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*The bone marrow stroma was originally
thought to function mainly as a structural framework for the hematopoietic component of the marrow. Since then, it
has become well established that the stroma
consists of a heterogeneous population of cells, a subset of which exerts
both positive and negative regulatory effects on the proliferation and
differentiation of hematopoietic stem cells
(HSC) in the marrow. The stroma also contains
other non-hematopoietic cells termed mesenchymal stem cells (MSC), that
are capable of both self renewal and differentiation into bone,
cartilage, muscle, tendon and fat. MSC are similar to HSC in that they
are very rare, existing at an estimated frequency of about 1 in 100,000. MSC also
give rise to a variety of mature cell types via a step-wise maturation
process similar to hematopoiesis, termed mesengenesis.
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