Introduction
•Bone marrow is a semi-solid tissue which may be found within the spongy or cancellous portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production or hematopoiesis.
•It is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. In adult humans, bone marrow is primarily located in the ribs, vertebrae, sternum, and bones of the pelvis. On average, bone marrow constitutes 4% of the total body mass of humans; in an adult having 65 kilograms of mass (143 lb.), bone marrow typically accounts for approximately 2.6 kilograms.
•Human marrow produces approximately 500 billion blood cells per day, which join the systemic circulation via permeable vasculature sinusoids within the medullary cavity.
•All types of hematopoietic cells, including both myeloid and lymphoid lineages, are created in bone marrow; however, lymphoid cells must migrate to other lymphoid organs (e.g. thymus) in order to complete maturation.
•Bone marrow transplants can be conducted to treat severe diseases of the bone marrow, including certain forms of cancer such as leukemia.
•Additionally, bone marrow stem cells have been successfully transformed into functional neural cells and can also potentially be used to treat illnesses such as inflammatory bowel disease.
Structure of bone marrow
•The composition of marrow is dynamic, as the mixture of cellular and non-cellular components (connective tissue) shifts with age and in response to systemic factors. In humans, marrow is colloquially characterized as “red” or “yellow” marrow (Latin: medulla osmium rubra, Latin: medulla osmium flava, respectively) depending on the prevalence of hematopoietic cells vs fat cells.
•While the precise mechanisms underlying marrow regulation are not understood, compositional changes occur according to stereotypical patterns.
•For example, a newborn baby’s bones exclusively contain haematopoietically active “red” marrow, and there is a progressive conversion towards “yellow” marrow with age.
•In adults, red marrow is found mainly in the central skeleton, such as the pelvis, sternum, cranium, ribs, vertebrae and scapulae, and variably found in the proximal epiphyseal ends of long bones such as the femur and humerus.
•In circumstances of chronic hypoxia, the body can convert yellow marrow back to red marrow to increase blood cell production
•Hematopoietic components
•At the cellular level, the main functional component of bone marrow includes the progenitor cells which are destined to mature into blood and lymphoid cells.
•Marrow contains hematopoietic stem cells which give rise to the three classes of blood cells that are found in circulation: white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (thrombocytes).
| Cellular constitution of the red bone marrow parenchyma | |||
| Group | Cell type | Average fraction | Reference range |
| Myelopoietic cells | Myeloblasts | 0.9% | 0.2–1.5 |
| Promyelocytes | 3.3% | 2.1–4.1 | |
| Neutrophilic myelocytes | 12.7% | 8.2–15.7 | |
| Eosinophilic myelocytes | 0.8% | 0.2–1.3 | |
| Neutrophilic metamyelocytes | 15.9% | 9.6–24.6 | |
| Eosinophilic metamyelocytes | 1.2% | 0.4–2.2 | |
| Neutrophilic band cells | 12.4% | 9.5–15.3 | |
| Eosinophilic band cells | 0.9% | 0.2–2.4 | |
| Segmented neutrophils | 7.4% | 6.0–12.0 | |
| Segmented eosinophils | 0.5% | 0.0–1.3 | |
| Segmented basophils and mast cells | 0.1% | 0.0–0.2 | |
| Cellular constitution of the red bone marrow parenchyma | |||
| Group | Cell type | Average fraction | Reference range |
| Myelopoietic cells | Myeloblasts | 0.9% | 0.2–1.5 |
| Promyelocytes | 3.3% | 2.1–4.1 | |
| Neutrophilic myelocytes | 12.7% | 8.2–15.7 | |
| Eosinophilic myelocytes | 0.8% | 0.2–1.3 | |
| Neutrophilic metamyelocytes | 15.9% | 9.6–24.6 | |
| Eosinophilic metamyelocytes | 1.2% | 0.4–2.2 | |
| Neutrophilic band cells | 12.4% | 9.5–15.3 | |
| Eosinophilic band cells | 0.9% | 0.2–2.4 | |
| Segmented neutrophils | 7.4% | 6.0–12.0 | |
| Segmented eosinophils | 0.5% | 0.0–1.3 | |
| Segmented basophils and mast cells | 0.1% | 0.0–0.2 | |
Hematopoietic precursor cells: promyelocyte in the center, two metamyelocytes next to it and band cells from a bone marrow aspirate.
•Stroma
•The stroma of the bone marrow includes all tissue not directly involved in the marrow’s primary function of hematopoiesis. Stromal cells may be indirectly involved in hematopoiesis, providing a microenvironment that influences the function and differentiation of hematopoietic cells. For instance, they generate colony stimulating factors, which have a significant effect on hematopoiesis.
•Cell types that constitute the bone marrow stroma include:
1.fibroblasts (reticular connective tissue)
2.macrophages, which contribute especially to red blood cell production, as they deliver iron for hemoglobin production.
3.adipocytes (fat cells)
4.osteoblasts (synthesize bone)
5.osteoclasts (resorb bone)
6.endothelial cells, which form the sinusoids. These derive from endothelial stem cells, which are also present in the bone marrow.
Function of bone marrow
•Mesenchymal stem cells
•The bone marrow stroma contains mesenchymal stem cells (MSCs), also known as marrow stromal cells. These are multipotent stem cells that can differentiate into a variety of cell types. MSCs have been shown to differentiate, in vitro or in vivo, into osteoblasts, chondrocytes, myocytes, marrow adipocytes and beta-pancreatic islets cells.
•Bone marrow barrier
•The blood vessels of the bone marrow constitute a barrier, inhibiting immature blood cells from leaving the marrow. Only mature blood cells contain the membrane proteins, such as aquaporin and glycophorin, that are required to attach to and pass the blood vessel endothelium. Hematopoietic stem cells may also cross the bone marrow barrier and may thus be harvested from blood.
•Lymphatic role
•The red bone marrow is a key element of the lymphatic system, being one of the primary lymphoid organs that generate lymphocytes from immature hematopoietic progenitor cells.
•The bone marrow and thymus constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes. Furthermore, bone marrow performs a valve-like function to prevent the backflow of lymphatic fluid in the lymphatic system.
•Compartmentalization
Biological compartmentalization is evident within the bone marrow, in that certain cell types tend to aggregate in specific areas. For instance, erythrocytes, macrophages, and their precursors tend to gather around blood vessels, while granulocytes gather at the borders of the bone marrow.