associated proteins are transient and cell cycle-specific while others, such as y-tubulin, are permanently associated with centrosome structure. During gametogenesis, the spermatozoon retains its proximal centriole while losing most of the PCM, whereas the oocyte degenerates centrioles while retaining centrosomal proteins. In most mammals including humans, the spermatozoon contributes the proximal centriole during fertilization. Biparental centrosome contributions to the zygote are typical for most species with some exceptions such as the mouse in which centrosomes are maternally inherited and centrioles are assembled de novo during the blastocyst stage. After nuclear transfer in reconstructed embryos, the donor cell centrosome complex is responsible for carrying out functions that are typically fulfilled by the sperm centrosome complex during normal fertilization, including spindle organization, cell cycle progression and development. In rodents, donor cell centrioles are degraded after nuclear transfer, and centrosomal proteins from both donor cell and recipient oocytes contribute to mitotic spindle assembly. However, questions remain about the faithful reprogramming of centrosomes in cloned mammals and its consequences for embryo development. The molecular dynamics of donor cell centrosomes in nuclear transfer eggs need further analysis. The fate and functions of centrosome components in nuclear transfer embryos are being investigated by using molecular imaging of centrosome proteins labeled with specific markers including, but not limited to, green fluorescent protein (GFP).

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PubMed2007

Nucleolar remodeling in nuclear transfer embryos.

Transcription of the ribosomal RNA (rRNA) genes occurs in the nucleolus and results in ribosome biogenesis. The rRNA gene activation and the associated nucleolus formation may be used as a marker for the activation of the embryonic genome in mammalian embryos and, thus serve to evaluate the developmental potential of embryos originating from varied nuclear transfer protocols. In bovine in vivo developed embryos, functional ribosome-synthesizing nucleoli become structurally distinct toward the end of the 4th post-fertilization cell cycle. In embryonic cell nuclear transfer embryos, fully developed nucleoli are not apparent until the 5th cell cycle, whereas in somatic cell nuclear transfer embryos the functional nucleoli emerge already during the 3rd cell cycle. Intergeneric reconstructed embryos produced by the fusion of bovine differentiated somatic cell to a nonactivated ovine cytoplast fail to develop fully functional nucleoli. In bovine in vivo developed embryos, a range of important nucleolar proteins (e.g., topoisomerase I, upstream binding factor and RNA polymerase I, fibrillarin, nucleophosmin and nucleolin) become localized to the nucleolar anlage over several cell cycles. This relocation is completed toward the end of the 4th cell cycle. A substantial proportion of bovine embryos produced by nuclear transfer of embryonic or somatic cells to bovine ooplasts display aberrations in protein localization in one or more blastomers. This information is indicative of underlying aberrations in genomic reprogramming and may help to explain the abnormalities observed in a proportion of fetuses and offspring derive from nuclear transfer embryos.

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