Everything about Cellular Differentiation totally explained
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. Cell differentiation causes its size, shape, polarity,
metabolic activity, and responsiveness to signals to change dramatically. These changes are largely due 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's 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's 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.
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 (10
14) cells in an adult human has its own copy or copies of the
genome except certain cell types, such as
red blood cells, that lack nuclei in their fully differentiated state. Most cells are
diploid; they've two copies of each
chromosome. Such cells, called somatic cells, make up most of 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 thus 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's 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 can't 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:
Dedifferentiation
Dedifferentiation is a cellular process often seen in lower 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.
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. Cellular differentiation during development can be understood as the result of a
gene regulatory network. A regulatory gene and its cis-regulatory modules are nodes in a gene regulatory network; they receive input and create output elsewhere in the network . The
systems biology approach to developmental biology emphasizes the importance of investigating how developmental mechanisms interact to produce predictable patterns (
morphogenesis).
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. Although the details of specific signal transduction pathways vary, these pathways often share the following general steps. A ligand produced by one cell binds to a receptor in the extracellular region of another cell, inducing a conformational change in the receptor. The shape of the cytoplasmic domain of the receptor changes, and the receptor acquires enzymatic activity. The receptor then catalyzes reactions that phosphorylate other proteins, activating them. A cascade of phosphorylation reactions eventually activates a dormant transcription factor or cytoskeletal protein, thus contributing to the differentiation process in the target cell . Cells and tissues can vary in competence, their ability to respond to external signals .
Induction refers to cascades of signaling events, during which a cell or tissue signals to another cell or tissue to influence its developmental fate investigated the role of the lens in eye formation in cave- and surface-dwelling fish, a striking example of induction.
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