The Embryonic origin of Bone Marrow Stem Cells
The ontogeny of Bone Marrow Stem Cells is the focus of much attention. In fetal development the manufacture of blood occurs in various organs but after birth this function is subsumed by the marrow. Current evidence identifies early hematopoietic cells both in the extraembryonic yolk sac as well as an intraembryonic site within the ventral wall of dorsal aorta–gonad–mesonephros.
At approximately 5 to 6 weeks of gestation, progenitors derived from the yolk sac colonize the fetal liver and become the major source for blood cells for the remainder of intrauterine life. At around 7 weeks of gestation, hematopoietic cells also colonize the spleen and multipotent progenitor cells are seen in the fetal circulation by the end of 12 to 14 weeks of gestation.
At around 8 to 9 weeks of gestation, while hematopoiesis within the yolk sac per se is becoming extinct, T-cell production starts in the thymus. T-cell precursors derived from the fetal liver colonize the thymus and undergo maturation and differentiation. Hematopoietic cells first appear in the medullary cavities of bone at around 14 weeks of gestation and by birth the marrow has become the primary site of hematopoiesis.
Change in Function of Bone Marrow Stem Cells with age
Unlike the commonly held notion that Bone Marrow Stem Cells compartments diminish either in number or function with age ultimately resulting in an inability to meet homeostatic demands, age-related Bone Marrow Stem Cells changes appear to be an exception, at least for murine species in which this question has been most directly addressed. Early work demonstrated that marrow serially transplanted could reconstitute hematopoietic function for an estimated 15 to 20 life spans.
Furthermore, the capacity for old marrow to reconstitute proved superior to that of young. Subsequently, a number of investigators using a variety of techniques have concluded that Bone Marrow Stem Cells frequency in old mice is approximately twice that found in young.
Some evidence suggests that the intrinsic function of Bone Marrow Stem Cells changes somewhat with age, most notably in a shift in lineage potential from lymphoid to myeloid development. This may contribute to an observed relative increase in neutrophils and decrease in lymphocytes in the blood of older people.