Stem cell niches in the bone marrow. Current data support 2 niches in the bone marrow. (A) Sinusoidal-megakaryocyte niche. The sinusoidal-megakaryocyte niche contains sinusoidal endothelial cells, megakaryocytes, and CAR cells. (B) Arteriolar-pericyte niche. The arteriolar niche includes arteriolar endothelial cells, NG2⁺ arteriolar pericytes, CAR cells, sympathetic nerves, and nonmyelinating Schwann cells. (A-B) A subset of HSCs localize near the endosteum, placing osteoblast lineage cells (OB) in these niches.