Henry J J, Raff R A
Department of Biology, Indiana University, Bloomington 47405.
Dev Biol. 1990 Sep;141(1):55-69. doi: 10.1016/0012-1606(90)90101-n.
Embryos of the indirect developing sea urchin, Heliocidaris tuberculata, and of Heliocidaris erythrogramma which develops directly without the formation of a pluteus larva, were bisected at the two- and four-cell stages. Paired half-embryos resulting from the bisection of H. tuberculata embryos along either the first or the second cleavage plane develop identically into miniature prism stage larvae. As in other indirect developing sea urchins, no differential segregation of developmental potential takes place as a result of the first and second cleavage divisions. Although half-embryos resulting from bisection along the second cleavage plane differentiate all cell types and develop equivalently in H. erythrogramma, the isolated first cleavage blastomeres do not. One of these two cells always forms significantly more mesodermal and endodermal cells. These patterns of differentiation are consistent with fate-mapping studies indicating that most mesodermal and endodermal cells are derived from the prospective ventral blastomere. Therefore, a differential segregation of developmental potential takes place at the first cleavage division in H. erythrogramma. When embryos of H. erythrogramma were bisected during the eight-cell stage, isolated tiers of animal blastomeres typically formed only ectodermal structures including the vestibule, whereas vegetal embryo halves formed all differentiated cell types. We propose that animal-vegetal cell determination and differentiation takes place along an axis which has been shifted relative to the pattern of cell cleavages in the embryos of H. erythrogramma. Vegetal morphogenetic potential for the formation of mesodermal and endodermal structures has become more closely associated with the prospective ventral side of the embryo during the evolution of direct development in Heliocidaris.
间接发育的海胆瘤海胆（Heliocidaris tuberculata）以及直接发育且不形成长腕幼虫的红纹海胆（Heliocidaris erythrogramma）的胚胎，在二细胞期和四细胞期被一分为二。瘤海胆胚胎沿第一或第二卵裂平面切开后产生的成对半胚胎，均发育成微型棱柱期幼虫。与其他间接发育的海胆一样，第一次和第二次卵裂不会导致发育潜能的差异分离。虽然沿第二卵裂平面切开产生的半胚胎能分化出所有细胞类型，并在红纹海胆中发育程度相当，但分离的第一次卵裂的卵裂球却并非如此。这两个细胞中的一个总是形成明显更多的中胚层和内胚层细胞。这些分化模式与命运图谱研究一致，表明大多数中胚层和内胚层细胞源自预期的腹侧卵裂球。因此，在红纹海胆的第一次卵裂时发生了发育潜能的差异分离。当红纹海胆的胚胎在八细胞期被一分为二时，分离的动物极卵裂球层通常仅形成包括前庭在内的外胚层结构，而植物极的胚胎半体则形成所有分化的细胞类型。我们认为，动物 - 植物极细胞的决定和分化沿着一条相对于红纹海胆胚胎细胞分裂模式发生了偏移的轴进行。在海胆直接发育的进化过程中，形成中胚层和内胚层结构的植物极形态发生潜能与胚胎预期的腹侧联系更为紧密。
