Thursday, October 8, 2009
Cell differentiation is a process in which a generic cell develops into a specific type of cell in response to specific triggers from the body or the cell itself. This is the process which allows a single celled zygote to develop into a multicellular adult organism which can contain hundreds of different types of cells. In addition to being critical to embryonic development, cell differentiation also plays a role in the function of many organisms, especially complex mammals, throughout their lives.
When a single cell has the capability of developing into any kind of cell, it is known as totipotent. In mammals, the zygote and the embryo during early stages of development are totipotent, for example. Cells which can differentiate into several different cell types, but not all, are considered to be pluripotent. In both cases, the nucleus is the same, containing all of the genetic information needed to encode the entire organism, but only certain genes are activated
Some organisms are capable of dedifferentiation, in which specialized cells become more basic. This process is involved in the regeneration of limbs in animals which are capable of this feat, with the basic cells differentiating again to construct the needed tissues, bones, and other types of cells for the replacement. The exact processes behind cell differentiation and dedifferentiation are not fully understood, although researchers have studied cells capable of these feats extensively, as the mechanics of this process could have valuable implications for the medical field.
Differentiated cells produce and use specific proteins characteristic of their differentiation type. For example, red blood cells produce hemoglobin to help transport oxygen, and muscle cells produce myosin to help with muscle contraction. Differentiated cells often assume characteristic shapes, such as columnar epithelial cells and star-shaped astrocytes.
Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues.  Totipotent cells formed during sexual and asexual reproduction include spores and zygotes. Zygotes are the products of the fusion of two gametes (fertilization). In some organisms, cells can dedifferentiate and regain totipotency. For example, a plant cutting or callus can be used to grow an entire plant.
Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. These changes are largely due to highly-controlled modifications in gene expression.
Dedifferentiation is a cellular process often seen in more basal life forms such as worms and amphibians in which a partially or terminally differentiated cell reverts to an earlier developmental stage, usually as part of a regenerative process. Dedifferentiation also occurs in plants. Cells in cell culture can lose properties they originally had, such as protein expression, or change shape. This process is also termed dedifferentiation.
Some believe dedifferentiation is an aberration of the normal development cycle that results in cancer, whereas others believe it to be a natural part of the immune response lost by humans at some point as a result of evolution.
A small molecule dubbed reversine, a purine analog, has been discovered that has proven to induce dedifferentiation in myotubes. These dedifferentiated cells were then able to redifferentiate into osteoblasts and adipocytes.