The Centrosome-Attracting Body, Microtubule System, and Posterior Egg Cytoplasm Are Involved in Positioning of Cleavage Planes in the Ascidian Embryo

Abstract

Many kinds of animal embryos exhibit stereotyped cleavage patterns during early embryogenesis. In the ascidian Halocynthia roretzi, cleavage patterns are invariant but they are complicated by successive unequal cleavages that occur in the posterior region. Here we report the essential roles of a novel structure, called the centrosome-attracting body (CAB), which exists in the posterior pole cortex of cleaving embryos, in generating unequal cleavages. By removing and transplanting posterior egg cytoplasm and by treatment with sodium dodecyl sulfate, we demonstrated that loss of the CAB resulted in abolishment of unequal cleavage, while ectopic formation of the CAB caused ectopic unequal cleavages to occur. Experiments with a microtubule inhibitor demonstrated that the centrosome and nucleus were attracted toward the posterior cortex, where the CAB is located, by shortening of microtubule bundles formed between the centrosome and the CAB. Consequently, the mitotic apparatus was positioned asymmetrically, resulting in unequal cleavage. Immunohistochemistry provided evidence that a microtubule motor protein, a kinesin or kinesin-like molecule, may be associated with the CAB. Formation of the CAB during the early cleavage stage was resistant to treatment with the microtubule inhibitor. In contrast, the integrity of the CAB was lost upon treatment with a microfilament inhibitor. We propose that the CAB plays key roles in the orientation and positioning of cleavage planes during unequal cell division.

Interdependence of Filamentous Actin and Microtubules for Asymmetric Cell Division

“…Depolymerization of the actin cytoskeleton with cytochalasin B prevented maintenance of the CAB and abolished nuclear migration towards the posterior cortex (Nishikata et al, 1999). Interestingly, kinesin -or a kinesin-like protein -was found to localize to the CAB in an actin-dependent, but microtubule-independent, manner (Nishikata et al, 1999). In analogy to the dynactin complex pulling on astral microtubules due to its motor activity (see above), kinesin anchored to the actin-dependent CAB may exert a pulling force on captured astral microtubules by inducing depolymerization from the plus end.…”

“…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). Depolymerization of the actin cytoskeleton with cytochalasin B prevented maintenance of the CAB and abolished nuclear migration towards the posterior cortex (Nishikata et al, 1999).…”

“…Depolymerization of the microtubules with nocodazole prevented formation of the thick microtubule bundle and posterior migration of the nucleus (Nishikata et al, 1999). Depolymerization of the actin cytoskeleton with cytochalasin B prevented maintenance of the CAB and abolished nuclear migration towards the posterior cortex (Nishikata et al, 1999). Interestingly, kinesin -or a kinesin-like protein -was found to localize to the CAB in an actin-dependent, but microtubule-independent, manner (Nishikata et al, 1999).…”

Germline cell formation and gonad regeneration in solitary and colonial ascidians

“…PEM is associated with the centrosome attracting body (CAB; Nishikata et al, 1999). The CAB remains localized to the cortex of the vegetal blastomeres until the gastrula stage, when the unequal cleavage forms B7.6 blastomeres.…”

Polarisation des oeufs et des embryons : principes communs

“…Cette polarisation est amplifiée au moment de la fécondation par une contraction corticale massive de microfilaments, conséquence de la vague calcique d'activation déclenchée par le spermatozoide fécon-dant ( Figure 3, Df) [45,46]. le réticulum cortical auquel ils sont liés s'accumulent au stade 8 cellules dans une petite zone corticale appelée CAB (centrosome-attracting body) [44,48,49].…”