These microtubules radiate towards the mitotic spindle, unlike astral microtubules. Kinetochores emit anaphase-inhibition signals until their attachment to the mitotic spindle. Spindle assembly is largely regulated by phosphorylation events catalyzed by mitotic kinases. Kinetochore microtubules are the third important subclass of mitotic microtubules. Parthenogenesis can occur without meiosis through mitotic oogenesis. Most of the microtubules that form the mitotic spindle originate from the centrosome. Clathrin aids in the congression of chromosomes by stabilizing kinetochore fibers of the mitotic spindle. These microtubules form direct connections with the kinetochores in the mitotic spindle. Around 95 percent of microtubules in the mitotic spindle can be characterized as interpolar. The eight spores are produced by meiosis followed by a mitotic division. Prophase II is very similar to mitotic prophase. The zygote divides by mitotic division and grows into a multicellular, diploid sporophyte. Most human cells are produced by mitotic cell division. Without the mitotic spindle, cellular division cannot occur. Calcium is a necessary ion in the formation of the mitotic spindle.