Henry Jonathan
Marine Biological Laboratory, Woods Hole, MA 02543, USA.
Dev Biol. 2002 Aug 15;248(2):343-55. doi: 10.1006/dbio.2002.0741.
Many members of the spiralian phyla (i.e., annelids, echiurans, vestimentiferans, molluscs, sipunculids, nemerteans, polyclad turbellarians, gnathostomulids, mesozoans) exhibit early, equal cleavage divisions. In the case of the equal-cleaving molluscs, animal-vegetal inductive interactions between the derivatives of the first quartet micromeres and the vegetal macromeres specify which macromere becomes the 3D cell during the interval between fifth and sixth cleavage. The 3D macromere serves as a dorsal organizer and gives rise to the 4d mesentoblast. Even though it has been argued that this situation represents the ancestral condition among the Spiralia, these inductive events have only been documented in equal-cleaving molluscs. Embryos of the nemertean Cerebratulus lacteus also undergo equal, spiral cleavage, and the fate map of these embryos is similar to that of other spiralians. The role of animal first quartet micromeres in the establishment of the dorsal (D) cell quadrant was examined in C. lacteus by removing specific combinations of micromeres at the eight-cell stage. To follow the development of various cell quadrants, one quadrant was labeled with DiI at the four-cell stage, and specific first quartet micromeres were removed from discrete positions relative to the location of the labeled quadrant. The results indicate that the first quartet is required for normal development, as removal of all four micromeres prevented dorsoventral axis formation. In most cases, when either one or two adjacent first quartet micromeres were removed from one side of the embryo, the cell quadrant on the opposite side, with its macromere centered under the greatest number of the remaining animal micromeres, ultimately became the D quadrant. Twins containing duplicated dorsoventral axes were generated by removal of two opposing first quartet micromeres. Thus, any cell quadrant can become the D quadrant, and the dorsoventral axis is established after the eight-cell stage. While it is not yet clear exactly when key inductive interactions take place that establish the D quadrant in C. lacteus, contacts between the progeny of animal micromeres and vegetal macromeres are established during the interval between the fifth and sixth round of cleavage divisions (i.e., 32- to 64-cell stages). These findings argue that this mechanism of cell and axis determination has been conserved among equal-cleaving spiralians.
许多螺旋动物门的成员（即环节动物、螠虫、须腕动物、软体动物、星虫、纽形动物、多肠目涡虫、颚口动物、中生动物）表现出早期的均等卵裂。对于进行均等卵裂的软体动物而言，第一四分体小卵裂球的衍生物与植物性大卵裂球之间的动物性 - 植物性诱导相互作用决定了在第五次和第六次卵裂之间的间隔期哪个大卵裂球成为3D细胞。3D大卵裂球作为背侧组织者，并产生4d中胚层母细胞。尽管有人认为这种情况代表了螺旋动物门的祖先状态，但这些诱导事件仅在进行均等卵裂的软体动物中有记录。纽形动物乳色脑纹涡虫（Cerebratulus lacteus）的胚胎也进行均等的螺旋卵裂，并且这些胚胎的命运图谱与其他螺旋动物相似。通过在八细胞期去除特定组合的小卵裂球，研究了乳色脑纹涡虫中动物性第一四分体小卵裂球在背侧（D）细胞象限建立中的作用。为了追踪各个细胞象限的发育，在四细胞期用碘化二苯甲酰（DiI）标记一个象限，并从相对于标记象限位置的离散位置去除特定的第一四分体小卵裂球。结果表明，第一四分体对于正常发育是必需的，因为去除所有四个小卵裂球会阻止背腹轴的形成。在大多数情况下，当从胚胎的一侧去除一个或两个相邻的第一四分体小卵裂球时，相对一侧的细胞象限，其大卵裂球以剩余最多的动物性小卵裂球为中心，最终成为D象限。通过去除两个相对的第一四分体小卵裂球产生了具有重复背腹轴的双胞胎。因此，任何细胞象限都可以成为D象限，并且背腹轴在八细胞期之后建立。虽然目前尚不清楚在乳色脑纹涡虫中建立D象限的确切关键诱导相互作用发生的时间，但动物性小卵裂球的后代与植物性大卵裂球之间的接触在第五轮和第六轮卵裂之间的间隔期（即32 - 64细胞期）建立。这些发现表明，这种细胞和轴的确定机制在进行均等卵裂的螺旋动物中是保守的。
