It is now very clear that adult stem cells exhibit remarkable plasticity: thus muscle and CNS derived stem cells can give rise to haematopoietic stem cells.12,13
Bone marrow stem cells themselves can contribute to hepatocyte, biliary, endothelial, myogenic, glial, and renal lineages.1–11
Our present results show that an organised network of myofibroblasts, the pericryptal and lamina proprial meshwork, can receive a major contribution from the bone marrow. In our mouse studies there is no doubt that the marrow was the origin of the new ISEMFs, but in the human studies although donor bone marrow is the likely source of the pericryptal myofibroblasts, it cannot be unequivocally established due to lack of knowledge of the patient's genetic background.
While we believe that this is the first study to report that bone marrow stem cells can contribute to a normal organised system of myofibroblasts such as the ISEMFs, there have been several previous reports that have suggested that myofibroblasts can be derived from bone marrow. Thus Grimm and colleagues,21
also analysing sex mismatched transplants in humans, demonstrated that some 30% of kidney myofibroblasts were of extrarenal origin. While bone marrow was not specifically documented as the origin of these cells, our own observations11
strongly suggest that renal interstitial myofibroblasts can be so derived. Other observations have recorded that myofibroblasts will differentiate from human bone marrow mesenchymal stem cells when they are exposed to colorectal carcinoma cell lines in vitro,22
and also when they are around foreign bodies introduced into the mouse peritoneal cavity.23
A further group of myofibroblasts in the intestine are the interstitial cells of Cajal. These cells are located close to mural neurones and act as pacemakers, propagating electrical events and modulating neurotransmission. They are said to be SMA positive and desmin positive, and are also immunopositive for c-kit and CD45.19
We have identified Y chromosome positive cells in the muscularis layers of the female mouse intestine (data not shown), the phenotype of which will be determined by further immunohistochemical investigation.
Also included in the myofibroblast family are the stellate cells of the liver and pancreas.20
Such cells express muscle and non-muscle genes, are contractile, and secrete a number of growth factors and cytokines, and of course synthesise extracellular matrix proteins.19,20
Their proliferation is dependent on platelet derived growth factors (PDGFs) and their receptors: indeed PDGF-AA and PDGFR-α knockout mice display defective ISEMFs and profound gastrointestinal abnormalities.24
An interesting possibility would be whether a bone marrow transplant might at least partially ameliorate these defects: Lagasse and colleagues5
were able to correct the metabolic defect in fumaryl acetoacetate hydrolase−/− mice by such a procedure—the transplanted bone marrow stem cells transdifferentiated into functional hepatocytes.
Krause and colleagues25
showed that a purified bone marrow stem cell population engrafted multiple organs, including lung and epithelial cells of the gut and skin. However, they did not report that the pericryptal sheath was so colonised; the absence of bone marrow stromal stem cells in the graft could account for this as we have transfused whole bone marrow. The origin of these engrafted cells could then be the stromal stem cells although while disputed, it is generally said that in humans transplanted stromal cells do not survive26
and definitive proof must await the transplantation of defined populations of stromal and haematopoietic stem cells. However, it is interesting to speculate that the engrafted cells come from the population of circulating “fibrocytes” which, in the human, form between 0.1% and 0.5% of the non-red blood cell population27
and which migrate to skin wounds in mouse and humans. These cells are certainly SMA positive in the wounds and express collagen I and III, fibronectin, CD45RO, CD13, and CD34. It is possible that these cells derive from the transplanted male marrow cells, enter the circulation, and colonise the intestine.
Our observations indicate that bone marrow, albeit in circumstances surrounding a bone marrow transplant, can contribute to the turnover of the pericryptal myofibroblast sheath. Following irradiation, there is prominent loss of epithelial cells while pericryptal myofibroblasts disintegrate within 24 hours in the rat, as evidenced by loss of cadherin and SMA staining.28
In humans, the pericryptal fibroblast count falls on a time scale very closely associated with that seen in epithelial cells, with only some immediate recovery occurring in fibroblasts, in contrast with the adjacent epithelial cells where full recovery seems to take place. Moreover, fibroblasts showed a gradually diminishing trend during the first year while the epithelial cell numbers appeared to be maintained.29
Thus in both the mouse and human situation, bone marrow cells are apparently replacing cells killed by whole body irradiation.
However, in human graft versus host disease, while there was no doubt that myofibroblasts were derived from the transplanted bone marrow, these were far fewer than in the mouse. Moreover, we experienced considerable difficulty in accurately quantifying the number of Y positive cells in control human tissue and therefore calculation of a correction factor was inappropriate.
We observed lines of Y positive cells (fig 1D–G), highly suggestive evidence of clonal expansion, possibly with the bone marrow stem cells initially seeding the lamina propria near the base of the crypt and ISEMFs proliferating and migrating from here. There has been speculation that ISEMFs migrate parri passu
with the crypt epithelial cells.17
Our observations are consistent with bone marrow cells engrafting in the crypt base and migrating upwards into the upper crypt and villi, as Y chromosome positive myofibroblasts were present in the villi at two and six weeks but not at seven days. In the column of myofibroblasts that were positive for the Y chromosome, some did extend upwards into the villus; this might suggest that migration was continuous, although Neal and Potten17
suggested that these cells moved laterally into the lamina propria at the top of the crypt, and became polyploid. We certainly did not see any SMA positive cells with multiple Y chromosomes, unlike the bone marrow derived polyploid hepatocytes we have encountered.30
Alternatively, it may be simply that a number of cells entered locally or have occupied particular niches. In the patients studied here, there was wide variation in the proportions of myofibroblasts derived from extraintestinal sources; this may reflect differences in the degree of intestinal damage and also technical factors such as tissue fixation.
In the intestine, myofibroblasts form a continuous meshwork extending from the adventitia of the submucosal blood vessels through the lamina propria to the pericryptal sheath.16
Our observations indicate that all of these cells can be derived from bone marrow.
The pericryptal myofibroblast sheath is closely associated with the epithelial cells of its crypt and is thought to influence epithelial behaviour by producing a number of growth factors. Stem cells within many different tissues are thought to reside within “niches” or groups of cells and extracellular substrates which provide an optimal microenvironment for stem cells to give rise to their differentiated progeny. ISEMFs surround the base of each crypt, a commonly proposed location for the intestinal stem cell niche, and it is well documented that ISEMFs influence epithelial cell proliferation and regeneration through epithelial:mesenchymal cross talk, and ultimately determine epithelial cell fate. Our observations suggest that bone marrow cells could be used as vehicles for gene delivery to the gut, to modify both myofibroblast and epithelial cell behaviour.
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