A self-renewing cell means that one of the daughter cells produced by cell division, retains the identical properties of th mother cell, e.g., every time a stem cell divides, one of the progeny remains a stem cell, while the other daughter cell may develop along a particular differentiative pathway. A few weeks after a human egg is fertilized, some of the stem cells which developed in the yolk sac, become erythroid progenitor cells. After about the 3rd month of fetal development, all of the stem cells migrate to the fetal liver and spleen and undergo differentiation events through cytokine stimulation. After about the 7th month of human fetal development, all of the stem cells leave the fetal liver and spleen to populate the bone-marrow. After this time and throughout a person's life, the bone-marrow is the site of stem cell renewal and differentiation.
Stromal cells consist of fibroblasts, fat cells, endothelial cells, and mature macrophages.
We will begin our specific discussion of hematopoiesis with the initial stem cell, and then discuss development of the various cells lineages. We'll talk about each of the mature cells in a moment. The stem cell initially differentiates along two basic pathways. The first pathway is called the myeloid stem cell, and the second differentiative pathway is called the lymphoid stem cell. We will examine the myeloid stem cell differentiative pathway first.