Genesis of the White Blood Cells
Early differentiation of the pluripotential hematopoi-etic stem cell into the different types of committed stem cells is shown in Figure 32–2.
Aside from those cells committed to form red blood cells, two major lineages of white blood cells are formed, the myelocytic and the lymphocytic lineages. The left side of Figure 33–1 shows the myelocyticlineage, beginning with the myeloblast; the rightshows thelymphocytic lineage, beginning with the lymphoblast.
The granulocytes and monocytes are formed only in the bone marrow. Lymphocytes and plasma cells are produced mainly in the various lymphogenous tissues—especially the lymph glands, spleen, thymus, tonsils, and various pockets of lymphoid tissue elsewhere in the body, such as in the bone marrow and in so-called Peyer’s patches underneath the epithelium in the gut wall.
The white blood cells formed in the bone marrow are stored within the marrow until they are needed in the circulatory system. Then, when the need arises, various factors cause them to be released (these factors are discussed later). Normally, about three times as many white blood cells are stored in the marrow as circulate in the entire blood.This represents about a 6-day supply of these cells.
The lymphocytes are mostly stored in the various lymphoid tissues, except for a small number that are temporarily being transported in the blood.
As shown in Figure 33–1, megakaryocytes (cell 3) are also formed in the bone marrow. These megakary-ocytes fragment in the bone marrow; the small frag-ments, known as platelets (or thrombocytes), then pass into the blood. They are very important in the initia-tion of blood clotting.