Differentiation (from the Latin differre 'to differ' ) describes in developmental biology the development of cells or tissues from a less to a more specialized state. This is a frequently irreversible and therefore seemingly predetermined change in individual cells and tissues. This change can take place in different (polyvalent) directions.
Differentiation processes occur on the one hand in the individual development of a multicellular organism, which develops from a zygote (fertilized egg cell) into a complex structure with many different cell types and tissue types. But differentiation processes also play an important role in maintaining body functions in adult individuals.
Differentiation and genome
In molecular biology , the differentiation of cells is expressed in the fact that not the entire genome is expressed , i.e. converted into proteins , but only the genes required for the respective cell type are active. In contrast to the short-term variable gene expression, which allows, for example, the reaction to hormones or stress , the differentiation thus represents a long-term stable form of gene regulation.
Differentiation and Determination
In living beings with sexual reproduction, development begins with a single cell, the fertilized egg cell (zygote), which can produce all cell types of the entire organism . This property is called " totipotency " (from Latin totus - everything and potentia - power, ability). Cell division gives rise to several daughter cells, which specialize in different roles depending on the cell origin .
In animals in particular , this process is associated with so-called determination . This means that the chosen direction of specialization will be passed on epigenetically to subsequent cell generations . A determined cell thus retains its development program even if, for example, it is transplanted to another location within the organism. As a result, the potency of the cell line is increasingly limited, from pluripotent embryonic stem cells (from Latin pluriens - multiple), which can produce all cell types of the embryo , via multipotent body stem cells ("somatic stem cells", Latin multus - much or ancient Greek) σῶμα soma , German 'body' ), which can only produce the cell types of a certain tissue, up to irreversibly differentiated, functional body cells. These usually lose the ability to divide and often only have a limited lifespan.
However, under certain circumstances cells can change their determination ( transdifferentiation ), lose their differentiation ( dedifferentiation ) or differentiate again after dedifferentiation ( transdifferentiation ). These processes play a role in wound healing and the development of cancer , for example .
Plants also contain so-called meristematic cells that specialize in the division and thus the generation of new cells and tissues ; however, differentiated cells are often not or only to a limited extent determined and retain the ability to regain themselves under certain circumstances, for example after being wounded divide and produce different cell types.
Regulation of differentiation
The way of differentiation, i.e. the decision as to which cell type a cell develops into, depends on various external and internal factors, for example the influence of:
- Growth factors and hormones
- Neighboring cells ( cell contacts )
- The origin of the cell from its precursors (determination)
Gene-controlled formation of proteins
The human geneticist Friedrich Vogel (1961) spoke of problems of differentiation during embryonic development that are related to the formation of gene-controlled proteins, which in turn depend on the substrate available. To illustrate this process, Vogel chooses an experimentally verifiable example of enzymatic adaptation : When yeast is fed with glucose , it shows no willingness to convert other sugars such as galactose . Only in the course of a few hours can this readiness due to an increase in galactokinase activity be demonstrated when galactose is accordingly available , cf. → enzyme induction . At the same time, however, the glucokinase activity then falls.
Comparative consideration of development stages
The comparative general consideration of the stages of development within phylogenesis is instructive for understanding the functional structure of our own nervous system, since it shows us both advantages and disadvantages and thus the biological meaning of such differentiation. Such an example with regard to the organization of the retina can be seen when comparing human development and that of the squid, see → Centralization . The blueprint of the inverse eye offers advantages in terms of blood supply.
Differentiation similar to bark
The human retina is also an example of the cortical-like differentiation of nerve tissue. The retina represents an upstream part of the brain and has the microscopic structure in layers ( lamination ) typical of cortical structures . Lamination is a gray matter building principle .
Cellular pre-stages of later differentiation
The so-called blasts represent not finally differentiated histogenetic preliminary stages of different lines of development . The uniformly designed wall cells lying in the wall of the embryonic neural tube ( zona nuclearis intermedia ), which are also referred to as neuroepithelium , may serve as a concrete example . The glioblasts as well as the neuroblasts and ependymal cells develop from these wall cells. Neuroblasts are to be regarded as the preliminary stages of the later neurons , glioblasts as the preliminary stages of the glia .
According to the theories of individual Gestalt psychologists such as Heinz Werner et al. a. Differentiation as a biological development principle is to be seen in close connection with the concept of centralization . While the differentiation can be understood as a steady state without significant morphological changes, the centralization of organs is more related to the functional networking of differently differentiated cellular elements in a spatial organ system, see Chap. Morphogenesis . Every newly acquired behavioral unit must be integrated into the whole of the organism in order to avoid a disorganization of the behavior through this integration . Integration requires differentiation. Even if integration and differentiation are to be viewed as opposing active principles, this increases the organism's ability to respond specifically to different stimuli.
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- Heinz Werner : Introduction to Developmental Psychology . 1926
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- Otto Grosser arr. by Rolf Ortmann: Outline of the human development history . 6th edition, Springer, Berlin 1966; P. 2, 24 on head. “Predetermination”.
- Friedrich Vogel : General human genetics . Springer, Berlin 1961; P. 418.
- Alfred Benninghoff and Kurt Goerttler : Textbook of Human Anatomy. Shown with preference given to functional relationships . Volume 3: Nervous System, Skin and Sensory Organs . 7th edition. Urban and Schwarzenberg, Munich 1964; (a) Re. “Comparative consideration of development stages”: p. 106; (b) Re. “Layer structure of the retina”: pp. 428–435; on “bark-like differentiation”: p. 189; (c) Re. “Neuroblasts”: pp. 73, 123.
- neuroblasts . In: Helmut Ferner : Human development history . 7th edition. Reinhardt, Munich 1965, pp. 125, 137 f.
- Peter R. Hofstätter (Ed.): Psychology . The Fischer Lexicon. Fischer-Taschenbuch, Frankfurt a. M. 1972, ISBN 3-436-01159-2 ; on “Centralization, Differentiation, Development” pp. 102, 164 f .; to Stw. “Gestalt psychology, basic assumptions” p. 164 f.
- Wilhelm Karl Arnold et al. (Ed.): Lexicon of Psychology . Bechtermünz, Augsburg 1996, ISBN 3-86047-508-8 ; to Lex.-Lemma: “Differentiation”: Col. 367; to Lex.-Lemma: "Integration": Col. 995 f.