The umbilical cord contains a major umbilical vein and an umbilical artery, but these blood vessels are embedded in a gel-like matrix called “Wharton’s jelly.” Wharton’s jelly is home to a population of mesenchymal stem cells that have peculiar properties.
You might first say, “what on earth is a mesenchymal stem cell?” Fair enough. Mesenchymal stem cells were first discovered in bone marrow. In bone marrow, mesenchymal stem cells (MSCs) do not make blood cells; that;’s the job of the hematopoietic stem cells (HSCs). MSCs in bone marrow serve an important support role for HSCs in bone marrow. Traditionally, MSCs have the capacity to differentiate into fat cells, bone cells, and cartilage cells. However, further has shown that MSCs can also form a variety of other cell types as well if manipulated in the laboratory. MSCs also express are characteristic cadre of cell surface proteins (CD10, CD13, CD29, CD44, CD90, and CD105 for those who are interested).
MSCs, however, are found in more places that just bone marrow. As it turns out, MSCs have been found in fat, muscle, liver, tendons, synovial membrane (the membranes that surround joints, skin, and so on. Some scientists think that every organ in the body may harbor a MSC population. Furthermore, these MSC populations differ in the genes they express, their capability to differentiate into different cell types, and their cell surface proteins (see this article on this website for a rather exhaustive foray into this topic).
Now that you are more savvy about MSCs, Wharton’s jelly contains a MSC population, but this population seems to have a younger profile than MSCs from other parts of the body. They are more plastic and more invisible to the immune system than other types of MSCs. For that reason, they might be good candidates for treating a sick heart after a heart attack. A recent paper by Wei Zhang and others from the TEDA International Cardiovascular Hospital and the Tianjin Medical Cardiovascular Clinical College examined the ability of MSCs from the Wharton’s jelly of human umbilical cords to heal the hearts of minipigs after a heart attack. Oh, before I forget – this paper was published in the journal Coronary Artery Disease.
Twenty-three minipigs were subjected to open-heart surgery and given heart attacks. Then the pigs were divided into three groups, a control group, a group that received injections of saline into their hearts, and a third group that received injections of 40 million human Wharton’s jelly derived MSCs into the region of the infarct. The animals were sewn up and given antibiotics to prevent infection.
Six weeks after surgery, each animal was examined by means of Technetium-sestamibi myocardial perfusion imaging, and electrocardiography. For those who do not know what Technetium-sestamibi myocardial perfusion imaging is for, it works like this. Cardiolite is the trade name of a large, fat-soluble molecule that flows through the heart in a fashion proportion to the blood flow through the heart muscle. Single photon emission computed tomography or SPECT is used to detect the Cardiolite. Areas of the heart without blood flow are the regions damaged during the heart attack. Therefore, this technique is extremely useful to determine the area of damage in the heart.
After the animals were examined, they were put down and their hearts were extracted, sectioned, and stained for areas or cell death, and the areas where the injected stem cells resided. All injected stem cells were labeled before injection so that they were easily detectable.
The results were clear. The heart injected with MSCs from umbilical cord did not show any decrease in ejection fraction, whereas the other two groups showed an average reduction in injection fraction of around 10%. In fact the stem cell-injected hearts showed an average 1 % increase in ejection fraction. The blood flow in the hearts was even more different. blood flow is measured as a ratio of dead heart tissue to total heart tissue. The control of saline-injected hearts had an average ratio of about 4%, whereas the stem cell-injected hearts had a slightly negative percentage. This is a significant difference. Echocardiography confirmed that the wall thickness of the stem cell-injected hearts was significantly thicker than the walls of the control or the saline-injected hearts; some 14 times thicker!!
When the dissected hearts were examined, the MSC-injected hearts had lots of stem cells still in them. The cells not only survived, but, according to Zhang and his colleagues, differentiated into heart muscle cells. Their rationale for this conclusion is three-fold – the cells had the same shape and form or native heart muscle cells, they expressed heart specific Troponin T and vWF proteins, and electrically coupled with other heart muscle cells by expressing connexin. Connexin is a protein that traverses the membranes of two closely apposed cells and forms small pores between two cells that allows the exchange of SMALL molecules such as ions, ATP, and things like that. These connexin constructed pores are called “gap junctions” and they are the reason heart muscle cells work as a single unit, since any electrochemical change in one cell immediately spreads to all other nearby, connected cells.
As much as I would like to believe Zhang and his colleagues, I remain skeptical that these cells differentiated into heart muscle cells. I say this because MSCs can be differentiated in culture to form cells that look and act like heart muscle cells. These cells will even express some heart-specific genes. However, they lack the calcium handling machinery of true heart muscle cells and do not function as true heart muscle. To convince that these Wharton jelly MSCs truly are heart muscle cells, they will need to show that they contain heart specific calcium handling proteins (see Shake JG, Gruber PJ, Baumgartner WA et al. Ann Thorac Surg 2002;73:1919–1925; Davani S, Marandin A, Mersin N et al. Circulation 2003;108(suppl 1):II253–258; Hou M, Yang KM, Zhang H et al. Int J Cardiol 2007;115:220 –228). If they can show this, then I will believe them.
However, there are two findings of this paper that are not in doubt. The number of blood vessels in the hearts of the MSC-treated animals far exceeded the number found in the control or the saline-treated hearts (3-4 times the number of blood vessels). Therefore, the Wharton’s jelly MSCs induced lots and lots of blood vessels. Many of these blood vessels contained labeled cells, which shows that the MSCs differentiated into endothelial and smooth muscle cells, Also, the Wharton’s jelly MSCs clearly induced resident cardiac stem cell (CSC) populations in the hearts of the minipigs, since several cells that expressed CSC surface molecules were found in the heart muscle tissue. Previous work by Hatzistergos and others showed that MSCs induce the endogenous CSC population and this is one of the ways that MSCs help heal ailing hearts (Circulation Research 2010 107:913-22).
Zhang’s paper is interesting and it shows that Wharton’s jelly MSCs are safe and efficacious for treating the heart after a heart attack. Also, none of the minipigs in this experiment were treated with drugs to suppress the immune system. No immune response against the cells was reported. Therefore, the invisibility of these cells to the immune system seems to last, at least in this experiment.