Cell differentiation may simply be described as the process through which a young and immature cell evolves in to a specialized cell, reaching its mature form and function. For such unicellular organisms like bacteria, various life functions occur within a single cell. That is, such processes as the transport of molecules, metabolism and reproduction all take place within a single cell given that they are single celled. However, multicellular organisms require different types of cells for these processes to be possible.
Here, different types of cells play a specific function given that they have varied structures. For instance, whereas the nerve cells play a crucial role in the transmission of signals to different parts of the body, blood cells play an important role carrying oxygen to different parts of the body.
The differences in structure and functions between the cells mean that they are specialized cells. To be able to perform different functions, cells have to become specialized. This becomes possible through the process referred to as cell specialization.
A cell capable of differentiating into any type of cell is known as "totipotent". For mammals, totipotent includes the zygote and products of the first few cell divisions. There are also certain types of cells that can differentiate into many types of cells. These cells are known as "pluripotent" or stem cells in animals (meristemic cells in higher plants).
While this type of cell can divide to produce new differentiated generations, they retain the ability to divide and maintain the stem cell population making them some of the most important cells.
Examples of stem and progenitor cells include:
Hematopoietic Stem Cells - These are from the bone marrow and are involved in the production of red and white blood cells as well as the platelets.
Mesenchymal Stem Cells - Also from the bone marrow, these cells are involved in the production of fat cells, stromal cells as well as a given type of bone cell.
Epithelial Stem Cells - These are progenitor cells and are involved in the production of certain skin cells.
Muscle Satellite Cells - These are progenitor cells that contribute to differentiated muscle tissue.
The process of cell differentiation starts with the fertilization of the female egg. As soon as the egg is fertilized, cell multiplication is initiated resulting in the formation of a sphere of cells known as the blastocyst. It is this sphere of cells that attach to the uterine wall and continues to differentiate.
As the blastocyst differentiates, it divides and specializes to form a zygote that attaches to the womb for nutrients. As it continues to multiply and increase in size, the differentiation process results in the formation of different organs.
Once the female egg has been fertilized, the cells formed after cell division contain DNA that is identical. That is, the DNA in all the cells will be identical. However, different regions of a chromosome (DNA is wound in to a chromosome) code for different functions and cell type. Here, it is only the regions that are required to perform a given function that are expressed in each cell.
The regions (genes) that are expressed determine the type of cell that will be created. While the different types of cells that are formed contain the same DNA, it is the expression of different genes that results in different types of cells. This is to say that not all genes are expressed during differentiation.
* Gene expression is the process through which information from a given gene is used to develop the structures of specific cells.
During the differentiation process, cells gradually become committed towards developing into a given cell type. Here, the state of commitment may be described as "specification" representing a reversible type of commitment or "determination" representing irreversible commitment.
Although the two represent differential gene activity, the properties of cells in this stage is not completely similar to that of fully differentiated cells. For instance, in the specification state, cells are not stable over a long period of time.
There are two mechanisms that bring about altered commitments in the different regions of the early embryo. These include:
Cytoplasmic Localization - This occurs during the earliest stage of embryo development. Here, the embryo divides without growth and undergoes cleavage divisions that produce blastomeres (separate cells). Each of these cells inherit a given region of the cytoplasm of the original cell that may contain cytoplasmic determinants (reuratory substances).
Once the embryo becomes a morula (solid mass of blastomeres) it is composed of two or more differently committed cell populations. The cytoplasmic determinants may contain mRNA or protein a given state of activation that influence specific development.
Induction - In induction, a substance secreted by one group of cells causes changes in the development of another group. During early development, induction tends to be instructive in that tissue assumes a given state of commitment in the presence of the signal.
In induction, inductive signals also evoke various responses at varying concentrations which results in the formation of a sequence of groups of cells, each being in a different state of specification.
During the final phase of cell differentiation, there is formation of several types of differentiated cells from one population of stem cells of the precursor. Here, terminal differentiation occurs both in embryonic development as well as in tissues during postnatal life.
Control of the process largely depends on a system of lateral inhibition. That is, cells differentiating along a given pathway send out signals which repress similar differentiation by the neighboring cells. A good example of this is with the developing CNS of vertebrates (central nervous system).
In this system, neurons cells from the tube of neuropithelium possess a surface receptor known as Notch and a cell surface molecule known as Delta that can bind to the Notch of adjacent cells and activate them.
This activation results in a cascade of intracellular events that ultimately result in the suppression of Delta production as well as the suppression of neuronal differentiation. As a result, the neuropithelium ends up only generating a few cells with high expression of Delta surrounded by a larger number of cells with low expression of Delta.
As previously mentioned cell differentiation is a process through which a generic cell evolves into a given type of cell (cell type) and ultimately allowing the zygote to gradually evolve in to a multicellular adult organism.
Cell differentiation is an important process through which a single cell gradually evolves allowing for development that not only results in various organs and tissues being formed, but also a fully functional animal.
While it plays a significant role in embryonic development, the process of cell differentiation is also very important when it comes to complex organisms throughout their lives. This is because of the fact that it causes changes in size, shape, metabolic activities as well as signal responsiveness of cells.
In cell differentiation, gene expression is particular important given that there are vital control systems that only ensure certain differentiation. Here, the process proves beneficial by controlling certain activities to guarantee both normal functioning tissues and organs, but also a full functional animal.
Knowledge of cell differentiation has also influenced stem cell research. Today, scientists and researchers are working to determine the best way they can use stem cells for the purposes of regenerating and repairing cellular damage.
As mentioned earlier, stem cells are important in that they can develop to any cell type. This makes them very special in that they can differentiate and be used for given treatment purposes. A good example of this is with cells among the older adults. In older years, many of the cells experience wear and tear. As a result, they lose their ability to divide or repair themselves.
Stem cells can continue differentiating into a number of specialized cells to renew and repair the tissue in question. In theory, it is supposed that there is no limit as to the type of diseases that can be treated using stem cell therapy. However, research is still ongoing to ensure that this type of treatment is both safe and effective.
Gene Structure - This is the most important factor when it
comes to cell differentiation. Each of the viable genes contains important
information that determine the cell type and physical attributes of the animal
(host). Any problem in the genetic material ultimately affects cell
differentiation and the development of the host.
Environmental Factors - Various environmental factors as changes in temperature and supply of oxygen etc can affect the release and production of hormones given that various proteins are involved in the transmission of information as well as triggering of hormones. If these molecules are affected, then cell differentiation and development is also affected.
And more advanced: Cytochemistry
Interesting and Related Articles:
Bora Zivkovic (2001) From Two Cells To Many: Cell Differentiation and Embryonic Development.
Browder, L. W. 1996. Cell determination and differentiation: The muscle paradigm. In L. W. Browder (ed.), Advanced Developmental Biology. The University of Calgary. Retrieved November 15, 2007.
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