Stem cells are unspecialized cells capable of forming, through cell division, either another stem cell, or a cell of a specific organ or tissue. Stem cells are responsible for the original creation of the body’s different organs and tissues during development, and also serve to regenerate and repair tissues in adults. Because of the regenerative properties of these cells, they have great potential in human health and disease treatment. Stem cells studied in the laboratory are generally one of three types:
- Embryonic stem cells: Stem cells derived from embryos. These embryos usually come from eggs that are fertilized in a laboratory. These cells can differentiate into any cell of the body.
- Adult (somatic) stem cells: Undifferentiated cells within a specialized organ or tissue. These cells usually function to repair or regenerate the organ in which they are found.
- Induced pluripotent stem cells: Adult cells that have been reprogrammed to act like embryonic stem cells, making them capable of dividing into any cell type.
New avenues of stem cell research involve the introduction of induced human pluripotent stem cells into animal embryos. Animals containing human cells are called ‘chimeras’. This type of research could allow for the modeling of organ function and human disease in animals. It also has the potential for growing human organs in animals for transplantation. Ethical concerns arise from this type of research as introduction of foreign cells into the animal could potentially affect other organ systems of the receiving animal, including the nervous system and the brain.
The timing of stem cell introduction into animal embryos is highly restricted. The process of cell division and differentiation that occurs after fertilization of an egg is called embryogenesis. Fertilization of an egg results in a single fused cell called a zygote. The first ten days of development is known as the germinal stage. These cells divide and compact, eventually forming a hollow ball of cells known as a blastocyst. The blastocyst has two layers: tightly compacted outer cells, called trophoblasts, and an inner cell mass, called embryoblasts. The inner cell mass consists of pluripotent cells. Following the blastocyst stage, these pluripotent cells begin to differentiate to form the germ layers of the body during gastrulation. There are three germ layers that form during gastrulation:
- Ectoderm: Develops into the skin and nervous system;
- Mesoderm: Develops into muscle, cartilage, blood, bone and connective tissue; and
- Endoderm: Develops into the digestive and respiratory systems.
Introduction of stem cells during gastrulation would allow the cells to divide and differentiate during gastrulation, effectively integrating into the receiving animal. Following gastrulation, organogenesis occurs during which time cells of the germ layers differentiate to form specific organs and tissues.
Certain cells are set aside during initial embryonic cleavage that will form gametes (eggs or sperm). These cells can be passed on to offspring and therefore would contribute to the germ line.