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Overview of Bone Marrow
1. Definition and Function
What it is: A spongy tissue found inside bones, specifically in the medullary canals of long bones and the cavities of cancellous bones.
Primary Function: The main site of blood cell formation (hematopoiesis) after the fifth month of gestation.
Other Functions:
Breaks down old red blood cells (RBCs) using macrophages, and recycles the iron.
Stores fat through adipocytes.
2. Hematopoiesis (Blood Cell Formation)
This process is dynamic and adjusts to the body's needs.
The lifespan of blood cells is limited:
White blood cells (WBCs): a few hours to days.
Platelets: up to 10 days.
Red blood cells (RBCs): 120 days.
Development Timeline:
1st month of gestation: Mesoderm of the yolk sac.
2nd–3rd month: Liver and spleen.
By the 5th month: Bone marrow becomes the dominant source.
3. Blood and Nerve Supply
Blood Supply: Provided by one or more nutrient arteries that penetrate the bone. These arteries branch out to form a network of sinusoids where hematopoiesis occurs.
Nerve Supply: Innervated by myelinated and non-myelinated nerves that enter through the nutrient canals.
4. Types of Bone Marrow
Red Bone Marrow:
Characteristics: Hematopoietic (blood-forming), rich in blood cells.
Location: Flat bones (skull, sternum, vertebrae, etc.) and the epiphysis (ends) of long bones.
Age-related change: The amount of red marrow decreases with age.
Yellow Bone Marrow:
Characteristics: Non-hematopoietic, filled with fat cells (adipocytes).
Location: Diaphysis (shaft) of long bones.
Age-related change: The amount of yellow marrow increases with age.
Developmental Note: In newborns, all bone marrow is red. Yellow marrow gradually replaces red marrow as a person ages. It can, however, revert to red marrow in cases of severe bleeding, anemia, or hypoxia.
5. Cellular Components of Bone Marrow
Hematopoietic Cells:
Hematopoietic Stem Cells (HSCs): Multipotent cells that can self-renew and differentiate into all types of blood cells.
Progenitor Cells: HSCs develop into Multipotent Progenitor (MPP) cells, then into oligopotent progenitors (Common Lymphoid Progenitors and Common Myeloid Progenitors), which finally mature into specific blood cells.
Mature Cells: Include granulocytes, monocytes, megakaryocytes (which become platelets), erythrocytes (RBCs), and lymphocytes.
Non-Hematopoietic Cells (Stroma):
Function: Provide a microenvironment that supports hematopoiesis.
Components:
Bone cells: Osteoclasts and osteoblasts.
Marrow stroma: Composed of stromal cells, macrophages, and the extracellular matrix. They provide growth factors and cytokines.
Adipocytes: Support late-stage blood cells and store fat.
Sinusoids: Lined with endothelial cells, they act as a barrier to prevent the release of immature blood cells into circulation. Mature cells pass through this barrier.
6. Hematopoietic Growth Factors
These are chemical stimuli that regulate the production and differentiation of blood cells.
Key Factors:
Erythropoietin (EPO): Stimulates RBC production.
Thrombopoietin (TPO): Stimulates platelet production.
Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF): Stimulates the differentiation of myeloid progenitor cells into granulocytes and monocytes.
Interleukins (ILs): A diverse group of cytokines with various functions, including stimulating different
Types of marrow
- Based on gross examination:
- Red bone marrow:
- Hematopoietic/blood-forming
- Abundant blood cells
- The level decreases with age.
- Located in: flat bones (skull, sternum, vertebrae, scapulae, and pelvic bones), epiphysis of long bones (femur, tibia, humerus)
- Red bone marrow:
cell lineages (e.g., IL-3 for myeloid progenitors, IL-7 for lymphoid stem cells).
