TGF-β in the BM niche. HSCs reside in the BM, in specialized niches that support and regulate HSC fate options. TGF-β is activated by Schwann cells that ensheath sympathetic nerves. Large amounts of TGF-β are also produced by megakaryocytes. HSCs in close proximity to Schwann cells and megakaryocytes become exposed to TGF-β and exhibit activation of SMAD2/3.