Kinoshita K, Asashima M
Laboratory of Biology, Nippon Medical School, Kawasaki, Japan.
Development. 1995 Jun;121(6):1581-9. doi: 10.1242/dev.121.6.1581.
Dorsoventral mesoderm patterning in the amphibian embryo involves a series of interactions mediated by several peptide growth factors. Animal blastomeres isolated at the 8-cell stage are useful for studying mesoderm patterning, since they contain the prospective (uninduced) mesoderm region and allow examination of the default state of animal cells. When activin is applied to these dorsal and ventral animal half explants, a competence prepattern for responding to activin is observed. In order to investigate the characteristics of prepatterning, we treated animal blastomeres with the embryo dorsalizing agent LiCl. Treatment with lithium alone did not induce normal trunk mesoderm in either blastomere. Lithium did, however, alter the competence of animal blastomeres to activin. Dorsal mesoderm was formed in the ventral blastomeres, as well as in the dorsal blastomeres. This result reveals that the early dorsoventral polarity in the animal hemisphere is not fixed. Using goosecoid(gsc) and Xwnt-8 genes as dorsal and ventral mesoderm markers, it was verified that lithium modifies the competence to activin. Unexpectedly, lithium treatment on its own resulted in gsc expression in the animal half explants. This suggests that embryo goosecoid expression may be induced by the effect of dorsal determination activity, but not by mesoderm induction. However, lithium induced also the expression of brachyury (Xbra) gene at very low levels. This would indicate the formation of dorsal-anterior mesoderm, which was not identified by the tissue observations. Expression of Xwnt-8, a ventral mesoderm marker usually induced in blastula animal caps by activin, was hardly induced in the blastomere explants. We isolated whole animal half explants at the 8-cell stage and exposed to activin at different stages. It was found that the same concentration of activin induces gsc before the midblastula stage, and induces Xwnt-8 at later stages. This suggests that the response of animal blastomeres alters depending on the stage of activin signaling.
两栖类胚胎的背腹侧中胚层模式形成涉及一系列由多种肽类生长因子介导的相互作用。在8细胞期分离的动物卵裂球对于研究中胚层模式形成很有用，因为它们包含预期的（未诱导的）中胚层区域，并且可以检查动物细胞的默认状态。当将激活素应用于这些背侧和腹侧动物半胚外植体时，观察到了对激活素作出反应的感受态预模式。为了研究预模式的特征，我们用胚胎背化剂LiCl处理动物卵裂球。单独用锂处理在任何一个卵裂球中都不会诱导出正常的躯干中胚层。然而，锂确实改变了动物卵裂球对激活素的感受态。在腹侧卵裂球以及背侧卵裂球中都形成了背侧中胚层。这一结果表明动物半球中的早期背腹极性并非固定不变的。使用鹅膏蕈氨酸（gsc）和Xwnt - 8基因作为背侧和腹侧中胚层标记，证实了锂改变了对激活素的感受态。出乎意料的是，单独的锂处理导致动物半胚外植体中gsc的表达。这表明胚胎鹅膏蕈氨酸的表达可能是由背侧决定活性的作用诱导的，而不是由中胚层诱导的。然而，锂也以非常低的水平诱导了短尾（Xbra）基因的表达。这将表明形成了背侧 - 前侧中胚层，这在组织观察中未被识别。激活素通常在囊胚动物帽中诱导表达的腹侧中胚层标记Xwnt - 8，在卵裂球外植体中几乎不被诱导。我们在8细胞期分离整个动物半胚外植体，并在不同阶段暴露于激活素。发现相同浓度的激活素在囊胚中期之前诱导gsc，而在后期诱导Xwnt - 8。这表明动物卵裂球的反应根据激活素信号传导的阶段而改变。
