The Mechanism of the 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，在侧板的早期规范中，通过二元决策来构造体腔。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的过程是按照前后顺序进行的，而且这个过程与躯体分裂的过程不协调。通过一系列的胚胎学操作，我们发现年轻的内脏中胚层仍然能够被重新指定为体细胞中胚层，而覆盖在侧板上的外胚层足以实现这种重定向。维持中胚层的体细胞特性也需要外胚层。因此，外胚层在侧板的早期细分中至少起着两个作用：规范和维持体细胞中胚层。我们还发现，后一种相互作用是由BMP分子介导的，这些分子位于外侧外胚层。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 by the 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 antero posterior order and this progression is not coordinated with that of the somitic segmentation. By a series of embryological manipulations we found that a young splanchnic mesoderm is still competent to be respecified to the somatic mesoderm, and the ectoderm overlying the lateral plate is sufficient for this redirection. The lateral ectoderm is also required for maintenance of the somatic character of the mesoderm. Thus, the ectoderm plays at least two roles in the early subdivision of the lateral plate : specification and maintenance of the somatic mesoderm. We also show that the latter interactions are mediated by BMP molecules that are localized in the lateral ectoderm. Evolutionary aspects of the coelom formation are also considered.