bone marrow

bone marrow is a semi-solid tissue found within the spongy or cancellous bone portions of bones. In birds and mammals, it is the primary site of new blood cell production, a process known as hematopoiesis. It comprises two main types: red marrow, which is hematopoietically active, and yellow marrow, which is rich in adipose tissue.

Structure and types

Bone marrow occupies the central cavities of long bones, such as the femur and humerus, and the interstices of cancellous bone in flat bones like the sternum, pelvis, and vertebrae. The distribution and composition change with age; in newborns, nearly all marrow is red, active marrow, but it is gradually replaced by yellow, fatty marrow in a process known as conversion. This conversion begins in the extremities, following a centripetal pattern described in studies at the Mayo Clinic. The stroma of the marrow, a supportive connective tissue framework, includes fibroblasts, macrophages, adipocytes, osteoblasts, and endothelial cells. This microenvironment, often termed the hematopoietic niche, is crucial for stem cell maintenance and is regulated by complex signaling involving the Notch signaling pathway and various cytokines. The vascular system within the marrow is extensive, centered around specialized sinusoidal capillaries that allow newly formed cells to enter the bloodstream.

Function

The primary function of red bone marrow is hematopoiesis, the production of all blood cell lineages. This includes erythropoiesis for red blood cells, leukopoiesis for white blood cells like lymphocytes and granulocytes, and thrombopoiesis for platelets. This process is driven by a small population of hematopoietic stem cells, which were first definitively identified in pioneering experiments by Ernest McCulloch and James Till at the Ontario Cancer Institute. The marrow also serves as a key component of the lymphatic system, being a primary site for B cell maturation and the generation of memory B cells, as outlined in research from the National Institutes of Health. Furthermore, yellow marrow acts as a significant energy reservoir due to its high triglyceride content and can be reconverted to red marrow in states of increased demand, such as severe anemia or following treatment with agents like granulocyte colony-stimulating factor.

Clinical significance

Bone marrow is frequently examined for diagnostic purposes through procedures like bone marrow aspiration and bone marrow biopsy, which are standard in hematology and oncology. These procedures are often performed on the posterior iliac crest, a site popularized for its safety and accessibility. Analysis of marrow is critical for diagnosing conditions such as leukemia, multiple myeloma, myelodysplastic syndrome, and aplastic anemia. The cellularity and myeloid-to-erythroid ratio are key metrics assessed by pathologists. Marrow function can be suppressed by chemotherapy, radiation therapy, or infections like parvovirus B19, leading to pancytopenia. Its status is also monitored in patients with chronic conditions like HIV or those taking medications such as zidovudine.

Harvesting and transplantation

Bone marrow can be harvested for hematopoietic stem cell transplantation, a potentially curative treatment for many malignant and non-malignant diseases. There are two primary methods: direct surgical aspiration from the iliac crest under general anesthesia, and peripheral blood stem cell collection following mobilization from the marrow into the bloodstream using drugs like filgrastim. The first successful human bone marrow transplant was performed by E. Donnall Thomas at the Fred Hutchinson Cancer Research Center, work for which he later received the Nobel Prize in Physiology or Medicine. Donor matching is primarily based on human leukocyte antigen typing, coordinated globally by organizations like the National Marrow Donor Program and the World Marrow Donor Association. The transplant process involves conditioning the recipient with regimens such as those developed at the Johns Hopkins Hospital to ablate the existing marrow before infusion of donor cells.

Diseases and disorders

A wide spectrum of diseases originate in or affect the bone marrow. Myeloproliferative neoplasms, such as polycythemia vera and myelofibrosis, involve the overproduction of blood cells. Leukemia represents a malignant proliferation of white blood cell precursors, with classifications like acute lymphoblastic leukemia being common in studies by the Children's Oncology Group. Multiple myeloma is a cancer of plasma cells within the marrow. Non-malignant disorders include aplastic anemia, where the marrow fails to produce cells, and myelodysplastic syndrome, a pre-leukemic state. Infiltrative disorders like Gaucher's disease and osteopetrosis can also compromise marrow function. Infections such as tuberculosis or histoplasmosis may rarely involve the marrow, as documented in cases from the Centers for Disease Control and Prevention. Category:Anatomy Category:Hematology