differentiation


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differentiation,

in biology, series of changes that occur in cells and tissues during development, resulting in their specialization. This, in turn, permits a greater variety of organisms. In plants, unspecialized cells, composing tissue called meristemmeristem
, a specialized section of plant tissue characterized by cell division and growth. Much of the mature plant's growth is provided by meristems. Apical meristems found at the tips of stems and roots increase the length of these sections.
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, differentiate into vascular tissue (xylem and phloem; see woodwood,
botanically, the xylem tissue that forms the bulk of the stem of a woody plant. Xylem conducts sap upward from the roots to the leaves, stores food in the form of complex carbohydrates, and provides support; it is made up of various types of cells specialized for each of
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), supportive tissue (sclerenchyma), and storage tissue (parenchyma). In animals, the tissues of the gastrula stage of the embryoembryo
, name for the developing young of an animal or plant. In its widest definition, the embryo is the young from the moment of fertilization until it has become structurally complete and able to survive as a separate organism.
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 differentiate into specialized tissues. While it is not fully understood what initiates this processs, it is known that each of the specialized cells in an organism carries a full set of genesgene,
the structural unit of inheritance in living organisms. A gene is, in essence, a segment of DNA that has a particular purpose, i.e., that codes for (contains the chemical information necessary for the creation of) a specific enzyme or other protein.
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, with all of the organism's genetic information, but each specialized cell expresses only part of it. That is, each cell only transcribes that DNA that it needs to do its specific tasks (see nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
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).

differentiation

(dif-ĕ-ren-shee-ay -shŏn) The process by which layers of different density are formed from originally homogeneous molten rock. It is generally accepted that melting and differentiation occurred very early in the history of the Earth and resulted from the rapid increase in the young planet's temperature; the heat was generated by decay of radioactive elements in the planetary material, from the impact of infalling material, and from contraction of the planet. The widespread melting led to massive differentiation of the Earth into the iron-rich core and the lighter silicate (rocky) regions of the mantle and crust (see Earth). Differentiation also occurred on other terrestrial planets and the Moon.

differentiation

see SOCIAL DIFFERENTIATION.

Differentiation

 

(in biology). Phylogenetic differentiation is the division in the evolutionary process of a unified group of organisms into two or more. This is one of the characteristic features in the evolution of organisms. The most important phylogenetic differentiation is the process of species development that leads to the appearance of new species. Phylogenetic differentiation is inevitably accompanied by the formation of a hierarchical system of forms (population, species, genus, family, order, class, and so forth). Differentiation is connected with integration: an entity becomes more complex in its manifestations of life, and the individual parts harmoniously supplement each other, leading to more specialized use of the environment (increase in the “sum of life,” to quote C. Darwin) and to the appearance of new possibilities in evolution. Differentiation has an adaptive nature. In the evolutionary process there is an accumulation of differentiation of general significance and the replacement of individual adaptations by general ones.

Other types of differentiation include ontogenetic and sexual differentiation.

REFERENCES

Shmal’gauzen, I. I. Organizm kak tseloe v individual’nom i istoricheskom razvitii. Moscow-Leningrad, 1942.
Shmal’gauzen, I. I. Problemy darvinizma, 2nd ed. Leningrad, 1969.

A. V. IABLOKOV


Differentiation

 

ontogenetic differentiation (biology), the appearance of differences among homogeneous cells and tissues and changes in them in the course of development, which lead to specialization.

Differentiation occurs principally in the process of embryonic development, when organs and tissues with cells that are varied in form and function are formed from identical, unspecialized embryonic cells. The developing embryo differentiates at first into germ layers, then into the rudiments of the principal systems and organs, and later into a large number of specialized tissues and organs characteristic of the adult organism. Differentiation also occurs in some organs of the adult body. (For example, bone-marrow cells differentiate into various blood cells.) The term “differentiation” is also frequently applied to the series of successive changes undergone by cells of a single type in the process of their specialization. (For example, in the course of differentiation of red blood cells the erythroblasts are transformed into reticulocytes, and reticulocytes are transformed into erythrocytes.) Differentiation is expressed as changes in the form of cells, their internal and external structure, and their interconnections. (For example, myoblasts become elongated and merge with one another, myofibrils develop in them, and so on; in neuroblasts the nucleus becomes enlarged and processes appear that connect the nerve cells with various organs and among themselves.) Differentiation is also expressed as changes in the functional properties of cells. (Muscle fibers acquire the capacity to contract, nerve cells to transmit nerve impulses, gland cells to secrete appropriate substances, and so on.)

Principal factors in differentiation are the differences in the cytoplasm of early embryonic cells, caused by the nonhomogeneity of the cytoplasm of the egg, and the specific influences of neighboring tissues, or induction. A number of hormones exert an influence on the course of differentiation. Many factors that determine differentiation are as yet unknown. Differentiation can occur only in cells that are prepared for it. The action of a factor in differentiation at first produces a condition of latent differentiation, or determination, when the external signs of differentiation have not as yet manifested themselves, but the subsequent development of the tissue is already capable of proceeding independently of the inciting factor. For example, differentiation of nerve tissue is induced by the rudiment of the chordamesoderm. But induction of differentiation is possible and takes place only in the ectoderm of the embryo at a definite stage of its development. The state of differentiation is usually irreversible, that is, the differentiated cells can no longer lose their specialization. However, in conditions of injury to tissue capable of regeneration, and also in the event of malignant degeneration, partial dedifferentiation occurs, in which the cells lose many characteristics acquired in the process of differentiation and externally resemble the slightly differentiated cells of the embryo. There are possible instances in which dedifferentiated cells may become differentiated in some other direction (metaplasia).

The molecular-genetic basis of differentiation is the activity of genes specific for each tissue. Each cell, including the differentiated cell, contains the entire genetic apparatus (all the genes). However, in each tissue only that portion of the genes responsible for a given differentiation is active. The role of differentiation factors thus amounts to the strictly selective activation (engagement) of those genes. The mechanism of that activation is being intensively studied. The activity of certain genes leads to synthesis of the appropriate proteins that determine differentiation. Thus, the specific protein of red blood cells, hemoglobin, is synthesized in the erythroblasts; myosin is synthesized in muscle cells; insulin, trypsin, and amylase are synthesized in differentiating cells of the pancreas; during differentiation of cartilage or bone tissue, enzymes are synthesized that ensure formation and accumulation around the cells of the mucopolysaccharides of cartilage and the salts of bone. It is presumed that the proteins of the cell surface play a decisive role in determining the form of cells, their capacity to be connected to one another, and their movements in the course of differentiation.

A. A. NEIFAKH

differentiation

[‚dif·ə‚ren·chē′ā·shən]
(mathematics)
The act of taking a derivative.
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