**4.1 Compositional microenvironment**

In the bone marrow microenvironment, MSCs function as dominating component and stromal cells for the homeostasis and regeneration of hematopoietic stem cells (HSCs) and the concomitant derived cells [30, 64, 65]. In the context of physiological hematogenesis, MSCs are competent for the maintenance or replenishment of the stem cell pool in damaged tissues, and thus help reconstruct the microenvironment for the subsequent hematopoietic reconstitution [22, 30]. As to OA, by conducting MSC infusion into the articular cavity, the hyperactivated inflammatory response caused by inflammatory cytokines is supposed to be effectively suppressed by the released antiinflammatory factors, extracellular organelles, and vesicles in the microenvironment [24, 66]. As to OA, the roles of MSCs are to orchestrate the spatiotemporal balance between the inflammation and cartilage tissue reconstruction via providing the damaged tissues including bone tissue and cartilage tissue with a relatively desirable environment for tissue repair [24, 67].
