The mechanism a single fertilized, undifferentiated cell undergoes to generate the diverse range of tissues that make up a multicellular organism is a fascinating process. During development, many types of cells arise from the fertilized egg. As cells differentiate, only portions of the total genetic information contained within the nucleus are expressed within each cell type. For example, even though all cells contain the DNA sequence coding for insulin, only cells of the islets of Langerhans in the pancreas activate this sequence to manufacture and secrete insulin. With that said, it is important to clarify the terminology used in describing the genetic flexibility of cells during the developmental pathway.
- Stem cell - A cell from an embryo, fetus, or adult that can reproduce itself for long periods of time and can give rise to specialized cells and tissues.
- Totipotent - A cell capable of expressing all the genes of the genome (can give rise to any part of the later embryo or adult). In humans, the fertilized egg is totipotent until the eight-cell-stage. Most plant cells retain totipotency even after becoming specialized.
- Pluripotent - A cell with the potential to generate cell types and tissues from all three primary germ layers of the body.
- Differentiation - Cell specialization that occurs at the end of the developmental pathway. Only certain genes are activated.
- Plasticity - The ability of a stem cell of one tissue type to generate cells from another tissue type.
- Progenitor or precursor cell - Occurs when a stem cell divides into two partially differentiated cells, neither of which can replicate itself
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