It's not a specific cell that breaks down the nucleus during mitosis. It's actually a process called nuclear envelope breakdown that occurs during prophase, the first stage of mitosis.

Here's how it happens:

* The nuclear envelope, a double membrane surrounding the nucleus, starts to fragment.

* This is caused by the phosphorylation of lamins, proteins that provide structural support to the nuclear envelope.

* The breakdown allows the chromosomes to condense and attach to the spindle fibers, which are responsible for separating the chromosomes during mitosis.

So, the cell itself doesn't "break down" the nucleus, but rather the nucleus undergoes a controlled breakdown as part of the mitotic process.