A 63-year-old man is brought to the emergency department 2 hours after sudden onset of right arm and leg weakness and difficulty speaking. CT scan of the head shows no hemorrhage. He receives IV thrombolytic therapy for an acute ischemic stroke. Four weeks later, he returns for follow-up; his strength has partially improved, but MRI shows a well-demarcated region of tissue loss in the left frontal lobe with surrounding dense glial proliferation. Which of the following cell types is most directly responsible for the process that fills the extracellular space left by degenerating neurons and forms a scar in the CNS after injury?