Oocytes are unique cells with the inherent capability to reprogram nuclei.The reprogramming of the somatic nucleus from its original cellular state to a totipotent state is essential for term development after somatic cell nuclear transfer.The nuclear-associated factors contained within oocytes are critical for normal fertilization by sperm or for somatic cell nuclear reprogramming.The chromatin of somatic nuclei can be reprogrammed by factors in the egg cytoplasm whose natural function is to reprogram sperm chromatin.The oocyte first obtains its reprogramming capability in the early fetal follicle,and then its capacity is enriched in the late growth phase and reaches its highest capability for reprogramming as fully-grown germinal vesicle oocytes.The cytoplasmic milieu most likely contains all of the specific transcription and/or reprogramming factors necessary for cellular reprogramming.Certain transcription factors in the cytoplast may be critical as has been demonstrated for induced pluripotent stem cells.The maternal pronucleus exerts a predominant,transcriptiondependent effect on embryo cytofragmentation,with a lesser effect imposed by the ooplasm and the paternal pronucleus.With deep analysis of transcriptomics in oocytes and early developmental stage embryos more maternal transcription factors inducing cellular reprogramming will be identified.
The Tibetan antelope is endemic to the Tibetan Plateau,China,and is now considered an endangered species.As a possible rescue strategy,the development of embryos constructed by interspecies somatic cell nuclear transfer(iSCNT)was examined.Tibetan antelope fibroblast cells were transferred into enucleated bovine,ovine and caprine oocytes.These cloned embryos were then cultured in vitro or in the oviducts of intermediate animals.Less than 0.5%of the reconstructed antelope-bovine embryos cultured in vitro developed to the blastocyst stage.However,when the cloned antelope-bovine embryos were transferred to caprine oviducts,about 1.6%of the embryos developed to the blastocyst stage.In contrast,only 0.7%of the antelope-ovine embryos developed to the morula stage and none developed to blastocysts in ovine oviducts.The treatment of donor cells and bovine oocytes with trichostatin A did not improve the embryo development even when cultured in the oviducts of ovine and caprine.When the antelope-bovine embryos,constructed from oocytes treated with roscovitine or trichostatin A,were cultured in rabbit oviducts 2.3%and 14.3%developed to blastocysts,respectively.It is concluded that although some success was achieved with the protocols used,interspecies cloning of Tibetan antelope presents difficulties still to be overcome.The mechanisms resulting in the low embryo development need investigation and progress might require a deeper understanding of cellular reprogramming.
