Graduate Program in Zoology, School of Biological Sciences, The University of Texas at Austin, Austin, TX 78712, USA.
Dev Biol. 2012 Aug 1;368(1):86-94. doi: 10.1016/j.ydbio.2012.05.021. Epub 2012 May 26.
In the leech Helobdella, the ectoderm exhibits a high degree of morphological homonomy between body segments, but pattern elements in lateral ectoderm arise via distinct cell lineages in the segments of the rostral and midbody regions. In each of the four rostral segments, a complete set of ventrolateral (O fate) and dorsolateral (P fate) ectodermal pattern elements arises from a single founder cell, op. In the 28 midbody and caudal segments, however, there are two initially indeterminate o/p founder cells; the more dorsal of these is induced to adopt the P fate by BMP5-8 emanating from the dorsalmost ectoderm, while the more ventral cell assumes the O fate. Previous work has suggested that the dorsoventral patterning of O and P fates differs in the rostral region, but the role of BMP signaling in those segments has not been investigated. We show here that suppression of dorsal BMP5-8 signaling (which effects a P-to-O fate change in the midbody) has no effect on the patterning of O and P fates in the rostral region. Furthermore, ectopic expression of BMP5-8 in the ventral ectoderm (which induces an O-to-P fate change in the midbody) has no effect in the rostral region. Finally, expression of a dominant-negative BMP receptor (which induces a P-to-O fate change in the midbody) fails to affect O/P patterning in the rostral region. Thus, the rostral segments appear to use some mechanism other than BMP signaling to pattern O and P cell fates along the dorsoventral axis. From a mechanistic standpoint, the OP lineage of the rostral segments and the O-P equivalence group of the midbody and caudal segments constitute distinct developmental modules that rely to differing degrees on positional cues from surrounding ectoderm in order to specify homonomous cell fates.
在医蛭 Helobdella 中，外胚层在体节之间表现出高度的形态同形性，但侧外胚层的模式元素是通过头区和中体区各节段的不同细胞谱系产生的。在四个头节中的每一节中，一套完整的腹外侧（O 命运）和背外侧（P 命运）外胚层模式元素都来自单个创始细胞 op。然而，在 28 个体节和尾节中，有两个最初不确定的 o/p 创始细胞；这些细胞中更背侧的细胞通过来自最背侧外胚层的 BMP5-8 诱导而采用 P 命运，而更腹侧的细胞则采用 O 命运。先前的工作表明，头区 O 和 P 命运的背腹模式不同，但 BMP 信号在这些节段中的作用尚未得到研究。我们在这里表明，抑制背侧 BMP5-8 信号（在中体段产生 P 到 O 命运的改变）对头区 O 和 P 命运的模式没有影响。此外，在外胚层腹侧异位表达 BMP5-8（在中体段诱导 O 到 P 命运的改变）对头区没有影响。最后，表达显性负 BMP 受体（在中体段诱导 P 到 O 命运的改变）未能影响头区的 O/P 模式形成。因此，头节似乎使用了 BMP 信号以外的某种机制来沿背腹轴对 O 和 P 细胞命运进行模式化。从机制的角度来看，头节的 OP 谱系和中体节和尾节的 O-P 等价群构成了不同的发育模块，它们在不同程度上依赖于周围外胚层的位置线索来指定同源细胞命运。
