Centrosome‐attracting body: A novel structure closely related to unequal cleavages in the ascidian embryo

Abstract

The mechanism of unequal cleavage is one of the most intriguing subjects in cell biology. Previous studies of unequal cleavage have focused on a limited number of organisms such as yeasts, nematodes, sea urchins and annelids. The cleavage pattern of the ascidian embryo is invariant. In the ascidian embryo, the posterior-most blastomeres divide unequally in three successive cleavages. In the present study, it was shown that the ascidian embryo provides another good experimental system with which to analyze the mechanism of unequal cleavage. A novel structure, designated as CAB (centrosome-attracting body), which was found specifically in the unequally cleaving blastomeres was described. In the course of unequal cleavages, first, a thick microtubule bundle appeared between CAB and one of the centrosomes. Then with the shortening of the microtubule bundle, the nucleus with the centrosome was drawn toward CAB, situated at the posterior cortex of the blastomere. Finally, a cleavage furrow formed in the middle of the asymmetrically located mitotic apparatus and produced two blastomeres of different size, generating a smaller cell that inherits CAB. The CAB seemed to play an essential role in the unequal cleavages in the ascidian embryo.

An Advanced Filter-Feeder Hypothesis for Urochordate Evolution

“…It attracts the posterior centrosome in the posterior-most blastomeres, thereby regulating the orientation of cell divisions and causing successive unequal cell divisions in the B-line descendants. The CAB is present in ascidians (Hibino et al, 1998;Nishikata et al, 1999), and a similar structure was also Figure S1 online for a similar diagram with blastomere names and cell fates superimposed. For details of the spatial arrangements of blastomeres, see Satoh (1979), Nishida (2005Nishida ( , 2008, and Fujii et al (2008).…”

Differences in embryonic pattern formation between Caenorhabditis elegans and its close parthenogenetic relative Diploscapter coronatus

“…A cortical structure resembling the CCR in D. coronatus was found in early ascidian embryos (Hibino et al, 1998). Orientation of mitotic spindles and their asymmetric translocation resulting in unequal divisions are induced by this structure called CAB ("centrosome-attracting body").…”

Interdependence of Filamentous Actin and Microtubules for Asymmetric Cell Division

“…This results in the production of small posterior-most blastomeres with a distinct developmental potential. A unique structure, called the centrosome-attracting body (CAB), was found at the posterior cortex of the posterior most blastomere pair (Hibino et al, 1998; Figure 5B). It was observed that a thick bundle of microtubules appeared between the posterior centrosome and the CAB (Hibino et al, 1998), which shortened as the nucleus was drawn towards the CAB ( Figure 5B).…”

“…A unique structure, called the centrosome-attracting body (CAB), was found at the posterior cortex of the posterior most blastomere pair (Hibino et al, 1998; Figure 5B). It was observed that a thick bundle of microtubules appeared between the posterior centrosome and the CAB (Hibino et al, 1998), which shortened as the nucleus was drawn towards the CAB ( Figure 5B). Depolymerization of the microtubules with nocodazole prevented formation of the thick microtubule bundle and posterior migration of the nucleus (Nishikata et al, 1999).…”