Stem cells are cells not yet differentiated to make them into specific kinds of cells. The structure of the stem cells allows them to become any type of cell, from blood to bone. These unspecified cells have the capability to change through generations and become specialized cells. In fact, the two characteristics of a stem cell are the ability to self-renew and develop into different types of mature cells.
In the 1960's, McCulloch and Till injected bone marrow cells into the spleens of mice. "Spleen colonies" of cells arose. Later, Andy Becker, a graduate student joined the two and with the help of Lou Siminovitch, they not only published work on their original theory of stem cells, they correlated the evidence to show that marrow cells self-renewed. This was the final piece of evidence to prove their theory of stem cells. Stem cells reproduce for repair and replenishing the systems in adults but in embryos they change to all types of specialized cells.
The location of embryonic stem cells is in the blastocysts and develops approximately the fifth day after fertilization of the human egg. There are 70 to 150 cells in it. The inner part of the cell contains the embryoblast that eventually forms the embryo. The outer part of the blastocyst is the trophoblast that becomes the placenta. In adults, the stem cells are in the tissue.
The stem cell is like all other cells without the DNA turned on to make it differentiate into specific cells. It contains a cell membrane, the outer covering of the cell. It has cytoplasm, which is a watery solution that contains many different necessary gases and organic materials to sustain the life of the cell. Cell organelles on the interior of the cell are ribosomes that synthesize protein, endoplasmic reticulum that transport material in the cell, golgi apparatus which feed the exterior of the cell, mitochondria in the cytoplasm that produce the energy to feed the cell, lysomes that destroy debris, and centrioles which allow the cell to divide. Also in the cytoplasm is the nucleus of the cell that contains the DNA, RNA and a protein that forms ribosomes. In a stem cell, there is no cell differentiation yet triggered.
Stem cells have several places of origin. Embryonic stem cells come either from fertilized eggs left over after a couple conceived with in vitro fertilization. Umbilical stem cells come from the umbilical cord, and resemble those in the bone marrow. Stem cells also can be fetal stem cells from the tissue of an aborted fetus, located where the gonads would eventually exist. Placental stem cells are similar to umbilical stem cells but there's a more. Adult stem cells come from the various organs they replicate.
The DNA in the nucleus of a stem cell needs to have specific types of chemicals before it transforms one of the daughter cells into a differentiated cell. These chemicals with the pattern given by the DNA stimulate the mechanisms of the cell to divide into the differentiated cell.
While most theories revolve around the use of embryonic stem cells, the ASCB, the American Cancer Society for Cell Biology reported that adult stem cells also self renew in a mammal at their San Francisco meetings December 13, 2008. They transplanted adult stem cells and the differentiated descendants into mice with muscle tissue damage to the hind limbs. These cells restored the limbs and made them functioning. The cells were mixed satellite cells from the skeletal muscle that grow just beneath the muscle membrane.