An embryo is formed after sexual reproduction, when a male's sperm fertilizes a females's egg to form a single cell, a zygote. This cell undergoes a series of cell division, forming two, four, eight cells and so on (Figure 1.3). After four to six days, a mass of cells is produced, which is called a blastocyst. It consists of an inner cell mass (ICM, embryoblast) and an outer cell mass (trophoblast), forming the embryo and placenta respectively after implantation to the uterus. ICM is the source of embryonic stem cells, which are totipotent (i.e. they have total potential to develop into any cell type in the body). In normal pregnancy, the blastocyst stage continues until implantation in the uterus, when it is called a fetus. This usually occurs at the end of the 10th week after fertilization, at which stage most of the major organs of the fetus have been created.
Source: Saxena, Singh and Gupta 2010
Embryonic stem cells can be harvested from a blastocyst of a four to five day old human embryo, which is usually created by in vitro fertilization (IVF). These embryos are usually donated by the IVF clinics where several eggs are fertilized in a test tube, and are not implanted into a woman. Most of these cells are completely unspecialized.
The extracted stem cells are then transferred to a culture dish containing a nutrient-rich broth and anchor medium. Without the signal or stimulation to differentiate, the cells begin to divide and replicate, maintaining an unspecialized state in the body. These undifferentiated cells can then be stimulated to create specialized cells under suitable conditions. The manipulation of the conditions or factors in differentiation is one of the most important areas for further investigation, because this is the key step in creating specific replacement cells, tissues or organs for therapeutic uses.
Activity 2 introduces how the inner cell mass (ICM) cells of blastocyst-stage early human embryos removed and cultured to create a cell line. These cells can be grown in a controlled environment indefinitely. Various growth factors induce the differentiation of these cells to develop into cells with a specific function, such as muscle or nerve cells.
At this point, test your understanding of embryonic stem cells with Self-test 2.
Watch the following video animation.
Then answer the following questions:
- What has been added to the culture environment to help the growth of stem cells?
- What kinds of signals trigger the differentiation of stem cells? Name two of them.
Activity 2 feedback
- Mouse embryonic fibroblast
- Growth factors, sonic hedgehog and retinoic acid
- At what stages of human development can the embryonic stem cells be found?
- Name one common source of embryonic stem cells for research.
Self-test 2 feedback
- Embryonic stem cells can be found at the blastocyst stage, 4–6 days after fertilization.
- A blastocyst of a human embryo is created by in vitro fertilization (IVF).