The aorta is the largest blood vessel in our bodies and it emerges from the left ventricle of the heart, takes a U-turn, and swings down toward the legs (descending or dorsal aorta). There are several branches of the aorta as it sharply turns that extend towards the head and upper extremities.
Sometimes, as a result of inflammation of the aorta or other types of problems, the elastic matrix that surrounds and reinforces the aorta breaks down. This weakens the wall of the aorta and it bulges out. This bulge is called an aortic aneurysm and it is a dangerous condition because the aneurysm can burst, which will cause the patient to bleed to death.
If an aneurysm is discovered through medical imaging techniques, drugs are given to lower blood pressure and take some of the pressure off the aorta. Also, drugs that prevent further degradation of the elastic matrix are also used. Ultimately, for large or fast-growing aneurysms, surgical repair of the aorta is necessary. For aneurysms of the abdominal aorta, a surgical procedure called abdominal aortic aneurysm open repair is the “industry standard.” For this surgery, the abdomen is cut open, and the aneurysm is repaired by the use of a long cylinder-like tube called a graft. Such grafts are made of different materials that include Dacron (textile polyester synthetic graft) or polytetrafluoroethylene (PTFE, a nontextile synthetic graft). The surgeon sutures the graft to the aorta, and connects one end of the aorta at the site of the aneurysm to the other end.
A “kinder, gentler” way to fix an aneurysm is to use a procedure called endovascular aneurysm repair (EVAR). EVAR uses these devices called “stents” to support the wall of the aorta. A small insertion is made in the groin and the collapsed stent is inserted through the large artery in the leg. Then the stent, which is long cylinder-like tube made of a thin metal framework and covered with various materials such as Dacron or polytetrafluoroethylene (PTFE), is inserted into the aneurysm. Once in place, the stent-graft will be expanded in a spring-like fashion to attach to the wall of the aorta and support it. The aneurysm will eventually shrink down onto the stent-graft.
In some cases, the patient is too weak for surgery, and is not a candidate for EVAR. A much better option would be to non-surgically repair the elastic support framework that surrounds the aorta, and stem cells are candidates for such repair.
To repair the elastic mesh work that surrounds the wall of the aorta, smooth muscle cells that secrete the protein “elastin” must be introduced into the wall of the aorta. Also, using the patient’s own stem cells offers a better strategy at this point, since this circumvents such issues as immune rejection of implanted tissues and so on. The sources of stem cells for smooth muscle cells include bone marrow stem cells, fat-based stem cells, and stem cells from peripheral blood. All three of these stem cell sources have problems with finding enough cells in the body and expanding them to high enough numbers in order to properly treat the aneurysm.
Fortunately, the use of induced pluripotent stem cells, which are made from a patient’s mature cells and have many, though not all of the characteristics of embryonic stem cells, can provide large quantities of elastin-secreting smooth muscle cells. Also, one laboratory in particular has reported differentiating human induced pluripotent stem cells into smooth muscle cells (Lee TH, Song SH, Kim KL, et al. Circ Res 106:120–128). While there are challenges to making functional elastin, there are possibilities that many of these can be overcome.
In addition to induced pluripotent stem cells, other laboratories have examined umbilical cord mesenchymal stem cells and their ability to decrease the inflammation within the aorta that leads to aneurysms. The researchers discovered that all the indicators of inflammation decreased, but the synthesis of new elastin was not examined. However, a Japanese laboratory used mouse mesenchymal stem cells from bone marrow and found that not only did these cells shut down enzymes that tend to degrade elastin, but also initiated new elastin synthesis in culture. The same study also showed that MSCs implanted into the vessel walls of an aorta that was experiencing an aneurysm stabilized the aneurysm by inhibiting the elastin-degrading enzymes, and increasing the elastin content of the vessel wall. This had the net effect of stabilizing the aneurysms and preventing them from growing further (see Hashizume R, Yamawaki-Ogata A, Ueda Y, et al. J Vasc Surg 54:1743–1752).
These experiments show that stem cell treatments for abdominal aneurysms are feasible and would definitely be a much-needed nonsurgical treatment option for the high-risk elderly demographic, which is rapidly growing in the developed world.
For more information on this interesting topic, see Chris A. Bashura, Raj R. Raob and Anand Ramamurthia. Perspectives on Stem Cell-Based Elastic Matrix Regenerative Therapies for Abdominal Aortic Aneurysms. Stem Cells Trans Med June 2013 vol. 2 no. 6 401-408.