Presented in the digital fluorescence microscopy image above are rat kangaroo (PtK2) kidney epithelial cells in various stages of prometaphase. The chromatin is stained with a bright red fluorescent probe (Alexa Fluor 568), while the microtubule network (mitotic spindle) is stained green (Alexa Fluor 488). During prometaphase, the mitotic spindle microtubules are now free to enter the nuclear region, and formation of specialized protein complexes known as kinetochores begins on each centromere. These complexes become attached to a subset of the spindle microtubules, which are then termed kinetochore microtubules. Other microtubules in the spindle (not attached to centromeres) are termed polar microtubules, and these help form and maintain the spindle structure along with astral microtubules, which remain outside the spindle.