Gardner R L
Department of Zoology, University of Oxford, UK.
Development. 1997 Jan;124(2):289-301. doi: 10.1242/dev.124.2.289.
At least one polar body, almost invariably the second, persists intact to the early blastocyst stage in nearly two-thirds of mouse conceptuses of the PO strain. The distribution in early blastocysts of these surviving polar bodies was highly non-random. Most not only lay in the mid-region of the embryonic-abembryonic axis but, on discovering that early blastocysts are bilaterally rather than radially symmetrical about this axis, were found to align with the bilateral axis. Cell marking experiments failed to detect movement of polar bodies relative to the surface of the conceptus during either cleavage or blastulation. That the distribution of degenerating polar bodies and their presumed debris was similar to intact ones also argued against their motility, as did the finding that at all stages second polar bodies were attached to conceptuses by a thin, extensible, weakly elastic 'tether'. Although the transfer of small fluorochromes between them was rarely observed beyond second cleavage, the second polar body and conceptus could remain coupled ionically up to the blastocyst stage. It is concluded that the second polar body normally remains attached to the conceptus through persistence of the intercellular bridge formed during its abstriction, and therefore provides an enduring marker of the animal pole of the zygote. Hence, according to the distribution of polar bodies, the axis of bilateral symmetry of the early blastocysts is normally aligned with the animal-vegetal axis of the zygote and its embryonic-abembryonic axis is orthogonal to it. Such relationships suggest that, at least in undisturbed development, specification of the axes of the blastocyst depends on spatial patterning of the zygote.
在PO品系近三分之二的小鼠受精卵中，至少有一个极体（几乎总是第二个极体）完整地持续到早期囊胚阶段。这些存活极体在早期囊胚中的分布极不随机。大多数极体不仅位于胚胎 - 反胚胎轴的中部区域，而且在发现早期囊胚围绕该轴是双侧对称而非径向对称后，发现它们与双侧轴对齐。细胞标记实验未能检测到在卵裂或囊胚形成过程中极体相对于受精卵表面的移动。退化极体及其推测碎片的分布与完整极体相似，这也反驳了它们的运动性，同样，在所有阶段第二个极体通过一个薄的、可伸展的、弱弹性的“系绳”附着于受精卵这一发现也反驳了其运动性。尽管在第二次卵裂之后很少观察到小荧光染料在它们之间转移，但第二个极体和受精卵在囊胚阶段之前可以保持离子耦合。得出的结论是，第二个极体通常通过其排出过程中形成的细胞间桥的持续存在而保持附着于受精卵，因此提供了合子动物极的持久标记。因此，根据极体的分布，早期囊胚的双侧对称轴通常与合子的动物 - 植物轴对齐，并且其胚胎 - 反胚胎轴与之正交。这种关系表明，至少在未受干扰的发育过程中，囊胚轴的特化取决于合子的空间模式形成。
