What is it?
Myeloproliferative disorders (MPD) - a subset of bone marrow disorders - are a group of four diseases centered in the bone marrow.  They are characterized by the overproduction of a precursor (immature form) of a marrow cell.  Bone marrow, located in the center of the body's bones, is a soft fatty tissue.  It contains a fibrous network that supports the body's production of blood cells.  When a particular type of blood cell is needed, undifferentiated pluripotent stem cells in the marrow begin to change, becoming the immature blast forms of whatever cell type is in short supply.  These blasts mature to become one of five types of white blood cells (grouped into lymphocytes or granulocytes), or mature to form red blood cells, or platelets.  Since only fully mature cells normally leave the bone marrow, it usually contains a mixture of cells in various stages of maturity. 

While white blood cells (WBCs) help the body to fight infection, red blood cells (RBCs) carry oxygen, and platelets - which are actually fragments of cells called megakaryocytes - help the blood to clot.  In MPD conditions, excessive production of a cell's precursor leads to an increased number of that type of mature cell, and a correspondingincrease or decrease in the number of other blood cells (which may be inhibited and crowded out). This results in symptoms related to blood cell overproduction, shortages, and dysfunction throughout the body. 

The four MPDs include:

• Chronic myelogenous (myeloid, myelocytic) leukemia, a disease that leads to an overproduction of myelocytes, precursor to granulocytes - white blood cells that digest and kill invading microorganisms.  Often these overproduced cells do not function normally.  (For more information on this disorder, see Leukemia)
  
  
• Polycythemia vera, a disease in which too many red blood cell precursors and erythrocytes (red blood cells) are made in the bone marrow.  When RBCs build up in the blood stream, they cause the spleen to swell and the patient to itch all over.
  
  
• Agnogenic myeloid metaplasia (also know as Myeloid metaplasia with fibrosis, or myelofibrosis), is a disease characterized by overproduction of fiber creating cells, leading to excess fibrous tissue in the marrow.  The dense network of fiber inhibits the normal formation and maturation of RBCs and myelocytes.  The red blood cells that do enter the bloodstream look like teardrops instead of circles.  There may be too few normal mature red blood cells to carry oxygen, causing anemia.  Fiber cells may also spread to the spleen, causing it to swell.
  
  
• Essential Thrombocythemia, is characterized by an increased number of megakaryocytes, precursor to platelets (also called thrombocytes).  Platelets are inherently "sticky" - this allows them to plug holes in leaking blood vessels, but excess platelets can make it hard for the blood to flow normally and can increase the patient's risk of developing blood clots or of having a stroke.  Essential thrombocythemia must be distinguished from secondary thrombocytosis, increased numbers of platelets caused by non-marrow disorders (such as iron deficiency, rheumatoid arthritis, bleeding, or removal of the spleen). 
  

MPDs are not curable, but their slow progression can usually be controlled, and their symptoms alleviated.  For each, there is the slight chance that the disease will develop into an acute leukemia.  If this occurs, the course of the disease will be accelerated, the symptoms will intensify, and the need for more aggressive treatment will be increased.