BONE MARROW

In the adult, red blood cells, many white blood cells, and platelets are formed in the bone marrow. In the fetus, blood cells are also formed in the liver and spleen, and in adults such extramedullary hematopoiesis may occur in diseases in which the bone marrow becomes destroyed or fibrosed. In children, blood cells are actively produced in the marrow cavities of all the bones. By age 20, the marrow in the cavities of the long bones, except for the upper humerus and femur, has become inactive . Active cellular marrow is called red marrow; inactive marrow that is infiltrated with fat is called yellow marrow.

The bone marrow is actually one of the largest organs in the body, approaching the size and weight of the liver. It is also one of the most active. Normally, 75% of the cells in the marrow belong to the white blood cell-producing myeloid series and only 25% are maturing red cells, even though there are over 500 times as many red cells in the circulation as there are white cells. This difference in the marrow reflects the fact that the average life span of white cells is short, whereas that of red cells is long.

The bone marrow contains multipotent uncommitted stem cells (pluripotential stem cells) that differentiate into one or another type of committed stem cells (progenitor cells). These in turn form the various differentiated types of blood cells. There are separate pools of progenitor cells for megakaryocytes, lymphocytes, erythrocytes, eosinophils, and basophils, whereas neutrophils and monocytes arise from a common precursor. The bone marrow stem cells are also the source of osteoclasts , Kupffer cells , mast cells, dendritic cells, and Langerhans cells (see below).

The pluripotential cells are few in number but are capable of completely replacing the bone marrow when injected into a host whose own bone marrow has been completely destroyed. The best current source for these hematopoietic stem cells is umbilical cord blood.

The pluripotential cells are derived from uncommitted, totipotent stem cells that at least in theory can be stimulated to form any cell in the body. There are a few of these in adults, but they are more readily obtained from the blastocysts of embryos. Totipotential cells from human embryos have now been cultured, and there is immense interest in stem cell research. However, there are ethical as well as scientific issues involved, and debate on these issues will undoubtedly continue.