Plickert Günter, Frank Uri, Müller Werner A
Biocenter Cologne, University of Cologne, Germany.
Int J Dev Biol. 2012;56(6-8):519-34. doi: 10.1387/ijdb.123502gp.
Hydractinia, a representative marine colonial hydroid, was the first organism in the history of biology in which migratory precursors of germ cells were described and termed "stem cells" (Weismann, 1883). These stem cells, now known as interstitial cells (i-cells), are thought to remain pluripotent throughout their life. Using animals depleted of their own stem cells and repopulated with allogeneic mutant donor stem cells, it was shown that Hydractinia i-cells differentiate into any cell type including epithelial cells and germ cells that express germ line markers such as Vasa, Piwi and Nanos. In Hydra, i-cells also provide germ cells and somatic cells with the exception of epithelial cells. The latter derive from two subpopulations of differentiated epithelial cells with self-renewal capacity. In Hydractinia, forced expression of the Oct4-like transcription factor, Polynem (Pln), in epithelial cells transforms them into stem cells that develop neoplasms. I-cells express the Wnt-receptor Frizzled and are Wnt responsive. Activation of Wnt signaling induces the production of numerous nematocytes (stinging cells) and nerve cells. In parallel, supernumerary tentacles develop. I-cells also express Myc and Nanos. Their misexpression causes severe developmental defects. Hydractinia polyp buds arise from aggregating stem cells, in contrast to Hydra buds, which derive from evaginating epithelial cells. Wnt activation increases budding frequency and the emergence of ectopic head structures. The potential of stem cells to invade neighbors may have provided selection pressure for the evolution of allorecognition and histo-incompatibility. Hence, Hydractinia have now attained the position of a powerful model in stem cell research, axis formation and allorecognition.
水螅虫是一种典型的海洋群体水螅，是生物学史上第一个描述生殖细胞迁移前体并将其称为“干细胞”的生物（魏斯曼，1883年）。这些干细胞，现在被称为间质细胞（i细胞），被认为在其一生中都保持多能性。通过使用耗尽自身干细胞并用同种异体突变供体干细胞重新填充的动物，研究表明水螅虫的i细胞可以分化为任何细胞类型，包括上皮细胞和表达生殖系标记如Vasa、Piwi和Nanos的生殖细胞。在水螅中，i细胞也提供生殖细胞和除上皮细胞外的体细胞。后者来源于具有自我更新能力的两个分化上皮细胞亚群。在水螅虫中，上皮细胞中强迫表达类似Oct4的转录因子Polynem（Pln）会将它们转化为发展成肿瘤的干细胞。i细胞表达Wnt受体卷曲蛋白并且对Wnt有反应。Wnt信号的激活诱导大量刺细胞（刺丝囊细胞）和神经细胞的产生。同时，会发育出多余的触手。i细胞也表达Myc和Nanos。它们的错误表达会导致严重的发育缺陷。与源自外翻上皮细胞的水螅芽不同，水螅虫的息肉芽来自聚集的干细胞。Wnt激活会增加出芽频率和异位头部结构的出现。干细胞侵入邻居的潜力可能为同种异体识别和组织不相容性的进化提供了选择压力。因此，水螅虫现在已在干细胞研究、轴形成和同种异体识别方面成为一个强大的模型。
