Each spindle fiber is composed of a bundle of microtubules, which are assembled from tubulin molecules that become available as the cytoplasmic microtubular network is disassembled for cell division. The microtubules converge at each end of the spindle to a point termed the spindle pole or mitotic center. In animal cells each spindle pole contains a pair of centrioles, which are self-replicating organelles about 150 nanometers in diameter, surrounded by a radial array of aster microtubules. Plant cells, in contrast, do not contain centrioles, and their mitotic spindles are not as clearly defined at the poles. Plants lack the radial astral array characteristic of animal cells.