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3) Totipotent, multipotent, pluripotent animal cells

Totipotency is the ability of a single cell to divide and produce all of the differentiated cells in an organism, including extraembryonic tissues.Totipotent cells include spores and zygotes.  In the spectrum of cell potency, totipotency represents the cell with the greatest differentiation potential.Toti comes from the Latin totus which means "entirely."

Research has helped demonstrate that cells can regain totipotency[5] but research has also shown that regaining totipotency is complex. Instead of regaining full totipotency, cells may instead differentiate through a complex cellular variation of totipotency.

Just how totipotent cells arise is described through the process of human development. Consider first that human development begins when a spermfertilizes an egg. The resulting fertilized egg creates a single totipotent cell, a zygote. In the first hours after fertilization, this cell divides into identical totipotent cells, which can later develop into any of the three germ layers of a human (endodermmesoderm, or ectoderm) and into cells of thecytotrophoblast layer or syncytiotrophoblast layer of the placenta. After reaching the 16-cell stage, the totipotent cells of the moruladifferentiate into cells that will eventually become either the blastocyst's Inner cell mass or the outer trophoblasts. Approximately four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize. The inner cell mass, the source ofembryonic stem cells, is pluripotent, not totipotent.

In cell biology, pluripotency (from the Latin plurimus, meaning very many, and potens, meaning having power)[11] refers to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous system).

Multipotency describes progenitor cells which have the gene activation potential to differentiate into multiple, but limited cell types. For example, a multipotent blood stem cell is a hematopoieticcell — and this cell type can itself differentiate into several types of blood cell types like lymphocytesmonocytesneutrophils, etc., but cannot differentiate into brain cellsbone cells or other non-blood cell types.

New research related to multipotent cells suggests that multipotent cells may be capable of conversion into unrelated cell types. In one case, fibroblasts were converted into functional neurons.[19]In another case, human umbilical cord blood stem cells were converted into human neurons.[21] Research is also focusing on converting multipotent cells into pluripotent cells.

Multipotent cells are found in many, but not all human cell types. Multipotent cells have been found in adipose tissue,[23] cardiac cells,[24] bone marrow, and mesenchymal stromal cells (MSCs) which are found in the third molar.

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