Cellular differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of tissues and cell types. Differentiation is a common process in adults as well: adult stem cells divide and create fully-differentiated daughter cells during tissue repair and during normal cell turnover. When a cell differentiates its size, shape, polarity, metabolic activity, and responsiveness to signals may change dramatically. These changes are largely do to highly-controlled modifications in gene expression. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome.

A cell that is able to differentiate into many cell types is known as pluripotent. These cells are called stem cells in animals and meristematic cells in higher plants. A cell that is able to differentiate into all cell types is known as totipotent. In mammals, only the zygote and early embryonic cells are totipotent, while in plants, many differentiated cells can become totipotent with simple laboratory techniques. In cytopathology the level of cellular differentiation is used as a measure of cancer progression. "Grade" is a marker of how differentiated a cell in a tumor is.

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Mammalian cell types

Three basic categories of cells make up the mammalian body: germ cells, somatic cells, and stem cells. Each of the approximately 100,000,000,000,000 (10¹⁴) cells in an adult human has its own copy, or copies, of the genome, with the only exception being certain cell types that lack nuclei in their fully differentiated state, such as red blood cells. The majority of the cells are diploid, meaning they have two copies of each chromosome. This category of cells, called somatic cells, includes most of the cells that make up the human body, such as skin and muscle cells.

Germ line cells are any line of cells that give rise to gametes—eggs and sperm—and are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development.

Development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. These cells are referred to as pluripotent.

Pluripotent stem cells undergo further specialization into multipotent progenitor cells that then give rise to functional cells. Examples of stem and progenitor cells include:

• Hematopoietic stem cells (adult stem cells) from the bone marrow that give rise to red blood cells, white blood cells, and platelets
• Mesenchymal stem cells (adult stem cells) from the bone marrow that give rise to stromal cells, fat cells, and types of bone cells
• Epithelial stem cells (progenitor cells) that give rise to the various types of skin cells
• Muscle satellite cells (progenitor cells) that contribute to differentiated muscle tissue

Dedifferentiation

Dedifferentiation is a cellular process commonly believed to be uniquely available to lower life forms such as worms and amphibians in which a partially or terminally differentiated cell reverts to an earlier developmental stage.^{[dubious – discuss]}

Cells in cell culture can lose properties they originally had, such as protein expression, or change shape. This process is also termed dedifferentiation^[1].

Dediferentiation also occurs in plants^[2].

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.

Mechanisms

Each specialized cell type in an organism expresses a subset of all the genes that constitute the genome of that species. Each cell type is defined by its particular pattern of regulated gene expression. Cell differentiation is thus a transition of a cell from one cell type to another and it involves a switch from one pattern of gene expression to another. A few evolutionarily conserved types of molecular processes are often involved in the cellular mechanisms that control these switches. The major types of molecular processes that control cellular differentiation involve cell signaling. Many of the signal molecules that convey information from cell to cell during the control of cellular differentiation are called growth factors. Another important strategy is to unequally distribute molecular differentiation control signals inside a parent cell. Upon cytokinesis, the amount of such intracellular differentiation control signals can be unequal in the daughter cells and this imbalance results in distinct patterns of differentiation for the different daughter cells. A well-studied example is this is for body axis patterning in Drosophila. RNA molecules are an important type of intracellular differentiation control signal.

See also

• Morphogenesis
• Multipotent
• Germ layer
• Cell fate determination

References

• What is a cell? 2004. A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI.