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Hematopoietic stem cells line the inner surface of the bone, attached by components of the extracellular matrix. On page 657, Kollet et al. show that osteoclasts, when induced to differentiate by osteoblasts, secrete various factors that cleave the extracellular matrix, resulting in the mobilization of stem cells from the bone marrow. Thus, bone homeostasis and hematopoiesis have been shown to be linked. The cover depicts a primary osteoclast stained for CXCR4 (blue), polymerized actin (red) and nuclear DNA (green).
Descended from a long line of cattle ranchers, Neal Barnard seems an unlikely advocate for animal rights. But this doctor is not afraid to take on the entire medical establishment.
Two studies examine the microbe-fighting peptides that keep infection at bay in the urinary tract and the colon. In the urinary tract, such peptides are deployed with lightning speed, and in the colon a short-chain fatty acid induces the peptides and fends off dysentery caused by Shigella species (pages 636 – 641).
Marijuana receptors are present in fibrogenic cells of the liver, and their expression is induced in cirrhosis. Blockade of the CB1 subtype is now shown to inhibit fibrogenesis, offering a new approach for the treatment of cirrhosis (pages 671 – 676).
Osteoclasts carve out the marrow space where hematopoietic stem cells reside, but they have been considered bystanders during hematopoiesis. Osteoclasts are now shown to mediate stem cell mobilization, enabling stem cell emigration from the marrow to the circulation (pages 657 – 664).
Massive numbers of T cells infiltrate the epithelial cells of the intestine in people with celiac disease, caused by hypersensitivity to gluten. New findings show how some of these T cells become dangerous by taking on the characteristics of natural killer cells.
The central position of estrogen in the physiological and pharmacological control of bone resorption is now challenged by evidence from mouse genetics of estrogen-independent control by pituitary FSH.
Being in constant contact with the environment, the lung has evolved special mechanisms for resisting infections such as influenza. A newly discovered mechanism assures that influenza-specific T cells do not enter the general circulation. Instead, they recirculate between the draining lymph node and the respiratory mucosa—so remaining close to the site of infection.
For years, clinicians have administered antibodies and other protein-based drugs intravenously. Findings in mice suggest that, for at least one such agent, a pill might also work (pages 627–635).