Question: which type of cell forms the blood brain barrier
[{'answer': 'endothelial cells', 'provenance': [{'wikipedia_id': '84936', 'title': 'Blood–brain barrier', 'start_paragraph_id': 1, 'start_character': 223, 'end_paragraph_id': 1, 'end_character': 240, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]