Bone Marrow

Bone marrow is a semisolid, highly specialised biological tissue located within the spongy, or cancellous, regions of bones. In mammals and birds, it serves as the principal site of haematopoiesis, producing the full complement of blood cells that sustain life. Composed of haematopoietic tissue, adipose components, and supportive stromal cells, it occupies around five per cent of total body mass in healthy adults. In a 73-kilogram adult, this corresponds to approximately 3.7 kilograms of bone marrow. Human marrow generates nearly 500 billion blood cells each day, releasing them into the circulatory system through the sinusoidal blood vessels of the medullary cavity.

Structure and Distribution

The composition of bone marrow shifts throughout life. Marrow is classically described as red or yellow, reflecting the relative proportions of haematopoietic cells and adipocytes. Newborns possess exclusively red marrow, which gradually converts to yellow marrow with age. Nevertheless, red marrow persists in adults within the axial skeleton, particularly the sternum, ribs, vertebrae, pelvis, skull, and scapulae, and in the proximal epiphyses of the femur and humerus. During chronic hypoxic states, yellow marrow can reconvert to red marrow to enhance haematopoietic output.
At the microscopic level, marrow contains progenitor cells that mature into the three blood cell lineages: erythrocytes, leukocytes, and thrombocytes. Pluripotential haematopoietic stem cells reside within specialised niches and maintain continual cell production. Surrounding these elements is the stromal framework, consisting of connective tissue, adipocytes, osteoblasts, osteoclasts, fibroblasts, and endothelial cells that form the sinusoid vasculature. Stromal cells produce growth factors such as colony-stimulating factors that regulate blood cell formation.

Haematopoietic Components

Normal haematopoiesis involves three major cell lines—erythroid, myeloid, and megakaryocytic—arising from common stem cells. Erythroid precursors mature into circulating red blood cells, myeloid progenitors generate granulocytes and monocytes, and megakaryocytes produce platelets. These populations develop within organised microenvironments: erythroblasts often associate with macrophages, granulocytes cluster near the cortical edges of the marrow, and megakaryocytes position themselves adjacent to sinusoids for platelet release into the bloodstream.
The selective release of cells is governed by the bone marrow barrier, which prevents immature cells from entering circulation. Only mature cells express the required membrane proteins, such as glycophorin and aquaporins, to adhere to and traverse sinusoidal endothelium. Haematopoietic stem cells, however, can cross this barrier, enabling clinical harvesting from peripheral blood.

Bone Marrow Stroma and Mesenchymal Stem Cells

The stromal compartment provides structural and biochemical support. It includes reticular cells that assist in iron supply for erythropoiesis, adipocytes that store lipids, and endothelial cells that give rise to sinusoids. Mesenchymal stem cells (MSCs) form a crucial part of this stroma. These multipotent cells can differentiate into osteoblasts, chondrocytes, myocytes, adipocytes, and even pancreatic beta cells under appropriate conditions. MSCs also contribute to tissue repair following injury and participate in immune regulation.

Immunological Functions

Bone marrow operates as a central organ for both haematopoiesis and immunity. It plays a significant role in antigen-responsive processes. Naive T cells circulating through the bloodstream can home to marrow sinuses, passing through the endothelium into the parenchyma, where dendritic cells capture, process, and present antigens. The interaction between antigen-presenting cells and T cells triggers activation, proliferation, and subsequent recirculation.
The marrow is also a reservoir for memory B cells, memory T cells, and plasma cells, which inhabit long-term survival niches sustained by stromal signals. This retention of immunological memory supports durable responses to pathogens and influences vaccine efficacy. The marrow contributes to osteogenesis, self–non-self discrimination, immune surveillance of the central nervous system, and metabolic adaptation during energy scarcity.

Lymphatic Role

Red marrow forms part of the primary lymphoid organs, along with the thymus. It generates lymphocytes from haematopoietic progenitors and supports early lymphocyte maturation prior to their functional development in secondary lymphoid tissues. Additionally, marrow helps maintain directional flow within the lymphatic system by functioning as a biological valve and preventing retrograde movement of lymph.

Compartmentalisation

Distinct spatial organisation characterises marrow physiology. Erythrocytes, macrophages, and erythroid precursors localise near vascular structures to facilitate nutrient exchange. Granulocytes populate peripheral zones, while lymphoid elements may cluster in defined niches. This compartmentalisation ensures efficient cell development and streamlined movement into circulation.

Bone Marrow in Cuisine

Across cultures, bone marrow has been valued as a nutrient-rich food source for millennia. It is a key ingredient in dishes such as ossobuco alla Milanese, where the marrow from veal shanks contributes flavour and richness.

Clinical Significance

Bone marrow integrity is vital for systemic health. Diseases such as aplastic anaemia, multiple myeloma, and various forms of leukaemia disrupt normal architecture and reduce blood cell production. Infectious agents, including tuberculosis, may infiltrate marrow and impair its function. Exposure to ionising radiation or chemotherapy damages rapidly dividing haematopoietic cells, leading to immunosuppression; many symptoms of radiation sickness stem from this injury.
Diagnosis of marrow disorders often involves bone marrow aspiration, commonly taken from the iliac crest using a hollow needle under local or general anaesthesia. Imaging studies contribute supplementary information. Radiographs reveal structural bone changes but provide little detail about marrow tissue. Computed tomography offers moderate assessment of marrow density, distinguishing fatty yellow marrow from more cellular red marrow. Magnetic resonance imaging, however, is particularly sensitive, detecting differences in fat and water content to characterise marrow composition accurately.