The elimination of the spermatid cytoplasm during spermiogenesis enables the sperm to acquire a streamlined architecture, which allows for unhindered swimming. While this process has been well described in vertebrates, it has rarely been reported in invertebrates. In this study, we observed the process of cytoplasm elimination during spermiogenesis in Octopus tankahkeei (Mollusca, Cephalopoda) using light microscopy, transmission electron microscopy, and immunofluorescence. In the early spermatid, the cell is circular, and the nucleus is centrally located. With spermatid development, the cell becomes polarized. The nucleus gradually elongates and moves toward the end of the cell where the tail is forming. As a result, the cytoplasm moves past the nucleus at the anterior region of the future sperm head (the foreside of the acrosome). Following this, during the late stage of spermiogenesis, the cytoplasm condenses and collects on the foreside of the acrosome until finally the residual body is discarded from the top of the sperm head. This represents a distinct directionality for the development of cytoplasmic polarity and discarding of residual body compared with that reported for vertebrates (in which the cytoplasm of the elongating spermatids is polarized toward the caudal region). The fact that the cytoplasm also becomes concentrated suggests that water pumps may be involved in the elimination of water from the cytoplasm before the residual body is discarded. Furthermore, we found that microtubules, forming a manchette-like structure, are involved not only in reshaping of the nucleus but also in the transport of mitochondria and vesicles to the foreside of the acrosome, subsequently allowing them to be discarded with the residual body. This study broadens our understanding of the development of polarization and elimination of cytoplasm from spermatids in animals.

中文翻译：

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八爪鱼精子发生过程中的细胞质消除过程：精子细胞的极化发育和残体的丢弃

在精子发生过程中精子细胞质的消除使精子获得流线型的结构，从而可以不受阻碍地游泳。虽然这个过程在脊椎动物中得到了很好的描述，但在无脊椎动物中却鲜有报道。在这项研究中，我们使用光学显微镜、透射电子显微镜和免疫荧光观察了八爪鱼（软体动物，头足类动物）在精子发生过程中细胞质消除的过程。在早期的精子细胞中，细胞是圆形的，细胞核位于中央。随着精子细胞的发育，细胞变得极化。细胞核逐渐拉长并移向尾部形成的细胞末端。结果，细胞质移动通过未来精子头部前部区域（顶体前部）的细胞核。按照此，在精子发生的后期，细胞质在顶体的前部凝结并聚集，直到最后残体从精子头顶部被丢弃。与报道的脊椎动物（其中伸长的精子细胞的细胞质向尾部区域极化）相比，这代表了细胞质极性发展和残留体丢弃的明显方向性。细胞质也变得浓缩的事实表明，在丢弃残留体之前，水泵可能参与了从细胞质中去除水的过程。此外，我们发现微管形成类似曼切特的结构，不仅参与细胞核的重塑，还参与线粒体和囊泡向顶体前部的运输，随后允许它们与残留的身体一起丢弃。这项研究拓宽了我们对动物精子细胞极化和消除细胞质发展的理解。