Primitive hematopoietic stem cells migrate from the bone marrow to the specialized environment of the thymus which is the site of T cell development. B cells by contrast, or at least immature B cells, are produced in the bone marrow, with only the later stages of B cell maturation and selection ocurring in the peripheral lymphoid tissues. The bone marrow stages of B cell development have been designated as stem cell, early pro-B cell, late pro-B cell, pre-B cell and immature B cell on the basis of sequential immunoglobulin gene rearrangements and the expression of surface immunoglobulin. The earliest B cell precursors appear to be dependent on adhesion to the bone marrow stroma and VCAM-1, VLA-4 and CD44 have been particularly implicated in B cell development. The multipotent stem cells can be regulated by a number of early-acting cytokines as discussed above, but at the late pro-B cell stage a requirement develops for the presence of IL-7, which is produced in small quantities by the stromal cells.

See also: Colony stimulating factors; Granulocyte colony stimulating factor (G-CSF); Granulocyte-macrophage colony stimulating factor (GM-CSF); Interleukin 1 and its receptors; Interleukin 3; Interleu-kin 4; Interleukin 5 and its receptor; Interleukin 6; Macrophage colony stimulating factor (CSF-1); Microenvironment.

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