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Keywords:

  • ascidian embryo;
  • centrosome-attracting body;
  • electron-dense matrix;
  • germ plasm;
  • unequal cleavage.

In ascidian embryos, three successive unequal cleavages occur at the posterior pole, generating a specific cleavage pattern. A recently reported novel structure designated the centrosome-attracting body (CAB) has been suggested to play essential roles in the unequal cleavages attracting centrosomes and the nucleus towards the posterior pole. To examine the morphological features of the CAB, the ultrastructure of the CAB of two ascidian species, Halocynthia roretzi and Ciona intestinalis was observed by transmission electron microscopy. Detailed observations clarified that the electron-dense matrix (EDM) was a CAB-specific component that was commonly observed in the CAB of both species but was not found in other areas of the embryo. Further observations of the CAB in various staged embryos revealed that the ultrastructure was quite stable, with no difference between points of a cell cycle or between each stage from the 8- to 64-cell stage when unequal cleavage occurred. Observations of extracted embryos implied that the EDM was the extraction-resistant component of the CAB and was tightly anchored to the plasma membrane. It has been proposed that the EDM functions as a physical attachment site at the cell cortex for microtubules emanating from centrosomes and provides a scaffold for the centrosome-attracting machinery. Interestingly, the ultrastructure of the CAB resembled germ plasm reported in other animals, raising the possibility that the CAB-containing posterior-most blastomeres are germ- line precursors.