HSC and lineage-restricted hematopoietic progenitor cells are found in the developing spleen on gestational day 13, at approximately the same time they are found in the fetal liver (Landreth 1993) (Figure 1.2). Although the expansion of hema-tolymphoid cells in the spleen never rivals that found in the developing liver, the spleen maintains this low level of hematopoietic activity well into postnatal life. For this reason, splenic hematopoiesis serves as a reserve of HSC following damage to the bone marrow organ following chemotherapy or radiation in postnatal animals. The persistence of the hematopoietic microenvironment in the spleen is revealed by the seeding of HSC to that organ following hematopoietic transplantation in rodents.
It is of some interest that lymphopoiesis has never been convincingly demonstrated in the spleen of postnatal mice under any experimental conditions tested (Paige et al. 1981). These observations led to the proposal of specific hematopoietic microenvironments in the bone marrow and spleen. The bone marrow hematopoietic microenvironment substantively differs from that found in the spleen such that it is uniquely required for lymphopoiesis in mice.
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With all the stresses and strains of modern living, panic attacks are become a common problem for many people. Panic attacks occur when the pressure we are living under starts to creep up and overwhelm us. Often it's a result of running on the treadmill of life and forgetting to watch the signs and symptoms of the effects of excessive stress on our bodies. Thankfully panic attacks are very treatable. Often it is just a matter of learning to recognize the symptoms and learn simple but effective techniques that help you release yourself from the crippling effects a panic attack can bring.