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The center (∼1 µm) where spindle fibers (microtubules) originate (a microtubule organizing center, MTOC) and develop from a pair of centrioles toward the centromeres during nuclear divisions in animals (and few lower plants). In normal mammalian cells there are two centrosomes at the opposite poles (see Fig. C57). The mother centrosome normally remains anchored to the hub of the Drosophila germ stem cell interface and is inherited by the stem cell, whereas the daughter centrosome moves away from the hub and is inherited by the cell that commits to differentiation (Yamashita YM et al 2007 Science 315:518). In the human mammary epithelial cells there may be split centrosomes as an anomaly. Loss of p16INK4a causes splitting of the centriole that can lead to the formation of aneuploidy and genomic instability because multipolar mitoses may take place. Normally, p16INK4a in cooperation with p21 regulate cyclin-dependent kinases and prevent the splitting of the centriole. Such a condition ma ...