Stem cells, unlike their somatic cell counterpart, are self-renewing, highly proliferative, and capable of multilineage differentiation. The ultimate stem cell is made at conception. After fertilization, the zygote consists of totipotent stem cells that are able to form all three germ layers as well as placental tissue. After the zygote becomes a preimplantation blastocyst, the inner cell mass consists of pluripotent stem cells that will give rise to all three germ layers—ectoderm, mesoderm, and endoderm—and can no longer form placental tissues. At that stage, the stem cells are embryonic. After day 8, the cells become either somatic cells (terminally differentiated) or stem cells committed to a specific lineage (multipotent). After that point, the stem cells are considered adult derived despite their presence in fetal tissues. Local niches of lineage-committed multipotent stem cells remain in adult tissue throughout life for normal tissue remodeling and repair. With increasing age, the number, expansion potential, differentiation potential, and so-called potency of stem cells decline; therefore there is increasing interest in allogeneic embryonic and fetus-derived stem cells as well as banking of autologous stem cells from postnatal samples.

The initial enthusiasm for stem cells in regenerative medicine was related to the capacity for tissue-specific differentiation, in that stem cells implanted in a cartilage lesion would engraft, become chondrocytes, and produce cartilage matrix. As both scientific and clinical data accumulate, it appears that the effects of stem cell therapy may also be, largely or in part, a result of local production of bioactive molecules and immune modulation rather than tissue-specific differentiation and long-term engraftment of the implanted cells. What treatment effects stem cells actually impart is an important question. The answers will likely change what conditions are treated with stem cells and by which stem cell source, when the cells are applied and by which route, how often they are administered, and the number of cells used. To answer these questions, additional clinical and experimental studies are needed.

Because of the difficulty in isolation, expansion, and cryopreservation of equine embryonic stem cells, they have not been investigated in the horse for regenerative medicine and will not be discussed in this chapter. In contrast, adult-derived stem cells (nonembryonic) are generally considered to be safe and to carry little risk for tumor formation, are easy to isolate and expand, and have been used extensively in the horse. This chapter will focus on adult-derived stem cells. Modifications to these classifications of stem cells (embryonic and adult derived) are also being investigated and will be briefly discussed. One modification is a fetus-derived stem cell that has been manipulated in vitro to act more like an embryonic stem cell. One such product has been developed and tested in the horse, but commercial availability is pending U.S. Food and Drug Administration (FDA) approval. An important benefit of this type of stem cell is its immediate availability as an off-the-shelf product and its increased potency because of its pluripotent-like (embryonic-like) state. Another modification is the induced pluripotent stem (iPS) cell, in which in vitro manipulations are applied to adult somatic cells, such as skin fibroblasts, to dedifferentiate them and induce a stem cell–like state. The iPS cell is currently being investigated by several equine research groups.

Adult-derived mesenchymal stem cells (MSCs) are considered an excellent stem cell source for musculoskeletal regenerative therapies because they are readily available from several tissues, allow for use of autologous cells as well as allogeneic cells because of immune tolerance to non-self MSCs, and are of mesodermal lineage and thus able to differentiate into cartilage, tendon, and bone. The immune-privileged status of MSCs may be in part a result of their lack of expression of major histocompatibility complex class II proteins and most of the classic costimulatory molecules of antigen-presenting cells. Recent evidence also suggests that in addition to being immune privileged, MSCs are immune modulatory, through secretion of chemoattractants followed by regulation of immune cell (T and B cells) activation. Finally, MSCs may also be antiinflammatory through inhibition of interferon-γ and tumor necrosis factor-α and stimulation of metalloproteinase inhibitors and antiinflammatory interleukins, such as interleukin-10. The most exciting ele­ment of the MSC is an exquisite responsiveness to their microenvironment, in that the cells behave according to the environment in which they are placed. In this manner, MSCs would respond appropriately to the degree of disease and modulate the local environment in favor of some combination of reduced inflammation, reduced apoptosis, and enhanced matrix synthesis of endogenous progenitors and tissue-specific cells.