Sproull F, David C N
J Cell Sci. 1979 Aug;38:155-69. doi: 10.1242/jcs.38.1.155.
Interstitial stem cells in Hydra are rapidly proliferating multipotent stem cells which continuously give rise to precursors for nerve and nematocyte differentiation. Growth of the stem cell population is controlled by the cell cycle time of the stem cells and the self-renewal probability, Ps (the fraction of stem cells in each generation which divide to yield more stem cells). In normal Hydra the stem cell generation time is 24 h and Ps = 0.6; under these conditions the stem cell population doubles in 3.5 days. In the present experiments we have systematically investigated the dependence of Ps on stem cell density. We culture stem cells in a feeder layer system consisting of aggregates of nitrogen-mustard (NM)-inactivated Hydra cells. In this system stem cell density can be varied over a wide range by changing the number of clone-forming units (CFU) added to the aggregates. We have measured the growth rate of the stem cell population and the cell cycle of stem cells in NM aggregates after 4--7 days of culture. From these data we calculate the value of Ps. The results indicate that the growth rate decreases 4-fold as the number of CFU seeded per aggregate increases from 10 to 400. Under these same conditions the cell cycle remains constant. The values of Ps calculated from these results indicate the Ps decreases from 0.75 in aggregates seeded with 10--30 CFU to 0.55 in aggregates seeded with 200--400 CFU. These results support a model in which Ps is controlled by negative feedback from neighbouring stem cells. In addition, our experiments indicate that Ps decreases during the growth of stem cell clones. When only a few stem cells are seeded in aggregates, they give rise to isolated clones distributed throughout the aggregate. Ps decreases markedly within such clones as they grow in size presumably due to increasing stem cell content of the clones. Since Ps in such isolated clones declines with growth, we infer that the local stem cell concentration is what controls Ps and that the spatial range of the negative feedback signal is short compared to the dimensions of NM aggregates.
水螅中的间质干细胞是快速增殖的多能干细胞，它们持续产生神经细胞和刺细胞分化的前体细胞。干细胞群体的生长受干细胞的细胞周期时间和自我更新概率(Ps)（每一代中分裂产生更多干细胞的干细胞比例）控制。在正常水螅中，干细胞的世代时间为24小时，(Ps = 0.6)；在这些条件下，干细胞群体在3.5天内翻倍。在本实验中，我们系统地研究了(Ps)对干细胞密度的依赖性。我们在由氮芥（NM）灭活的水螅细胞聚集体组成的饲养层系统中培养干细胞。在这个系统中，通过改变添加到聚集体中的克隆形成单位（CFU）数量，可以在很宽的范围内改变干细胞密度。我们测量了培养4 - 7天后NM聚集体中干细胞群体的生长速率和干细胞的细胞周期。根据这些数据我们计算出(Ps)的值。结果表明，随着每个聚集体接种的CFU数量从10增加到400，生长速率降低了4倍。在相同条件下，细胞周期保持不变。根据这些结果计算出的(Ps)值表明，(Ps)从接种10 - 30个CFU的聚集体中的0.75降至接种2００ - 400个CFU的聚集体中的0.55。这些结果支持了一个模型，即(Ps)受相邻干细胞的负反馈控制。此外，我们的实验表明，在干细胞克隆生长过程中(Ps)会降低。当仅在聚集体中接种少数干细胞时，它们会产生分布在整个聚集体中的孤立克隆。随着这些克隆体积的增大，(Ps)在其中会显著降低，这可能是由于克隆中干细胞含量增加所致。由于此类孤立克隆中的(Ps)随生长而下降，我们推断局部干细胞浓度控制着(Ps)，并且与NM聚集体的尺寸相比，负反馈信号的空间范围较短。
