All Things Stem Cell

Salamanders have the amazing ability to re-grow a limb after it has been cut off. It is thought that by better understanding this regenerative ability, researchers will be able to apply this knowledge to humans and improve wound healing. Recently it was reported that salamander limb regeneration may occur in a different way than was previously thought; in short, the severed limb may not need pluripotent stem cells to regenerate, as was believed, but only multipotent or unipotent stem cells, stem cells with relatively restricted fates.

In salamanders, when a limb is severed the resultant limb bud undergoes a distinct process to regenerate the lost limb. The epithelial layer quickly spreads across the amputation site, closing the wound within 24 hours (Mescher, 1996). This epithelial layer thickens and becomes what is referred to as the wound epithelium (WE). As the immune system responds to the injury, macrophages and neutrophils arrive to clean up the wound site beneath the WE. The existing injured tissues and cells are broken down as well as the extracellular matrix, which is made up of proteins that surround cells to hold them together and stimulate normal cellular functions. It was thought that at this time in the regenerative process other resident cells below the WE become multipotent mesenchymal stem cells (MSCs) (see Figure). These eventually form a mass of MSCs called a blastema (Mescher, 1996; Brockes and Kumar, 2005). The blastema was thought to contain a homogenous group of pluripotent stem cells that had “dedifferentiated” or “redifferentiated,” meaning they had reverted back from their committed fates to function as very potent stem cells in order to recreate the limb. The WE stimulates the cells in the blastema to proliferate, making new cells and extracellular matrix, though more than is required for simple repair; the WE signals the blastema cells to regenerate the entire lost limb (Mescher, 1996; Kragl et al., 2009).

Limb regeneration in the salamander after limb amputation (time course going from the top down). Shortly after the limb is amputated, the epithelium layer covers the exposed limb bud, forming the wound epithelium (WE). A group of stem cells collects below this layer, forming the blastema (at the tip of the bud). The WE signals the stem cells below it to rebuild the limb, recreating the limb from the point of injury out towards the hand. The final regenerated limb is indistinguishable from the original.

Read more…

admin Mesenchymal Stem Cells, Non-Mammalian Stem Cells history, news, regenerative medicine

Human embryonic stem cells (hESCs) recently celebrated the 10th anniversary of their discovery, and in the decade since their isolation they have possibly received more press coverage, both over their many potential applications as well as ethical concerns, than any other type of stem cell. In the last decade, much progress has been made in better understanding these cells and their capabilities. hESCs hold much promise not only for being cellular models of human development and function, but also for use in the field of regenerative medicine. However, due to ethical and application concerns, only recently have these cells made it to clinical trials.

Figure 1: The Blastocyst. Human embryonic stem cells are isolated from early-stage embryos in the late blastocyst stage, about four or five days after fertilization. The blastocyst is a hollow sphere made of approximately 150 cells and contains three distinct areas: the trophoblast, which is the surrounding outer layer that later becomes the placenta, the blastocoel, which is a fluid-filled cavity within the blastocyst, and the inner cell mass, also known as the embryoblast, which can become the embryo proper, or fetus, and is where hESCs are isolated from.

Though human embryonic stem cells were isolated just over a decade ago, embryonic stem cells were successfully isolated from other animals before this. Nearly 30 years ago, two groups independently reported the isolation of mouse embryonic stem cells (mESCs) (Martin, 1981; Evans and Kaufman, 1981). The mESCs were isolated from early-stage mouse embryos, approximately four to six days post-fertilization (out of 21 days total for mouse gestation). At this point in development, the embryo is in the late blastocyst stage (see figure 1). It was not until the mid-1990s that this feat was accomplished with non-human primates by Dr. James Thomson’s group (Thomson et al., 1995). Only a few years later, embryonic stem cells isolated from humans, once again by Thomson’s group, in 1998 (Thomson et al., 1998).

It is important to understand where hESCs come from in order to understand the ethical arguments that surround them, as well as their enormous, innate biological potential. Like mESCs, hESCs are isolated from early-stage embryos that are, specifically, in the late blastocyst stage, about four or five days after fertilization. After the fertilized egg cell starts cell division, what is referred to as the “blastocyst” occurs once the cell has divided into a hollow sphere made up of approximately 150 cells (see figure 1). At this point, the embryo has not even yet been implanted in the uterus. The blastocyst contains three distinct areas: the trophoblast, which is the surrounding outer layer that later becomes the placenta, the blastocoel, which is a fluid-filled cavity within the blastocyst, and the inner cell mass, also known as the embryoblast, which can become the embryo proper, or fetus. Embryonic stem cells can be created from cells taken from the inner cell mass (Stem Cell Basics: What are embryonic stem cells?, 2009). Because these cells are taken from such an early stage in development, they have the ability to become cells of any tissue type (except for the whole embryo itself), making them pluripotent. The pluripotency of hESCs is probably the trait that contributes most to their enormous potential, both as models of cell function and human development and, potentially, for uses in regenerative medicine. Being pluripotent and seemingly unlimited in supply separates hESCs from adult stem cells, which are multipotent or unipotent, able to become a more select group of cell types, and more limited in their cellular lifespan.

Read more…

admin Embryonic Stem Cells clinical trials, embryonic, history, news, regenerative medicine

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.

The uterus is lined by a layer of cells called the endometrium. During the menstrual cycle, the endometrium cycles between thickening and being broken down if fertilization does not occur. The break down and expulsion of the endometrium is called menstruation, or menstrual bleeding, and is the source of endometrial regenerative cells (ERCs).

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).

Read more…

admin Endometrial Regenerative Cells adult, clinical trials, embryonic, hematopoietic, mesenchymal, news, regenerative medicine

Perhaps containing more different cell types than any other stem cell category, mesenchymal stem cells (MSCs) can be isolated from a wide variety of tissues in the human body. These cells have been grouped and labeled as “mesenchymal” because they are thought to have a common progenitor in the mesenchyme, an embryonic tissue (Caplan, 2005). In the developing vertebrate embryo, there are three distinct “germ layers,” or layers of cells: the endoderm, the mesoderm, and the ectoderm. Together with the germ cells, these three layers pattern out the entire body (see figure). The mesenchyme is a collection of cells mostly derived from the mesoderm that later becomes supportive structures throughout the body, including bone, cartilage, connective tissue, smooth muscle, adipose tissue, as well as the lymphatic and hematopoietic systems. Most MSCs are thought to contain progenitors in the mesenchyme (Gilbert, 2003; Conrad et al., 2009; Caplan, 2005).

The endoderm layer later becomes skin (epidermis) and the nervous system, the ectoderm becomes the digestive tract and respiratory system, and the mesoderm becomes bone, blood, muscles, connective tissue, and several organs (heart, kidney, and gonads).

However, calling MSCs “mesenchymal” can be misleading. Because this term refers to a precursor of the large MSC family, it is referring to an embryonic tissue, though the descendant MSCs can be found in both fetal and adult tissues. MSCs have been isolated from adult muscle, bone marrow, adipose tissue, cartilage, bone, potentially teeth (Caplan, 2005) as well as some fetal tissues (fetal liver, lung, amniotic fluid, and umbilical cord) (Phinney and Prockop, 2007). The MSCs isolated from any one of these tissues are multipotent and are usually shown to be MSCs by being able to differentiate into at least three different, standard mesenchymal cell types: osteocytes (bone), chondrocytes (cartilage), and adipocytes (fat) (Baksh et al., 2004). There is much evidence, though somewhat inconsistent, showing that MSCs can also differentiate into neuronal cells, which may be from mesenchyme derived from the endoderm instead of the mesoderm (Gilbert, 2003; Phinney and Prockop, 2007). Overall, MSC differentiation potentials can vary depending on what mesenchyme-derived tissue the MSCs were harvested from (Phinney and Prockop, 2007). However, MSCs cannot become hematopoietic cells (which are derived from hematopoietic stem cells), even though these cells are derived from the mesenchyme, making the label “mesenchymal” more deceptive (Gilbert, 2003; Caplan, 2005).

Read more…

admin Mesenchymal Stem Cells adult, embryonic, hematopoietic, history, mesenchymal, regenerative medicine

The tooth is made up of four primary components: dental pulp, dentin, enamel, and cementum (Cate, 1998)

A relatively recent addition to the stem cell family and holding promise as an easily obtained source of adult stem cells, dental pulp stem cells (DPSCs) were discovered at the beginning of this decade (Gronthos et al., 2000). DPSCs, also known as SHED cells (stem cells harvested from exfoliated deciduous teeth (Miura et al., 2003)), are one of the few discovered stem cells available from a naturally molted human tissue.

Dental pulp, which is living tissue at the center of the tooth made up of cells called “odontoblasts,” contains the aptly-named DPSCs. DPSCs were originally identified as being able to produce odontoblast-like cells as well as a tissue similar to dentin, which normally surrounds and protects the dental pulp (Gronthos et al., 2000; 2002). Although these studies found DPSCs able to produce two of the primary components of teeth, pulp and dentin, studies have not reported that DPSCs are able to create the other two factors, which are enamel and cementum (Cate, 1998).

Read more…

admin Dental Pulp Stem Cells adult, history, mesenchymal, neural crest, news, regenerative medicine