Figure 2. Model for acentrosomal spindle assembly.

The emphasis of this model is that the accumulation and bundling of microtubules are regulated to occur in close proximity to the chromosomes. A) Oocyte before nuclear envelope breakdown. In most organisms, microtubules are visible outside the nuclear envelope. In mouse, these cluster in MTOCs. B) After nuclear envelope breakdown, there may be bundles or clusters of microtubules in close proximity, but distinct from, the chromosomes. This is seen in mouse and nematodes but not in fly or human. C) In all systems, the initial interactions between the chromosomes and microtubules may appear quite disorganized or appear as a ball of microtubules. D) A bipolar spindle forms and then lateral attachments predominate. E) Prior to anaphase, end-on attachments are stabilized. This last step is not clear in nematodes which lack point centromeres. In B and C, the kinetochores (red) are drawn relative to the chromosomes (blue) to show they are not stretched by the microtubules (green). In D and E, the kinetochores are drawn relative to the chromosomes to indicate they are stretched towards the poles.