Key Terms: Stem Cells

blastocyst- the term for a fertilized egg which, after approximately five days, has developed into a hollow, fluid-filled sphere of around 150 cells. It contains an inner cell mass of pluripotent cells. Embryonic stem cells used for research are cultured from these pluripotent cells.

embryonic stem cell- a non-specialized cell that can divide indefinitely and eventually give rise to all the cell types in the body.

multipotent cells- unspecialized cells found in certain tissues that can renew themselves and naturally differentiate into other specialized cells. However, the kind of cell they can become is limited to those of the tissue or organ in which they are found. For example, multipotent cells in the bone marrow can become several different kinds of blood cells, but they cannot become skin cells or brain cells. Multipotent cells are often referred to as “adult" stem cells.

stem cell plasticity- “adult” stem cells from one kind of tissue creating cells for another kind of tissue (i.e., brain stem cells becoming blood cells). So far, this has only worked in limited circumstances and is difficult to achieve.

pluripotent cells- cells with the potential to develop into almost any kind of cell, but not capable of creating an entire organism (unlike totipotent cells, they cannot form a placenta or other cells necessary for the development of a fetus). A few days after an egg is fertilized, the totipotent cells begin to specialize and form a blastocyst. The specialized cells within the blastocyst are then known as pluripotent cells, and they continue to form the rest of the components of the human body. Embryonic stem cells are pluripotent cells.  

totipotent cells- cells that can develop into any kind of cell, with the potential to create an entire organism. A zygote, or a female egg cell fertilized by a male sperm cell, is a totipotent cell, as are the cells that are created as the zygote continues to divide in the very early stages of development.