Bone marrow is the soft tissue occupying cavities and spaces within bones. In humans, it is the principal site of hematopoiesis, the production of blood cells after birth, and an important component of the immune system. It contains blood-forming cells, fat cells, blood vessels, and supporting connective tissue. Its activity supplies the circulation with oxygen-carrying red blood cells, infection-fighting white blood cells, and platelets involved in blood clotting. (medlineplus.gov)
Structure and distribution
Marrow occupies the central cavities of long bones and the spaces between the trabeculae of spongy bone. It is conventionally divided into red marrow, which contains abundant blood-forming tissue, and yellow marrow, which has a higher proportion of fat cells. These categories describe differences in tissue composition rather than two completely separate organs; both form part of the marrow compartment within the skeleton. (cancer.gov)
During growth, much of the red marrow is progressively replaced by fatty yellow marrow. In adults, active blood formation is concentrated in the vertebrae, pelvis, ribs, sternum, skull, and proximal portions of the femur and humerus. The distribution and relative abundance of red and yellow marrow therefore vary with age and skeletal location. Yellow marrow is relatively inactive in blood-cell production, whereas red marrow contains numerous developing blood cells. (ncbi.nlm.nih.gov)
Marrow also contains stromal cells and a vascular network. Together, these structures provide more than mechanical support: they create local environments that regulate the maintenance and development of blood-forming cells. (pubmed.ncbi.nlm.nih.gov)
Blood-cell production
Blood cells arise from hematopoietic stem cells, which can maintain their own population while generating progenitors that develop into specialized blood-cell types. These descendants undergo successive stages of proliferation and differentiation. The marrow thus contains a mixture of stem cells, committed progenitors, and cells at different stages of maturation rather than simply a store of finished blood cells. (pubmed.ncbi.nlm.nih.gov)
The major products have distinct functions:
- Red blood cells, or erythrocytes, carry oxygen throughout the body.
- White blood cells, or leukocytes, contribute to immune defense.
- Platelets participate in clot formation and help prevent bleeding.
Maintaining production of these components is essential because circulating blood cells require continual replacement. Marrow stem cells and their progeny sustain blood-cell counts and immune function throughout adult life. (medlineplus.gov)
Blood production is regulated by signals originating both inside and outside the marrow. For example, erythropoietin, a hormone produced chiefly by the kidneys, signals the marrow to make red blood cells. Reduced erythropoietin production in kidney disease can contribute to anemia, illustrating that impaired blood production does not necessarily originate in the marrow itself. (niddk.nih.gov)
Stem-cell environments and immune development
The stem-cell niche is the local cellular and molecular environment that supports stem-cell survival and regulates their behavior. In marrow, vascular endothelial cells, stromal cells, bone-associated cells, and other neighboring cells contribute to these environments. Different marrow microenvironments support different aspects of stem-cell maintenance and blood-cell development; the marrow is therefore functionally heterogeneous. (pubmed.ncbi.nlm.nih.gov)
Bone marrow is also a primary lymphoid organ. B cells undergo early development there, supported by interactions with stromal cells, before completing maturation in peripheral lymphoid tissues. Precursors of T cells likewise originate in marrow but migrate to the thymus, where their major developmental and selection processes occur. (ncbi.nlm.nih.gov)
Developing B cells assemble their antigen receptors through rearrangement of receptor-encoding genes. Immature B cells then undergo selection that limits potentially harmful recognition of the body's own components. This is part of central immune tolerance, which acts during lymphocyte development before these cells participate in immune responses in peripheral tissues. (ncbi.nlm.nih.gov)
Disorders involving marrow
Marrow disorders may reduce normal blood production, increase particular cell populations, or disrupt their maturation. In aplastic anemia, damage to marrow stem cells results in insufficient production of new blood cells. Its consequences can include fatigue, persistent infections, and easy bruising or bleeding, reflecting deficiencies in different blood-cell populations. Causes include immune-mediated stem-cell destruction, certain medicines or toxic exposures, and, less commonly, inherited conditions. (nhlbi.nih.gov)
Marrow examination is relevant to several forms of cancer, including leukemia, lymphoma, and multiple myeloma. It is also used in evaluating myelodysplastic syndromes and other disorders of blood-cell production. Some cancers originating elsewhere can spread to marrow. These conditions differ substantially in the cells involved, their tissue effects, and their diagnostic findings. (medlineplus.gov)
Examination and transplantation
Two complementary procedures obtain marrow samples. Aspiration withdraws fluid containing marrow cells, while a biopsy removes a small core of marrow-containing tissue. Samples are commonly taken from the back of the pelvic bone. Laboratory examination assesses cell numbers, appearance, and tissue composition to investigate abnormal blood counts, establish diagnoses, or monitor disease and treatment. (medlineplus.gov)
Hematopoietic stem-cell transplantation restores blood-forming cells in selected cancers and other blood disorders. Transplanted cells may come from marrow, circulating blood, or umbilical cord blood. They are infused into the bloodstream and travel to marrow, where they can establish blood production. Autologous transplantation uses the recipient's own cells; allogeneic transplantation uses donor cells. The term “bone marrow transplant” specifically describes transplantation using marrow-derived cells, although it is sometimes used more broadly. (cancer.gov)
Marrow donation collects cells through needles inserted into the hip bones under anesthesia. Peripheral blood stem-cell donation instead uses apheresis to separate stem cells from circulating blood. These are distinct collection methods for obtaining cells capable of rebuilding the recipient's blood-forming system. (cancer.gov)