|The production of blood cells in bone marrow begins roughly 4-5 months after conception. Stem cells immigrate from the liver into the bone marrow, where the "microenvironment" is decisive for the development of stem cells. This stroma consists of endothelial cells, fat cells, osteoblasts and fibrocytes. Here, among others, mature the B lymphocytes.
Macrophages also colonize the stroma, but they stem from hematopoietic stem cells. This creates an environment that, according to need, stimulates the proliferation and differentiation of the precursor cells. As soon as these cells are mature they proceed through the openings in the sinusoids from the bone marrow into the blood stream.
||Fig. 7 - Normal blood-forming bone marrow