Lansdorp P M
Terry Fox Laboratory, British Colombia Cancer Agency, Vancouver, Canada.
Vox Sang. 1998;74 Suppl 2:91-4. doi: 10.1111/j.1423-0410.1998.tb05402.x.
The limited life-span of most blood cells requires continuous production of cells which in adults may exceed 1012 cells/day. This impressive production of cells (approximately 4.1015 cells over a life time) is achieved by the proliferation and differentiation of committed progenitor cells which themselves are derived from a population of pluripotent stem cells with self-renewal potential. In adults, the large majority of stem cells are found in the bone marrow among cells with a CD34 + CD38- phenotype. Interestingly, small but significant numbers of such cells can be found in the circulation. The frequency of circulating CD34 + CD38- cells can be dramatically increased by treatment with certain compounds including cytokines. Such "mobilized" peripheral blood stem cells have become an important alternative to bone marrow in stem cell transplantation procedures primarily because engraftment is more rapid. The latter is almost certainly related to the increased numbers of primitive CD34 + CD38- cells capable of engrafting the bone marrow in blood versus bone marrow stem cell grafts [1]. Paradoxically, the large majority of "candidate" stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 52 divisions can yield 4.1015 cells. First, it was shown that "candidate" stem cells purified from fetal and adult tissue display marked functional differences in turn-over time and the ability to produce cells with stem cell properties [2]. Secondly, these functional differences were found to correlate with a measurable loss of telomere repeats [3], despite the presence of low but readily detectable levels of telomerase in all purified cell fractions [4,5]. In order to address questions about the role of telomeres in normal and malignant hematopoiesis, we developed quantitative fluorescence in situ hybridization [6]. With this technique the length of telomere repeats at individual chromosome ends can be reliably estimated using optical density measurements from digital images of metaphase chromosomes after fluorescence in situ hybridization with directly labeled (CCCTAA)3--Peptide Nucleic Acid Probe [6,7]. Furthermore, we recently showed that this method can be adapted to measure the total telomere repeat content of cells by flow cytometry [8]. Here some issues in studies of hematopoietic stem cells are discussed in relation to rapidly accumulating information about telomere biology.
大多数血细胞的寿命有限，这就需要持续产生细胞，在成年人中，每天产生的细胞数量可能超过10¹²个。如此惊人的细胞产量（一生中约4×10¹⁵个细胞）是通过定向祖细胞的增殖和分化实现的，而这些祖细胞本身来源于具有自我更新潜力的多能干细胞群体。在成年人中，绝大多数干细胞存在于骨髓中，其表型为CD34⁺CD38⁻。有趣的是，在循环血液中也能发现少量但数量可观的这类细胞。通过使用某些化合物（包括细胞因子）进行治疗，循环中CD34⁺CD38⁻细胞的频率可显著增加。这类“动员的”外周血干细胞已成为干细胞移植手术中骨髓的重要替代物，主要是因为其植入速度更快。后者几乎可以肯定与血液中能够植入骨髓的原始CD34⁺CD38⁻细胞数量增加有关，而不是骨髓干细胞移植 [1]。矛盾的是，成年骨髓中绝大多数“候选”干细胞是静止细胞。一种可能性是，干细胞与其他体细胞一样，其复制潜力有限（<100次分裂）。这一假设得到了两个关键观察结果的支持，并且从理论上讲，52次分裂可以产生4×10¹⁵个细胞。首先，研究表明，从胎儿和成人组织中纯化的“候选”干细胞在周转时间和产生具有干细胞特性细胞的能力方面表现出明显的功能差异 [2]。其次，尽管在所有纯化细胞组分中都存在低但易于检测到的端粒酶水平 [4,5]，但发现这些功能差异与端粒重复序列的可测量损失相关 [3]。为了解决端粒在正常和恶性造血中的作用问题，我们开发了定量荧光原位杂交技术 [6]。使用这种技术，在与直接标记的(CCCTAA)₃ - 肽核酸探针进行荧光原位杂交后，通过对中期染色体数字图像的光密度测量，可以可靠地估计单个染色体末端端粒重复序列的长度 [6,7]。此外，我们最近表明，这种方法可以通过流式细胞术来测量细胞的总端粒重复序列含量 [8]。在此，结合关于端粒生物学的快速积累的信息，讨论造血干细胞研究中的一些问题。
