Question: what type of cell contributes to the blood-brain barrier in the central nervous system
[{'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.'}]}, {'provenance': [{'wikipedia_id': '84936', 'title': 'Blood–brain barrier', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 562, 'bleu_score': 0.700457535021148, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9419404594078451816}, 'section': 'Section::::Abstract.'}]}]