Bone marrow niche. Pre-pro-B cells share a common niche with plasma cells based on the expression of CXC chemokine ligand-12 (CXCL12) by bone marrow stromal cells. Upon further differentiation into pro-B cells, cells switch to a different niche, which is based on interleukin-7 (IL-7)-expressing bone marrow stromal cells. Further differentiation of B cells into pre-B cells makes them independent from bone marrow niches before leaving bone marrow to secondary lymphatic organs. Plasma cells re-entering the bone marrow share the CXCL12-triggered bone marrow niche with pre-pro B cells, as described above. Hematopoietic stem cells (HSCs) are linked to bone marrow niches created by osteoblasts.