Role of MKs in bone remodeling and metastatic niches. Functional osteoblastic and vascular/hematopoietic niches are required for successful establishment of bone metastases.⁷⁴  MKs and hematopoietic stem cells are located in the vascular niche. MKs control bone mass through the secretion of bone formation factors (BMP-2, −3, −4, TGF-β) and production of mostly inhibitors of osteoclast function (OPG, IL-10, IL-13, GM-CSF).⁶⁵  In turn, osteoblasts secrete a series of cytokines that control hematopoiesis (stem cell factor, LIF, IL-6) and MK proliferation and differentiation.⁶⁴  In a bone metastasis animal model, MKs inhibited skeletal destruction and induced apoptosis of prostate cancer cells,⁶⁷  suggesting that the presence or absence of MKs might convert the vascular/osteoblastic niche from a nonpermissive to a permissive environment for successful establishment of metastases. Figure was generated using the database of images from Servier Medical Art from Servier (http://creativecommons.org/licenses/by/3.0/fr/).