Localization and functions of mesenchymal stromal cells in vivo

The study of mesenchymal stromal cells (MSCs) is a very topical problem. Numerous experiments in vitro advanced the understanding of MSC biology to a great extent. However, many aspects of their behavior in vivo still remain unclear. This review deals with MSC localization and functioning in an organism. MSCs are present in various tissues, changing their numbers and traits during ontogenesis. Pericytes, or adventitial cells, can be considered possible equivalents of MSCs in vivo. Self-maintenance, proliferation, and differentiation of MSCs are controlled by their tissue microenvironment, which includes the surrounding cells, soluble molecules, and extracellular matrix. At early stages of ontogenesis, MSCs probably migrate throughout an organism and populate various tissues. The migration occurs also through a mature organism when tissues happen to be damaged. MSCs move pointedly to the damaged parts and render a reparative effect that is due primarily to the paracrine production of bioactive molecules and immunomodulatory properties of MSCs rather than their differentiation. An important function of MSCs is the creation of hematopoietic microenvironment by the secretion of cytokines and chemoattractants, as well as by direct interaction with hemopoietic cells. It is possible that MSCs sustain the stable functioning of not only hematopoietic but also other tissues. Their unique features make them quite attractive for clinical use, although the successful introduction of MSC into medical practice requires further study of their interaction with the recipient organism and their effect on regeneration.