Mechanisms for Coelom formation : Binary dicision of the lateral plate mesoderm

包括脊椎动物在内的大多数三胚层动物都有一个体腔，将身体的内外部分分开。体腔由两种不同的组织组成，躯体胸膜和植骨胸膜，它们来自侧板区域。因此，体腔是在侧板的早期规范期间二元决策的结果。在本报告中，我们研究了这种二元决策的分子机制。我们首先证明了侧板分裂成两个细胞片是按照前后顺序进行的，这个过程与体分裂的分裂不协调。通过一系列的胚胎学操作，我们发现原始侧板下的外胚层也是胸膜中胚层所必需的。屏障实验表明，诱导活性与侧板内胚层分泌因子有关。我们还表明，诱导活性是由位于外侧内胚层的SHH分子介导的。体腔形成的进化方面也被考虑。

Most triploblastic animals including vertebrates have a coelomic cavity that separates the outer and inner components of the body. The coelom is lined by two different tissue components, somatopleure and splanchnopleure, which are derived from the lateral plate region. Thus, the coelom is constructed as a result of a binary decision during early specification of the lateral plate. In this report we studied the molecular mechanisms of this binary decision. We first demonstrate that the splitting of the lateral plate into the two cell sheets progresses in an anteroposterior order and this progression is not coordinated with that of the somitic segmentation. By a series of embryological manipulations we found that the ectoderm underneath the primitive lateral plate is also required for the Splanchnopleural mesoderm. From the barrier experiment, it was suggested that this inductive activity is depend on the secreting factor(s) from lateral plate endoderm. We also show that the inductive activity is mediated by SHH molecules that are localized in the lateral endoderm. Evolutionary aspects of the coelom formation are also considered.