ANATOMY

Detailed explanation-1: -The structure involved in the formation of spindle fibres are centrioles. They exist in a pair close to the nucleus and are present only in the animal cell.

Detailed explanation-2: -At the beginning of nuclear division, two wheel-shaped protein structures called centrioles position themselves at opposite ends of the cell forming cell poles. Long protein fibers called microtubules extend from the centrioles in all possible directions, forming what is called a spindle.

Detailed explanation-3: -Just before mitosis, the two centrosomes move apart until they are on opposite sides of the nucleus. As mitosis proceeds, microtubules grow out from each centrosome with their plus ends growing toward the metaphase plate. These clusters of microtubules are called spindle fibers.