Bone marrow is composed of hematopoietic cells as well as structural components, including trabecular bone, adipose tissue, a sinusoidal system, supporting reticular cells, and an extracellular matrix.
The hematopoietic system is composed of erythrocytes, leukocytes, and platelets; their precursor cells; and the tissues that support the continuous cycles of cell differentiation required for maintenance of oxygen delivery, protection from infectious agents, and hemostasis.
A single multipotent stem cell is the precursor for all hematopoietic cells. From this cell arise progenitor cells directed toward various cell lineages. Postnatally, hematopoiesis to replenish circulating erythrocytes, granulocytes, monocytes, and platelets predominantly occurs in the bone marrow. Although lymphocytic precursor cells continue to travel from the bone marrow to the thymic cortex and enteric mucosa for differentiation postnatally, most circulating lymphocytes are associated with the extramedullary lymphoid organs.Constantly replenishing blood cells, the bone marrow is one of the most active tissues in the body. Blood cell turnover is rapid; for example, neutrophils have a circulatory half-life of approximately 10 hours. The life span of platelets, on average, is approximately 7 to 10 days, and the life span of large animal erythrocytes is approximately 5 months, with slightly shorter half-lives reported in some breeds.1-3 Readers are referred to a veterinary hematology textbook for more in-depth information on the hematopoietic system.4
Multipotent mesenchymal stromal cells are progenitor cells that give rise to the structural components of bone marrow and show great promise in regenerative medicine. Bone marrow-derived mesenchymal stem cells (BMSCs) have shown higher osteogenic capacity compared to mesenchymal stem cells (MSCs) from adipose tissue, umbilical cord tissue, and cord blood,5 although they seem to proliferate at a slower rate than MSCs originating from other tissue types.6 Bone marrow aspirates are routinely performed in equine practice to obtain bone marrow concentrates and BMSCs. Bone marrow aspirates without processing, concentrates, and BMSCs are frequently used to treat tendon, ligament, and meniscal injuries. Intra-articular administration is also used to enhance healing of cartilage defects and treat subchondral bone cystic lesions in horses. ,8