The Multiplicity of the Mitotic Centers and the Time-Course of Their Duplication and Separation

Abstract

In this study, the reproduction of the mitotic centers in the eggs of a sea urchin, Strongylocentrotus purpuratus and a sand dollar Dendraster excentricus has been studied by means of experimental designs that do not depend on the actual visualization of centrioles. The centers are defined in operational terms as potential poles. Blockage of mitosis by mercaptoethanol, it was found, inhibits the duplication of the centers, but does not inhibit the splitting and separation of centers that have already duplicated and thus potential poles could be realized as actual poles in multipolar divisions. At all times, the center is at least a duplex structure; that is, it contains two potential poles. The actual duplication process is the earliest event in a given mitotic cycle, taking place at very early interphase or in late telophase of the previous division. The splitting of the centers following duplication is a distinct process, dissociable from the duplication as such. Duplication and splitting normally occur at about the same time in the mitotic cycle, with a precession of the former. That is, as the two members of a pair of "old" centers split, each one gives rise to a new one, which remains associated with it until the next phase of splitting and duplication occurs. The results are consistent with what is termed a "generative" model of the self-reproduction of an intracellular body. According to this, the body does not immediately produce a full-fledged copy of itself, with simultaneous fission, but the primary duplication event involves only a part of the parent structure. This gives rise to a "germ" or "seed" which then grows to be equivalent to the parent body, and finally splits from it.

One to only two: a short history of the centrosome and its duplication

“…A singularly important step forward in our understanding of centrosome duplication was an elegantly thought out study of the functional properties of centrosome duplication in early-stage sea urchin zygotes [18]. At the time, nothing was known about how centrosome duplication was controlled and few, if any, were thinking about this issue.…”

“…Although Mazia [18,24] indicated that centrioles could be morphological correlates for the mitotic centres, this had not been tested. This was substantiated in a serial section electron microscopy characterization of individual mercaptoethanoltreated sea urchin zygotes that had been previously followed in vivo [25].…”

“…Mazia [18,24] was careful to separate the concept of the mitotic centre (an activity) from the centriole (a morphological structure in animal cells). Although he did not say at the time why he was so deliberate in doing this, his later writings suggested that he was thinking about integrating his findings with spindle assembly in higher plant and acentriolar animal cells (see [30]).…”

Post‐karyokinesis centrosome movement leaves a trail of unanswered questions

“…First, an increasing number of studies are identifying proteins that are associated with centriole splitting. Since centriole splitting has historically been thought to be confined to the onset of centriole/centrosome duplication [Mazia et al, 1960;Robbins et al, 1968;Kuriyama and Borisy, 1981], these proteins have therefore been assigned a role in the duplication process. The new findings now raise the possibility that some of these proteins may actually be involved in splitting that occurs either prior to movement, in association with duplication, or both.…”

Centrosomal protein centrin is not detectable during early pre-implantation development but reappears during late blastocyst stage in porcine embryos

“…Centrosome studies were started in gametes at the beginning of the last century (Boveri 1900, Wilson 1924, revived in gametic cells in the middle of the century (Mazia et al 1960), and now being advanced in the ooplasmic cell-free system (Palazzo et al 1992, Heald et al 1997, Hinchcliffe et al 1999. The centrosome undergoes remarkable changes during gametogenesis and fertilization, unparalleled in other cellular systems.…”