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Stem Cells

Author(s): Joseph G. Marx, Ph.D

What is An Embryonic Stem Cell?

Embryogenesis is the process by which a single cell, formed from the union of the nucleus from a sperm and an egg, produces a complete organism. The fertilized egg is said to be totipotent, meaning that it has the potential to generate all the specialized cells and tissues of the body, as well as the tissues for its development in the uterus.

The DNA within the fertilized egg instructs the cell to divide (usually 24 - 36 hours after fertilization). The cell divides into two cells, and then each cell divides again, producing four cells. This process continues for about five to six days. At that point, the single cell has become a hollow ball of cells, called a blastocyst, that has not formed an attachment within the uterus. All mammals produce a hollow ball of cells during embryogenesis. The blastocyst is composed of two distinct cell types: 1) cells that will become the placenta; and 2) the inner cell mass. The inner mass of cells (in red) are stem cells. These cells are unique and described as pluripotent, since they retain the ability to read all information contained in the DNA within their nuclei and the capacity to generate cells from all three primary germ layers (mesoderm, ectoderm, and endoderm).

A very specific event occurs after about 5-6 days: the blastocyst changes very subtly and the cells in the inner cell mass lose their ability to read all of their DNA. In other words, cells are no longer pluripotent after this point in development.