Viamontes G I, Kirk D L
J Cell Biol. 1977 Dec;75(3):719-30. doi: 10.1083/jcb.75.3.719.
Inversion is a dominant aspect of morphogenesis in Volvox. In this process, the hollow, spheroidal Volvox embryo turns inside-out through a small opening called the phialopore to bring flagella from its inner to its outer surface. Analyses of intact, sectioned, and fragmented embryos by light, scanning electron, and transmission electron microscopy, suggest that shape changes preprogrammed into the cells cause inversion. First, cells throughout the embryo change from pear to spindle shape, which causes the embryo to contract and the phialopore to open. Then cells adjacent to the phialopore become flask-shaped, with long, thin stalks at their outer ends. Simultaneously, the cytoplasmic bridges joining all adjacent cells migrate from the midpoint of the cells to the stalk tips. Together, these changes cause the lips of cells at the phialopore margin to curl outward. Now cells progressively more distal to the phialopore become flask-shaped while the more proximal cells become columnar, causing the lips to curl progressively further over the surface of the embryo until the latter has turned completely inside-out. Fine structural analysis reveals a peripheral cytoskeleton of microtubules that is apparently involved in cellular elongation. Cell clusters isolated before inversion undergo a similar program of shape changes; this suggests that the changes in cellular shape are the cause rather than an effect of the inversion process.
内陷是团藻形态发生的一个主要方面。在这个过程中，中空的球形团藻胚胎通过一个叫做瓶口孔的小开口由内向外翻转，使鞭毛从其内部表面转移到外部表面。通过光学显微镜、扫描电子显微镜和透射电子显微镜对完整、切片和破碎的胚胎进行分析表明，细胞中预先编程的形状变化导致了内陷。首先，整个胚胎中的细胞从梨形变为纺锤形，这导致胚胎收缩，瓶口孔打开。然后，与瓶口孔相邻的细胞变成烧瓶状，在其外端有细长的柄。同时，连接所有相邻细胞的细胞质桥从细胞的中点迁移到柄的尖端。这些变化共同导致瓶口孔边缘的细胞唇向外卷曲。现在，离瓶口孔越来越远的细胞逐渐变成烧瓶状，而离瓶口孔较近的细胞变成柱状，导致细胞唇在胚胎表面逐渐卷曲得更远，直到胚胎完全由内向外翻转。精细结构分析揭示了一个由微管组成的外周细胞骨架，它显然参与了细胞伸长。在倒转之前分离的细胞团经历了类似的形状变化程序；这表明细胞形状的变化是内陷过程的原因而非结果。
