Martindale M Q, Henry J Q
University of Chicago, Department of Organismal Biology and Anatomy, IL 60637, USA.
Development. 1995 Oct;121(10):3175-85. doi: 10.1242/dev.121.10.3175.
The nemerteans belong to a phylum of coelomate worms that display a highly conserved pattern of cell divisions referred to as spiral cleavage. It has recently been shown that the fates of the four embryonic cell quadrants in two species of nemerteans are not homologous to those in other spiralian embryos, such as the annelids and molluscs (Henry, J. Q. and Martindale, M. Q. (1994a) Develop. Genetics 15, 64-78). Equal-cleaving molluscs utilize inductive interactions to establish quadrant-specific cell fates and embryonic symmetry properties following fifth cleavage. In order to elucidate the manner in which cell fates are established in nemertean embryos, we have conducted cell isolation and deletion experiments to examine the developmental potential of the early cleavage blastomeres of two equal-cleaving nemerteans, Nemertopsis bivittata and Cerebratulus lacteus. These two species display different modes of development: N. bivittata develops directly via a non-feeding larvae, while C. lacteus develops to form a feeding pilidium larva which undergoes a radical metamorphosis to give rise to the juvenile worm. By examining the development of certain structures and cell types characteristic of quadrant-specific fates for each of these species, we have shown that isolated blastomeres of the indirect-developing nemertean, C. lacteus, are capable of generating cell fates that are not a consequence of that cell's normal developmental program. For instance, dorsal blastomeres can form muscle fibers when cultured in isolation. In contrast, isolated blastomeres of the direct-developing species, N. bivittata do not regulate their development to the same extent. Some cell fates are specified in a precocious manner in this species, such as those that give rise to the eyes. Thus, these findings indicate that equal-cleaving spiralian embryos can utilize different mechanisms of cell fate and axis specification. The implications of these patterns of nemertean development are discussed in relation to experimental work in other spiralian embryos, and a model is presented that accounts for possible evolutionary changes in cell lineage and the process of cell fate specification amongst these protostome phyla.
纽形动物属于真体腔蠕虫门，其细胞分裂模式高度保守，称为螺旋卵裂。最近研究表明，两种纽形动物胚胎的四个细胞象限的命运与其他螺旋卵裂胚胎（如环节动物和软体动物）的不同（亨利，J.Q.和马丁代尔，M.Q.（1994a）《发育遗传学》15，64 - 78）。均等分裂的软体动物利用诱导相互作用在第五次卵裂后建立象限特异性细胞命运和胚胎对称特性。为了阐明纽形动物胚胎中细胞命运是如何确立的，我们进行了细胞分离和缺失实验，以研究两种均等分裂的纽形动物——双线纽虫（Nemertopsis bivittata）和乳白角吻纽虫（Cerebratulus lacteus）早期卵裂球的发育潜能。这两个物种表现出不同的发育模式：双线纽虫通过非摄食幼虫直接发育，而乳白角吻纽虫发育形成摄食的帽状幼虫，该幼虫经历剧烈变态发育成幼体蠕虫。通过研究这两个物种中某些具有象限特异性命运特征的结构和细胞类型的发育，我们发现间接发育的纽形动物乳白角吻纽虫的分离卵裂球能够产生并非该细胞正常发育程序结果的细胞命运。例如，背侧卵裂球在单独培养时可形成肌肉纤维。相比之下，直接发育物种双线纽虫的分离卵裂球在相同程度上不会调节其发育。在该物种中，一些细胞命运以早熟的方式被指定，比如那些发育成眼睛的细胞命运。因此，这些发现表明均等分裂的螺旋卵裂胚胎可以利用不同的细胞命运和轴特化机制。结合其他螺旋卵裂胚胎的实验工作讨论了纽形动物发育模式的意义，并提出了一个模型，该模型解释了这些原口动物门之间细胞谱系和细胞命运特化过程中可能的进化变化。
