From arteries, veins and capillaries right down to arterioles, venules and beyond, the bloodstream and its network of vessels rank among the body's most complicated organs.
The blood these vessels transport contains a plethora of cells, proteins and other elements that range from the immune system's antibodies - and the B cells that make them - to cytokines and chemokines that resist infection, to the oxygen and nutrients that fuel the body in a swap of carbon dioxide and surplus debris. Where do all these components come from? They're fashioned in all their diversity by hematopoietic stem cells (HSCs). As a fetus develops in the womb, these HSCs sally forth to fabricate the blood constituents as needed. Now that scientists can manipulate stem cells, they leach or filter the HSCs out of the blood's bone marrow, where they originate and reside, then assign them to other gigs. But it's not simple.
Molecular and medical geneticist John Dick, at the University of Toronto, has turned from blood to bone - which harbors the bone marrow in which these stem cell-generated elements grow and reside. He and his colleagues have identified the new human stem cells, derived from umbilical cord blood, after injecting a batch of them directly into the femur of SCID mice. Instead of the traditional method of intravenous injection of the HSCs into the teeming bloodstream, this complex process requires circulation through blood, recognition and extravasation (blood leakage) through bone-marrow vasculature, and migration to a supportive microenvironment. Thus, some classes of hematopoietic stem cells may remain undetected.
"We observed how this new subpopulation of stem cells rapidly repopulated the blood-producing system of the mice," Dick recounted. "They produced high levels of blood cells within the first week or two after bone marrow transplant.
"The intravenous group of mice, as reported previously, followed very low levels of engraftment in the first three weeks, followed by rapid increases between weeks three and six. By contrast, intrafemoral delivery provided rapidly and significantly higher (bone marrow, 30-fold; peripheral blood, 16-fold; spleen, eightfold) engraftment at early time points, but similar levels beyond five weeks. The percent engraftment from 23 mice in six experiments was six- to 12-fold higher (right and left femurs, respectively) in the intrafemoral group than in the intravenous cohort."
Bone Delivery Beats Out Intravenous Injection
"We observed this new subpopulation of stem cells rapidly repopulate the blood-producing system of the mice," Dick recounted. "They produced high levels of blood cells within the first week or two after bone marrow transplant. This is one to two weeks earlier than the normal rate. Different classes of human blood cells quickly grew from a previously undescribed class of HSCs."
Dick is senior author of an article in Nature Medicine released online June 8, 2003, and scheduled for publication in the journal's July 2003 issue. It bears the title: "Rapid myeloerythroid repopulation after intrafemoral transplantation of NOD [non-obese] SCID mice reveals a new class of human stem cells."
"Our findings," Dick said in an press statement, "are a major advancement in human stem cell research with possible significant clonal implications for designing more effective cancer therapies. This is an exciting discovery," he added, "because for the first time we have found human stem cells that rapidly rebuild a blood system in SCID mice. The potential," he continued, "is that it may allow transplant patients to quickly regain their blood cells, which are critical to their immune system."
Sparing Patients Lingering Infections
"The discovery," he went on, "could have far-reaching implications for cancer and transplant patients whose immune systems are weakened by their clinical treatments. These patients are very vulnerable to infection, for as long as three weeks after their therapy - until their blood system recovers enough to fight off risk of pathogenic invasion.
"Transplanting umbilical cord blood into the bone marrow," Dick said, "is an efficient method to identify and characterize stem cells. A major problem hampering effective stem cell-based therapies," he noted, "is the absence of a clear understanding of the human hematopoietic stem cell-pool composition. Our study also promises," Dick said, "to further knowledge about human stem cells, which are much less understood than mouse stem cells.
"NOD-SCID xenograft mouse models using IBMT [intrafemoral bone marrow transplant] procedures provide powerful tools for the detection of new classes of HSCs that are either rare, poorly detected using traditional intravenous-based assays, or from atypical sources, such as embryonic stem cells or nonhematopoietic tissues."
He described the procedure: "Briefly, after anesthetizing the mouse, the knee was flexed and 25 microliters or cells were injected with a 28.5-gauge needle through the joint into the right femur."
Dick summed up "that intrafemoral transplantation is more reliable and sensitive for rapid engraftment. Because the SRC [SCID repopulating cell] is quantitative, we would expect that intravenous injection would require tenfold more cells to generate early engraftment levels comparable to that achieved with intrafemoral transplantation. Overall, these results suggest the existence of R-SRCs," he concluded, "with the capacity to swiftly generate mature cells and progenitors of the myeloid and erythroid lineages."