The microscopic appearance of ependymomas is dominated by the presence of radially-arranged tumor cells forming rosettes (black arrows), which are named for their resemblance to the flower-like architecture of rose windows in gothic cathedrals. The perivascular pseudorosette, characterized by tumor cells radially arranged around a central vessel, is more common by far, but it is not specific to ependymomas. Conversely, true ependymal rosettes (pictured here) have a central clearing that resembles a tubule lumen (white arrows), which represent an attempt to recapitulate the ependymal lining of a ventricle. Although true ependymal rosettes are only found in a minority of ependymomas, they are considered to be a marker of ependymal differentiation.
For further study on this topic and others related to neuropathology and neuroanatomy, the following books come highly recommended, and a small portion of the proceeds (or any Amazon purchase through these links) help with expenses of hosting this site:
Adventures in Neuropathology 
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