A 36-year-old woman comes to the emergency department because

of a 4-week history of worsening headache, nausea, and vomiting. The

headache is worse at night. A CT scan of the brain shows a mass of

tissue in the third ventricle. The patient undergoes surgical excision of this

structure. Pathologic examination of the surgical specimen confirms that

the tumor originates from special cells lining the ventricle. How are

these cells called?