Boring L
Friday Harbor Laboratories, University of Washington, Seattle 98195.
Dev Biol. 1989 Nov;136(1):239-53. doi: 10.1016/0012-1606(89)90145-0.
Dorsoventral polarity in molluscan embryos can arise by two distinct mechanisms, where the mechanism employed is strongly correlated with the cleavage pattern of the early embryo. In species with unequal cleavage, the dorsal lineage, or "D quadrant", is determined in a cell-autonomous manner by the inheritance of cytoplasmic determinants. However, in gastropod molluscs with equal cleavage, cell-cell interactions are required to specify the fate of the dorsal blastomere. During the fifth cleavage interval in equally cleaving embryos, one of the vegetal macromeres makes exclusive contacts with the animal micromeres, and this macromere will give rise to the mesodermal precursor cell at the next division, thereby identifying the dorsal quadrant. This study examines D-quadrant determination in an equally cleaving species from a group of previously uninvestigated gastropods, the subclass Opisthobranchia. Blastomere ablation experiments were performed on embryos of Haminoea callidegenita to (i) determine the developmental potential of macromeres before and after fifth cleavage, and (ii) examine the role of micromere-macromere interactions in the establishment of bilateral symmetry. The results suggest that the macromeres are developmentally equivalent prior to fifth cleavage, but become nonequivalent soon afterward. The dorsoventral axis corresponds to the displacement of the micromeres over one macromere early in the fifth cleavage interval. This unusual cellular topology is hypothesized to result from constraints imposed on micromere-macromere interactions in an embryo that develops from a large egg and forms a stereoblastula (no cleavage cavity). Ablation of the entire first quarter of micromeres results in embryos which remain radially symmetrical in the vegetal hemisphere, indicating that micromere-macromere interactions are required for the elaboration of bilateral symmetry properties. Therefore, inductive interactions between cells may represent a general strategy for dorsoventral axis determination in equally cleaving gastropods.
软体动物胚胎的背腹极性可通过两种不同机制产生，所采用的机制与早期胚胎的卵裂模式密切相关。在卵裂不均等的物种中，背侧谱系或“D象限”通过细胞质决定因子的遗传以细胞自主方式确定。然而，在卵裂均等的腹足纲软体动物中，需要细胞间相互作用来确定背侧卵裂球的命运。在均等卵裂胚胎的第五次卵裂间期，一个植物性大卵裂球与动物性小卵裂球进行排他性接触，这个大卵裂球将在下一次分裂时产生中胚层前体细胞，从而确定背侧象限。本研究考察了一组此前未被研究过的腹足纲动物（后鳃亚纲）中一个均等卵裂物种的D象限确定过程。对美丽织纹螺的胚胎进行了卵裂球消融实验，以（i）确定第五次卵裂前后大卵裂球的发育潜能，以及（ii）研究小卵裂球 - 大卵裂球相互作用在建立双侧对称中的作用。结果表明，大卵裂球在第五次卵裂前发育上是等同的，但之后很快变得不等同。背腹轴对应于在第五次卵裂间期早期小卵裂球在一个大卵裂球上的移位。据推测，这种不寻常的细胞拓扑结构是由于在一个由大卵发育而来并形成实心囊胚（无分裂腔）的胚胎中，小卵裂球 - 大卵裂球相互作用受到限制所致。消融整个四分之一的小卵裂球会导致胚胎在植物性半球保持径向对称，这表明小卵裂球 - 大卵裂球相互作用对于细化双侧对称特性是必需的。因此，细胞间的诱导相互作用可能代表了均等卵裂腹足纲动物中确定背腹轴的一种普遍策略。
