Desjeux I, Price D J
Physiology Department, Edinburgh University Medical School, Teviot Place, Edinburgh EH8 9AG, UK.
Dev Genes Evol. 1999 May;209(5):284-93. doi: 10.1007/s004270050254.
Different species of leech vary greatly in body size but all have 32 body segments. It is unclear how the development of this precise number of segments is regulated, although it is known that the teloblasts of the early leech embryo initially produce more than the required numbers of segment founder cells (blast cells). We used fluorescent dextrans to show that the M teloblast of the Helobdella robusta embryo produces a variable number of additional (supernumerary) cells. These cells fail to enter the germinal band (which contains cells of all lineages and gives rise to the adult leech), but detach from its posterior end and disappear. Our observations suggest that some suffer an increase in membrane permeability while others fuse with the M teloblasts, but that they do not undergo apoptosis. The supernumerary cells of different lineages detach from the germinal band at different times, suggesting that detachment is not triggered by a global signal acting simultaneously on all lineages. We tested the hypothesis that the elimination of the supernumerary m blast cells results from a requirement of m blast cells for close interactions with cells of the other lineages for their survival, a condition that would not be achieved by the last-born m blast cells that fail to enter the germinal band. We cultured isolated M teloblasts and found that they do produce blast cells that themselves divide, indicating that cells of the M lineage can survive in the absence of any interactions with cells of the other lineages.
不同种类的水蛭身体大小差异很大，但都有32个身体节段。目前尚不清楚这种精确节段数目的发育是如何调控的，尽管已知早期水蛭胚胎的端细胞最初产生的节段起始细胞（胚细胞）数量超过所需数量。我们使用荧光葡聚糖来显示强壮黄蛭胚胎的M端细胞会产生数量可变的额外（多余）细胞。这些细胞未能进入生发带（包含所有谱系的细胞并发育成成年水蛭），而是从其后端脱离并消失。我们的观察表明，一些细胞的膜通透性增加，而另一些细胞则与M端细胞融合，但它们不会发生凋亡。不同谱系的多余细胞在不同时间从生发带脱离，这表明脱离并非由同时作用于所有谱系的全局信号触发。我们测试了这样一个假设，即多余的m胚细胞的消除是由于m胚细胞需要与其他谱系的细胞密切相互作用才能存活，而最后生成的未能进入生发带的m胚细胞无法满足这一条件。我们培养了分离的M端细胞，发现它们确实会产生能自行分裂的胚细胞，这表明M谱系的细胞在没有与其他谱系的细胞发生任何相互作用的情况下也能存活。
