differentiation
Development and Differentiation
The entire process of development is the progressive proliferation and differentiation of cells from the zygote to the adult. Hence, the number of differentiated cells and the general degree of differentiation both increase throughout development. The process begins with the totipotent zygote and early blastomeres. As development progresses stem cell populations become restricted to produce some cell types but not others. Hence they lose their totipotency and become pluripotent. This is an intermediate level of differentiation. Eventually, daughter cells will be produced with no ability to differentiate into any other cell types. These are the end point of the process, completely differentiated cells.
Regeneration and Differentiation
As cells become differentiated their ability to divide diminishes and so they lose the ability to proliferate. If they cannot proliferate, they cannot replenish themselves after damage. Therefore, tissues composed of highly differentiated cells (e.g. muscle, nerve) have little to no ability to regenerate. Less differentiated tissues (e.g. epithelial, hepatic) regenerate more readily.
Levels of Differentiation
Differentiation is not an all or none process. Some cells are more differentiated than others. For this reason differentiated and undifferentiated are relative terms. For instance, a zygote is completely undifferentiated because it will produce all cell types. A hemopoietic cell is much more differentiated because it can only transform into blood cells. However, it is undifferentiated relative to a muscle cell, because a hemopoietic cell can differentiate into any of a number of blood cell types, while a myocyte loses the ability to transform into any other cell type. Generally, the term undifferentiated refers to generalized cells that retain the ability to divide and/or differentiate into other types.
Dedifferentiation
to reverse the process of differentiation. For a specialized cell to transform into less specialized cell type.
Characteristics of Differentiated Cells
1.) Specialized cellular structures and/or functions 2.) Slow or arrested cell cycle progression 3.) Impaired ability to transform into other cells types
Neoplasia
abnormal, new growth (e.g. tumor formation). Note, most neoplasms are anaplastic.
Blastomeres
cells from cleavage stage embryos, either morulae or blastocysts
Undifferentiated cell types
cells that have not transformed into a specialized cell type.
Progenitor Cells (precursor cells)
cells that must differentiate into other cells types when they divide. Because progenitors do not produce more of themselves when they divide, they cannot propagate their own population. The population is maintained by stem cells that differentiate into progenitors.
Generalized cell types
cells with common traits, lacking specialized features.
Specialized cell types
cells with distinctive morphological characteristics and/or molecular processes.
anaplasia
for cells to lose specialized features, making them unrecognizable.
Growth Factors
hormones that control cell cycle progression, cellular differentiation or morphogenesis during development.
Autonomous Specification
intracellular signals (within a cell) controlling cellular differentiation.
Mesenchyme
loosely organized stem cells. These are usually thought of as mesodermal, but they may be derived from the two other layers (e.g. ectodermal neural crest cells).
Hormones
non-nutrient chemicals secreted by one cell to induce a response in another cell.
Stem Cells -
pluripotent, generalized cells that can differentiate into other cell types (e.g. spermatogonia, mesenchymal stem cells). When they divide they either produce two stem cells to propagate their own population, or they differentiate. When most cells types differentiate, one daughter cell will be differentiated while the other remains a stem cell.
Intercellular Induction
signals between cells controlling cellular differentiation.
Differentiated cell types
specialized cells with limited or no ability to transform into other cell types.
Blast Cells
stem or progenitor cells that differentiate into specific cells types. Note, blast cells are usually named after the cell types they produce. For instance, a neuroblast differentiates into neurons, but no other cell types.
Determination
the final phase, when the cell's fate becomes irreversible. It must differentiate into specific cell types; it has lost the ability to differentiate into other cell types. Note, commitment may occur before differentiation becomes evident. Some cells are committed early in development, before there are any detectable changes in a cell's morphology or biochemistry.
Specification
the first phase of commitment, when the fate is still reversible. For instance, an ectodermal cell in the medial region of a trilaminar germ disc is specified to develop into a neural cell. However, if a micromanipulator is used to transfer it to the lateral region of the disc, it will differentiate into a dermal cell instead.
Differentiation
the process of transformation into a different cell type.
Totipotency
to have the potential to differentiate into any cell type and to produce an entire organism (germ line, gametes, zygotes and early blastomeres).
Pluripotency, Multipotency
to have the potential to differentiate into multiple cell types, e.g. hemopoietic cells, fibroblasts. (Note, the term multipotency tends to be used for more differentiated cells, but there is no standardized criteria for distinguishing which cells should be called multipotent and which cells are pluripotent. For instance, many references refer to hemopoietic cells as multipotent, but others call these same cells pluripotent.)
Metaplasia
transformation of one differentiated cell type into another (e.g. smoking can induce the pseudostratified epithelium of the bronchi to become stratified squamous, as would normally occur lining more external surfaces such as skin or esophagus).
Commitment -
when the developmental fate of a cell becomes restricted, so that it will differentiate in a specific manner