Freeman G
Kristineberg Marine Research Station, Fiskebackskil, Sweden.
Dev Biol. 2000 Nov 1;227(1):219-38. doi: 10.1006/dbio.2000.9857.
A fate map has been constructed for the embryo of Crania. The animal half of the egg forms the ectodermal epithelium of the larva's apical lobe. The vegetal half of the egg forms endoderm, mesoderm, and the ectoderm of the mantle lobe. The vegetal pole is the site of gastrulation; this site becomes the posterior ventral region of the mantle lobe of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg; it bears no relationship to the future plane of bilateral symmetry of the larva. The timing of regional specification was examined by isolating animal, vegetal, or meridional halves from oocytes, eggs, or embryos from prior to germinal vesicle breakdown through gastrulation. Animal halves isolated from oocytes formed either the epithelium of the apical lobe or a larva with all three germ layers. Animal halves isolated from unfertilized eggs and eight-cell embryos formed only apical lobe epithelium. Beginning at the blastula stage, animal halves formed mantle in addition to apical lobe epithelium. In animal halves isolated after gastrulation, the mantle lobe was always truncated. Vegetal halves isolated at all stages prior to gastrulation gastrulated and formed apical and mantle lobes with endoderm and mesoderm; however, the relative size of the apical lobe that formed decreased substantially when vegetal halves were isolated at later developmental stages. When meridional halves were isolated from unfertilized eggs and two- to four-cell embryos, both halves frequently formed normally proportioned larvae. Beginning at the blastula stage, a number of pairs frequently had a member that lacked dorsal setae on its mantle lobe while the other member of the pair formed setae, indicating that the dorsoventral axis had been set up. The process of regional specification in Crania is compared to those of Discinisca and Glottidia in the brachiopod subphylum Linguliformea and Phoronis in the phylum Phoronida.
已构建了头盘虫胚胎的命运图谱。卵的动物极一半形成幼虫顶叶的外胚层上皮。卵的植物极一半形成内胚层、中胚层和套膜叶的外胚层。植物极是原肠胚形成的部位；该部位成为幼虫套膜叶的后腹侧区域。第一次卵裂平面穿过卵的动物 - 植物轴；它与幼虫未来的两侧对称平面无关。通过从卵母细胞、卵或从生发泡破裂前到原肠胚形成阶段的胚胎中分离动物极、植物极或子午面一半来研究区域特化的时间。从卵母细胞分离的动物极一半形成顶叶上皮或具有所有三个胚层的幼虫。从未受精卵和八细胞胚胎分离的动物极一半仅形成顶叶上皮。从囊胚期开始，动物极一半除了形成顶叶上皮外还形成套膜。在原肠胚形成后分离的动物极一半中，套膜叶总是截断的。在原肠胚形成前的所有阶段分离的植物极一半进行原肠胚形成并形成带有内胚层和中胚层的顶叶和套膜叶；然而，当在较晚发育阶段分离植物极一半时，形成的顶叶的相对大小显著减小。当从未受精卵和二细胞至四细胞胚胎分离子午面一半时，两半通常都形成比例正常的幼虫。从囊胚期开始，许多对中经常有一对中的一个成员在其套膜叶上没有背刚毛，而该对中的另一个成员形成刚毛，这表明背腹轴已经建立。将头盘虫中的区域特化过程与舌形贝亚门中的盘形贝和舌形贝以及帚虫动物门中的帚虫的区域特化过程进行了比较。
