Bone Marrow

• Bone marrow is the spongy portion inside bones and consists of a meshwork of bone trabeculae with spaces containing haematopoietic marrow elements and fat cells
• The ratio of haematopoietic elements versus fat cells in bone marrow depends on the age and activity of the bone marrow
• Marrow cellularity is roughly inversely proportional to age, with newborns having almost 100% cellular bone marrow and older individuals having 20-30% cellularity
• Bone marrow is where all blood cells are made from stem cells in post-natal life under normal physiologic mechanisms
• Neutrophils, erythrocytes, and platelets are made in bone marrow
• Lymphocytes are made in bone marrow, lymph nodes, and spleen
• Stem cells differentiate into primitive cells called blasts, which are precursors of each cell type
• Blasts divide and mature under the influence of proteins called growth factors
• Mature neutrophils and erythrocytes enter the blood, while megakaryocytes break into small fragments (platelets) that also enter the blood
• A stem cell is a cell that can reproduce itself indefinitely and differentiate into one or more types of functional mature cells
• Haematopoietic stem cells can differentiate into erythrocytes, neutrophils, megakaryocytes, or lymphocytes
• In theory, a single haematopoietic stem cell could reconstitute the entire bone marrow
• Haematopoietic tissue is discontinuous, produced in bone marrow but mature cells leave and function elsewhere
• Growth factors regulate the growth, differentiation, and function of cells of the haematopoietic and immune systems
• Erythropoietin stimulates red blood cell production when oxygen is low
• Thrombopoietin stimulates megakaryocyte production and platelets
• 1. CSF stimulates granulocyte (neutrophil) production and activates neutrophil function
• Haematopoietic tissue (red marrow) is found in the medullary cavity in all areas of spongy bone in infants and in the diploë of flat bones in adults