Often the feasibility of using stem cells for regenerative therapies is limited by two factors: obtaining a significant number of cells and doing so in a relatively noninvasive manner. Because our bodies freely shed a limited and select number of cells, many stem cell types must be obtained using a rather invasive procedure. However, around the beginning of last year two laboratories independently reported the discovery of a new type of stem cell that may overcome both obstacles; stem cells were found to reside in menstrual blood (Meng et al., 2007; Patel et al., 2008). These stem cells, termed endometrial regenerative cells (ERCs), are not only harvested in a noninvasive manner and relatively readily available in large quantities, but they potentially overcome the problem of immune rejection in many female patients as well.
Researchers suspected stem cells to be present in menstrual blood because stem cells were previously found to be present in the lining of the uterus. The wall of the uterus is lined by a layer of cells called the endometrium (see figure). To create ideal conditions for the uterus to accept and nurture an embryo, the endometrium lining becomes thicker and increases the number of blood vessels and glands within it. However, if implantation does not occur, the endometrium lining is broken down and shed. Overall, the endometrium is quite a hyperproliferative tissue, continuously being broken down and rebuilt; it is an ideal tissue to investigate for the presence of stem cells. In the menstrual cycle, the shedding is known as menstruation, or menstrual bleeding; the excreted menstrual blood is made up of blood as well as cells from the endometrium layer. Researchers previously reported the presence of stem cells in the intact endometrium lining of the uterus (Cho et al., 2004; Schwab et al., 2005; Du and Taylor, 2007). Because stem cells were found in the endometrium, researchers thought it likely that stem cells could also be found in the shed endometrium in the form of menstrual blood, which can be obtained in relatively large quantities in a much less invasive manner. However, the stem cells discovered in menstrual blood, ERCs, appear to be rather different from stem cells derived from the intact endometrium.
While stem cells from the intact endometrium appear to be mesenchymal stem cells (MSCs, as discussed earlier), ERCs do not; they are distinctly different not only in their undifferentiated state, but in the cells they can differentiate into as well. Researchers categorize stem cells into certain groups based off of, among other factors, their cell morphology and the proteins they express. An established stem cell group usually expresses a distinct set of proteins. ERCs, though morphologically appearing mesenchymal, were found to express only some, but not all, proteins characteristic of MSCs. Additionally, ERCs were reported to be able to differentiate into, or become, cells from the three different germ layers (see the previous post on MSCs for more details): mesoderm (muscle, bone, fat, cartilage, and endothelial cells), ectoderm (neurons), and endoderm (liver, pancreas, and lung cells) (Meng et al., 2007; Patel et al., 2008). However, the mesenchymal stem cells from the intact endometrium cannot generate cells from all three germ layers. Overall, ERCs were determined to be functionally distinct from endometrium MSCs (Meng et al., 2007; Hida et al., 2008).