Formation of Blood Cells (Hemopoiesis)
All blood cells have a lifetime and therefore must continually be replaced. Some lymphocytes have a lifetime measured in years, but most formed elements of blood have a lifetime measured in hours, days, or weeks. Regulation of RBC and platelets are by negative feedback. WBCs are regulated primarily by immune responses in response to invading pathogens and foreign antigens. Hemopoiesis aka Hematopoiesis : the process by which formed elements of blood develop. This process occurs in different locations through our life: before birth occurs in the yolk sac of an embryo, later in the liver, spleen, thymus, and lymph nodes. From the last 3 months and throughout our lifetime, red bone marrow becomes the primary site of hemopoiesis. Red Bone Marrow : the site of hemopoiesis. It is a highly vascularized connective tissue located in the microscopic spaces between trabeculae of spongy bone tissue. It is present mainly in the axial skeleton, proximal humerus and femur. Approx. 0.05-0.1% of red bone marrow cells are derived from mesenchyme and are called Pluripotent Stem Cells Aka Hemocytoblasts. These cells have the capacity to develop into many types of cells. In newborns all bone marrow is red and very active in blood cell production. As we grow, the rate of blood cell production slows, and the red bone marrow located in the medullary cavity of long bones becomes inactive and is replaced by yellow bone marrow – largely composed of fat cells. In certain situations, such as severe trauma, yellow bone marrow cells can revert back to red bone marrow cells. Stem cells in red bone marrow reproduce themselves, proliferate, and differentiate into cells that give rise to blood cells, macrophages, reticular cells, mast cells and adipocytes. Some of the stem cells can also form osteoblasts, chondroblasts, and muscle cells. This is a possible source of bone, cartilage and muscle tissue replacement for tissue and organ replacement.
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