Hematopoietic System

In the embryo the blood cells are formed from the mesenchyme of the blood islands. Later this mesenchyme forms the endo­thelium of blood capillaries of bone marrow, lymph nodes, spleen and liver but retains its erythropoietic functions. The mesenchyme associated with this endothe­lium forms reticulum and gives rise to granulocytes, monocytes and lymphoid ele­ments. In the bone marrow the red blood cells arise within the capillaries from endothelium, while the granulocytes are formed extravascularly from reticulum. The stromal elements about embryonal lymphatics are the antecedents of lymphoid tissue. Reticu­lum cells in this stroma form large lymphoblasts which develop into small lympho­cytes. Large phagocytic cells or histiocytes can arise either from this reticulum, from undifferentiated connective tissue or from the endothelial lining of lymphatic or venous sinuses, in the liver, spleen, lymph nodes, etc. Collectively these tissues which furnish histiocytes are referred to as the reticuloendothelial system.
The hematopoietic organs and their associated structures are divided into two classes, 1. the erythropoietic tissues and, 2. the leukopoietic tissues.
The diseases of the erythropoietic tissues include polycythemia, the various forms of anemia, the purpuras and megakaryocy-tosis. Primary neoplasia of the erythro­poietic system is not definitely established.The diseases of the leukopoietic tissue include a variety of chronic infections and neoplastic diseases which involve myeloid and lymphoid elements.

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