What structure forms spindle fibers during cellular division?

The structure that forms spindle fibers during cellular division is the centrioles. Centrioles are cylindrical organelles found in animal cells and play a crucial role in organizing the mitotic spindle, which is essential for separating chromosomes during cell division. As the cell prepares to divide, centrioles replicate and move to opposite poles of the cell. They help to organize microtubules into spindle fibers that attach to the centromeres of chromosomes, ensuring that the chromosomes are properly aligned and distributed to the daughter cells.

The nucleus, while vital for containing genetic material, does not play a direct role in forming spindle fibers. Chromosomes, which contain the DNA, are the structures that are being separated but do not contribute to the formation of spindle fibers themselves. Vesicles are membrane-bound structures that transport materials within the cell and are not involved in the spindle fiber formation process. Thus, centrioles are the key players in creating the spindle apparatus necessary for successful cell division.