• Korean Journal of Plant Tissue Culture
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• v.27 no.6
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• pp.423-428
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• 2000

In 1997 when cloned sheep Dolly and soon after Polly were born, it had become head-line news because in the former the nucleus that gave rise to the lamb came from cells of six-year-old adult sheep and in the latter case a foreign gene was inserted into the donor nucleus to make the cloned sheep produce human protein, factor IX, in e milk. In the last few years, once the realm of science fiction, cloned mammals especially in livestock have become almost commonplace. What the press accounts often fail to convey, however, is that behind every success lie hundreds of failures. Many of the nuclear-transferred egg cells fail to undergo normal cell divisions. Even when an embryo does successfully implant in the womb, pregnancy often ends in miscarriage. A significant fraction of the animals that are born die shortly after birth and some of those that survived have serious developmental abnormalities. Efficiency remains at less than one % out of some hundred attempts to clone an animal. These facts show that something is fundamentally wrong and enormous hurdles must be overcome before cloning becomes practical. Cloning researchers now tent to put aside their effort to create live animals in order to probe the fundamental questions on cell biology including stem cells, the questions of whether the hereditary material in the nucleus of each cell remains intact throughout development, and how transferred nucleus is reprogrammed exactly like the zygotic nucleus. Stem cells are defined as those cells which can divide to produce a daughter cell like themselves (self-renewal) as well as a daughter cell that will give rise to specific differentiated cells (cell-differentiation). Multicellular organisms are formed from a single totipotent stem cell commonly called fertilized egg or zygote. As this cell and its progeny undergo cell divisions the potency of the stem cells in each tissue and organ become gradually restricted in the order of totipotent, pluripotent, and multipotent. The differentiation potential of multipotent stem cells in each tissue has been thought to be limited to cell lineages present in the organ from which they were derived. Recent studies, however, revealed that multipotent stem cells derived from adult tissues have much wider differentiation potential than was previously thought. These cells can differentiate into developmentally unrelated cell types, such as nerve stem cell into blood cells or muscle stem cell into brain cells. Neural stem cells isolated from the adult forebrain were recently shown to be capable of repopulating the hematopoietic system and produce blood cells in irradiated condition. In plants although the term$\boxDr$ stem cell$\boxUl$is not used, some cells in the second layer of tunica at the apical meristem of shoot, some nucellar cells surrounding the embryo sac, and initial cells of adventive buds are considered to be equivalent to the totipotent stem cells of mammals. The telomere ends of linear eukaryotic chromosomes cannot be replicated because the RNA primer at the end of a completed lagging strand cannot be replaced with DNA, causing 5' end gap. A chromosome would be shortened by the length of RNA primer with every cycle of DNA replication and cell division. Essential genes located near the ends of chromosomes would inevitably be deleted by end-shortening, thereby killing the descendants of the original cells. Telomeric DNA has an unusual sequence consisting of up to 1,000 or more tandem repeat of a simple sequence. For example, chromosome of mammal including human has the repeating telomeric sequence of TTAGGG and that of higher plant is TTTAGGG. This non-genic tandem repeat prevents the death of cell despite the continued shortening of chromosome length. In contrast with the somatic cells germ line cells have the mechanism to fill-up the 5' end gap of telomere, thus maintaining the original length of chromosome. Cem line cells exhibit active enzyme telomerase which functions to maintain the stable length of telomere. Some of the cloned animals are reported prematurely getting old. It has to be ascertained whether the multipotent stem cells in the tissues of adult mammals have the original telomeres or shortened telomeres.

• Journal of Chest Surgery
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• v.40 no.3 s.272
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• pp.180-192
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• 2007

Background: As determined from the recent investigations of discordant cardiac xenotransplantation, hyperacute rejection occurs mainly at the endothelial cells in donor microvascular systems, but this does not occur at cardiac valve leaflets or at medium-to-large caliber vessels. On the basis of this background, this study was performed to look into the biocompatibility for transplantation of a middle or large diameter xenogenic blood vessel by conducting xenogenic arterial transplantation with the carotid artery in a pig-to-goat model. Material and Method: The experimental group was composed of 10 pairs of pig-to-goat combinations. They were divided into each period of 1 week, and 1, 3, 6 and 12 months. Four carotid artery grafts obtained through collection of the bilateral carotid arteries from two pigs were preserved at $-70^{\circ}C$ without other treatment, and then they were transplanted into the bilateral carotid arteries of two goats. Doppler ultrasonography was done on a periodic basis after transplantation to evaluate the patency of the grafted blood vessel. At the ends of a predetermined period, the grafts were explanted from the goats and they underwent gross examination. Hematoxylin-eosin and Masson's trichrome staining were conducted. In addition, in order to examine the immunological rejection of the grafted xenogenic blood vessel, immunohistochemical staining was conducted with T-lymphocyte indicator and von Willebrand factor. Result: Two goats at the each one-week period and the one-year period died during the experimental period because of a reason unrelated to the experimental procedure, and the remaining 8 goats survived until the end of each experiment period. On Doppler ultrasonography, unilateral carotid artery occlusion was found in a goat, whose period was specified as 3 months, among the 8 survived goats. However, the vascular patency was maintained well and there was no graft that formed aneurysms in the other goats. On gross examination, the region of vascular anastomosis was preserved well, and calcification of the grafted blood vessel was not shown. Histologically, the endothelial cells of the graft disappeared one week after transplantation, and then there was progressive spread of the recipients' endothelial cells from the anastomotic site. The reendothelialization occurred over the whole graft at one month after transplantation. The neointimal thickening and adventitial inflammation became severe by 3 months after transplantation, but this lessened at 6 months and 12 months, respectively. The rate of CD3 positive cells was very low among the infiltrated inflammatory cells. Conclusion: The fresh-frozen xenogenic artery kept its patency without being greatly influenced by xenogenic immune reaction.