Astrocytes are found throughout the brain and spinal cord, and their main roles include maintaining the blood-brain barrier, providing nourishment to neurons, and recycling neurotransmitters.
Ammonia appears to mainly target astrocytes, a type of glial cell in the brain, and although the mechanisms aren't fully understood, ammonia seems to cause astrocyte swelling and oxidative damage.
Because astrocytes are found through the brain and spinal cord, astrocytomas can form throughout those locations, but juvenile pilocytic astrocytomas are mostly infratentorial tumors in the cerebellum or near the brainstem.