Telomere lengthening from oocyte to embryonic stem cell

Telomeres are repeated DNA regions that provide protection from enzymatic end-degradation and maintain chromosome stability during DNA replication. In most mammalian somatic cell types, telomeres shorten with each cell cycle. Telomerase, a reverse transcriptase that can elongate telomeres, adds telomeric repeats into chromosome ends, and is involved in maintaining telomere length in germ-line tissues, in adult stem cell, and in most immortal cancer cells. Telomere length is already determined at the moment of fertilization, and during preimplantation; and a detailed understanding of these telomeres elongation program could be important for studying embryo quality, and the origin of the embryonic stem cells; moreover, it has implications for the study of stem cell, regenerative medicine, ageing and cancer. Oocytes have shorter telomeres than somatic cells, but their telomeres lengthen extraordinarily during early cleavage development; however before blastocyst formation there are very low levels of expression of telomerase; even in telomerase-null mice, telomeres may elongate during preimplantation. Recent studies suggest that telomeres lengthen during early preimplantation use a recombination mechanism, and that from the blastocyst stage onwards, telomerase maintains the telomere length established by this recombinant mechanism. Telomere rebuilding subsequent to somatic cell nuclear transfer appears to vary according to species and type of donor cell used. It is speculated that the rate of telomere erosion and the incidence of chromosome abnormalities affect developmental potential of early embryos and may be potential predictors of developmental outcome. The purpose of this chapter is to review characteristic differences of telomere lengthening during the growing face of the oocyte and the spermatozoa, in early development of in vivo and in vitro (cloned, manipulated) produced embryos, in male and female embryos and in embryonic stem cells generated from these embr