The bone marrow hosts skeletal progenitor cells which have most widely been referred to as
mesenchymal stem or stromal cells (MSCs), a heterogeneous population of adult stem cells possessing
the potential for self-renewal and multilineage differentiation. A consensus agreement on minimal criteria
has been suggested to define MSCs in vitro, including adhesion to plastic, expression of typical
surface markers and the ability to differentiate towards the adipogenic, osteogenic and chondrogenic
lineages but they are critically discussed since the differentiation capability of cells could not always be
confirmed by stringent assays in vivo. However, these in vitro characteristics have led to the notion that
progenitor cell populations, similar to MSCs in bone marrow, reside in various tissues. MSCs are in the
Recent advances in terms of genetic animal models enabled a couple of studies targeting skeletal progenitor
cells in vivo. Accordingly, different skeletal progenitor cell populations could be identified by
the expression of surface markers including nestin and leptin receptor. While there are still issues with
the identity of, and the overlap between different cell populations, these studies suggested that specific
microenvironments, referred to as niches, host and maintain skeletal progenitor cells in the bone marrow.
Dynamic mutual interactions through biological and physical cues between niche constituting cells
and niche inhabitants control dormancy, symmetric and asymmetric cell division and lineage commitment.
Niche constituting cells, inhabitant cells and their extracellular matrix are subject to influences of
aging and disease e.g. via cellular modulators. Protective niches can be hijacked and abused by metastasizing
tumor cells, and may even be adapted via mutual education. Here, we summarize the current
knowledge on bone marrow skeletal progenitor cell niches in physiology and pathophysiology. We
discuss the plasticity and dynamics of bone marrow niches as well as future perspectives of targeting
niches for therapeutic strategies.