Stem cells: Cells that are able to (1) self-renew (can create more stem cells indefinitely) and (2) differentiate into (become) specialized, mature cell types.

Embryonic stem cells: Stem cells that are harvested from a blastocyst. These cells are pluripotent, meaning they can differentiate into cells from all three germ layers.

Embryonic stem cells are isolated from cells in a blastocyst, a very early stage embryo. Once isolated from the blastocyst, these cells form colonies in culture (closely packed groups of cells) and can become cells of the three germ layers, which later make up the adult body.

Adult stem cells (or Somatic Stem Cell): Stem cells that are harvested from tissues in an adult body. These cells are usually multipotent, meaning they can differentiate into cells from some, but not all, of the three germ layers. They are thought to act to repair and regenerate the tissue in which they are found in, but usually they can differentiate into cells of completely different tissue types.

Adult stem cells can be found in a wide variety of tissues throughout the body; shown here are only a few examples.

The Three Germ Layers: These are three different tissue types that exist during development in the embryo and that, together, will later make up the body. These layers include the mesoderm, endoderm, and ectoderm.

The three germ layers form during the gastrula stage of development. The layers are determined by their physical position in the gastrula. This stage follows the zygote and blastocyst stages; the gastrula forms when the embryo is approximately 14-16 days old in humans.

Endoderm: One of the three germ layers. Specifically, this is the inner layer of cells in the embryo and it will develop into lungs, digestive organs, the liver, the pancreas, and other organs.

Mesoderm: One of the three germ layers. Specifically, this is the middle layer of cells in the embryo and it will develop into muscle, bone, blood, kidneys, connective tissue, and related structures.

Ectoderm: One of the three germ layers. Specifically, this is the outer layer of cells in the embryo and it will develop into skin, the nervous system, sensory organs, tooth enamel, eye lens, and other structures.

Differentiation, Differentiated: The process by which a stem cell turns into a different, mature cell. When a stem cell has become the mature cell type, it is called differentiated and has lost the ability to turn into multiple different cell types; it is also no longer undifferentiated.

Directed differentiation: To tightly control a stem cell to become a specific mature cell type. This can be done by regulating the conditions the cell is exposed to (i.e. specific media supplemented with different factors can be used).

The differentiation of stem cells can be controlled by exposing the cells to specific conditions. This regulation can cause the cells to become a specific, desired mature cell type, such as neurons in this example.

Undifferentiated: A stem cell that has not become a specific mature cell type. The stem cell holds the potential to differentiate, to become different cell types.

Potential, potency: The number of different kinds of mature cells a given stem cell can become, or differentiate into.

Totipotent: The ability to turn into all the mature cell types of the body as well as embryonic components that are required for development but do not become tissues of the adult body (i.e. the placenta).

A totipotent cell has the ability to become all the cells in the adult body; such cells could theoretically create a complete embryo, such as is shown here in the early stages.

Pluripotent: The ability to turn into all the mature cell types of the body. This is shown by differentiating these stem cells into cell types of the three different germ layers.

Embryonic stem cells are pluripotent cells isolated from an early stage embryo, called the blastocyst. These isolated cells can turn into cells representative of the three germ layers, all the mature cell types of the body.

Multipotent: The ability to turn into more than one mature cell type of the body, usually a restricted and related group of different cell types.

Mesenchymal stem cells are an example of multipotent stem cells; these stem cells can become a wide variety, but related group, of mature cell types (bone, cartilage, connective tissue, adipose tissue, and others).

Unipotent: The ability to give rise to a single mature cell type of the body.

Tissue Type: A group of cells that are similar in morphology and function, and function together as a unit.

Mesenchyme Tissue: Connective tissue from all three germ layers in the embryo. This tissue can become cells that make up connective tissue, cartilage, adipose tissue, the lymphatic system, and bone in the adult body.

Mesenchyme tissue can come from all three of the germ layers (ectoderm, mesoderm, and endoderm) in the developing embryo, shown here at the gastrula stage. The mesenchyme can become bone, cartilage, connective tissue, adipose tissue, and other components of the adult body.

Hematopoietic Stem Cells: Stem cells that can create all the blood cells (red blood cells, white blood cells, and platelets). These stem cells reside within bone marrow in adults and different organs in the fetus.

Hematopoietic stem cells can become, or differentiate into, all the different blood cell types. This process is referred to as hematopoiesis.

Bone marrow: Tissue within the hollow inside of bones that contains hematopoietic stem cells and mesenchymal stem cells.

Development: The process by which a fertilized egg (from the union of a sperm and egg) becomes an adult organism.

Zygote: The single cell that results from a sperm and egg uniting during fertilization. The zygote undergoes several rounds of cell division before it becomes an embryo (after about four days in humans).

When an egg is fertilized by a sperm, the resultant single cell is referred to as a zygote.

Blastocyst: A very early embryo (containing approximately 150 cells) that has not yet implanted into the uterus. The blastocyst is a fluid-filled sphere that contains a group of cells inside it (called the inner cell mass) and is surrounded by an outer layer of cells (the trophoblast, which forms the placenta).

The blastocyst contains three primary components: the inner cell mass, which can become the adult organism, the trophoblast, which becomes the placenta, and the blastocoele, which is a fluid-filled space. The blastocyst develops into the gastrula, a later stage embryo.

Inner Cell Mass: A small group of cells that are attached inside the blastocyst. Human embryonic stem cells are created from these cells in blastocysts that are four or five days post-fertilization. The cells from the inner cell mass have the potential to develop into an embryo, then later the fetus, and eventually the entire body of the adult organism.

Cells taken from the inner cell mass of the blastocyst (a very early stage embryo) can become embryonic stem cells.

Embryo: The developing organism from the end of the zygote stage (after about four days in humans) until it becomes a fetus (until 7 to 8 weeks after conception in humans).

Models: A biological system that is easy to study and similar enough to another, more complex system of interest so that knowledge of the model system can be used to better understand the more complex system. Such systems can include cells and whole organisms.

Model organism: An organism that is easy to study and manipulate and is similar enough to another organism of interest so that by understanding the model organism, a greater understanding of the other organism may be gained. For example, rats and mice can be used as model organisms to better understand humans.

Shown are several different model organisms frequently used in laboratory studies.

Severe Combined Immune-Deficient (SCID) mouse: A mouse lacking a functional immune system, specifically lacking or abnormal T and B lymphocytes. This is due to inbreeding or genetic engineering. They are extensively used for tissue transplants, because they lack an immune system to reject foreign substances, and for studying an immunocompromised system.

Cellular models: A cell system that can be used to understand normal, or diseased, functions that the cell has within the body. By taking cells from the body and studying them outside of the body, in culture, different conditions can be manipulated and the results studied, whereas this can be much more difficult, or impossible, to do within the body.

Stem cells obtained from different tissues of the body can be used as models to study normal, or diseased, cells in these tissues.

Cell Types:

Somatic Cell: Any cell in the body, developing or adult, other than the germline cells (the gametes, or sperm and eggs).

Gametes: The cells in the body that carry the genetic information that will be passed to the offspring. In other words, these are the germline cells: an egg (for females) or sperm (for males) cell.

Other terms:

Regenerative Medicine: A field of research that investigates how to repair or replace damaged tissues, usually by using stem cells. In this manner, stem cells may be differentiated into, or made to become, the type of cell that is damaged and then used in transplants. Also see clinical trials.

Clinical trials: A controlled test of a new drug or treatment on human subjects, normally performed after successful trials with model organisms. ClinicalTrials.gov lists many stem cell clinical trials.

Stem cells have great potential to treat a wide variety of human diseases and medical conditions.

Cell Surface Marker proteins, or simply Cell Markers: A protein on the surface of a cell that identifies the cell as a certain cell type.

Somatic Cell Nuclear Transfer (SCNT): A technique that uses an egg and a somatic cell (a non-germline cell). The nucleus, which contains the genetic material, is removed from the egg and the nucleus from the somatic cell is removed and combined with the egg. The resultant cell contains the genetic material of the nucleus donor, and is turned into a totipotent state by the egg. This cell has the potential to develop into an organism, a clone of the nucleus donor.

Dolly the sheep was cloned through somatic cell nuclear transfer (SCNT). An adult cell from the mammary gland of a Finn-Dorset ewe acted as the nuclear donor; it was fused with an enucleated egg from a Scottish Blackface ewe, which acted as the cytoplasmic (or egg) donor. An electrical pulse acted to fuse the cells and activate the oocyte after injection into the surrogate mother ewe. A successfully implanted oocyte developed into the lamb Dolly, a clone of the nuclear donor, the Finn-Dorset ewe.

Clone: A genetic, identical copy of an individual organism through asexual methods. A clone can be created through somatic cell nuclear transfer.

Other stem cell glossaries:

Image creditsImages of Endoderm, Mesoderm, Ectoderm, Bone Marrow, Neurons, Cartilage, Hand Skeleton, Connective and Adipose Tissue, Gastrula, Clinical Trials, Mouse, Rat, Drosophila, C. Elegans, Arabidopsis, Sea Urchin, Xenopus, Somatic Cell Nuclear Transfer to Create Dolly and other images were taken from the Wikimedia Commons and redistributed and altered freely as they are all in the public domain. The image of Hematopoiesis was also taken from the Wikimedia Commons and redistributed according to the GNU Free Documentation License.

2009. Teisha Rowland. All rights reserved.

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