Embryogenesis The process of embryo initiation and development

Endocrine cell? In animals, a cell that produces hormones, growth factors or other signalling substances for which the target cells, expressing the corresponding receptors, are located at a distance. See also 'Autocrine' and 'Paracrine'.

Eptgenetic event? Any change in a phenotype that does not result from an alteration in DNA sequence. This change may be stable and heritable and includes alteration in DNA methylation, transcriptional activation, translational control and post-translational modifications.

Epigenetic variation? Phenotypic variability that has a non-genetic basis.

Epithelial-Ike? Resembling or characteristic of, having the form or appearance of epithelial cells. In order to define a cell as an epithelial cell, it must possess characteristics typical of epithelial cells. Often one can be certain of the histological origin and/or function of the cells placed into culture and, under these conditions, one can be reasonably confident in designating the cells as epithelial. It is incumbent upon the individual reporting on such cells to use as many parameters as possible in assigning this term to a culture. Until such time as a rigorous definition is possible, it would be most correct to use the term epithelial-like.

Euploid: The situation that exists when the nucleus of a cell contains exact multiples of the haploid number of chromosomes.

Expiant: Tissue taken from its original site and transferred to an artificial medium for growth or maintenance.

Expiant culture? The maintenance or growth of an expl tni m culture.

Feeder layer: A layer of cells (usually lethally irradiated toi mimai cell culture) upon which are cultured a fastidious cell type.

Fibroblast-like? Resembling or characteristic of, having the form or appearance of fibroblast cells. In order to define a cell as a fibroblast cell, it must possess characteristics typical of fibroblast cells. Often one can be certain of the histological origin and/or function of the cells placed into culture and, under these conditions, one can be reasonably confident in designating the cells as fibroblast. It is incumbent upon the individual .reporting on such cells to use as many parameters as possible in assigning this term to a culture. Until such time as a rigorous definition is possible, if. would be most correct to use the term fibroblast-like.

Finite cell culture: A culture that is capable of only a limited number of population doublings after which the culture ceases proliferation. See also 'Ira vitro senescence'.

Habituation: The acquired ability of a population of cells to grow and divide independently of exogenously supplied growth regulators.

Heterokaryon: A cell possessing two or more genetically different nuclei in a common cytoplasm, usually derived as a result of cell-to-cell fusion.

Heteroploid: The term given to a cell culture when the cells comprising the culture possess nuclei containing chromosome numbers other than the diploid number. This is a term used only to describe a culture and is not used to describe individual cells. Thus, a heteroploid culture would be one that contains aneuploid uTi

Histotypic: The in vitro resemblance, of cells in culture, to a tissue in form oi 1 unction or both. For example, a suspension of fibroblast-like cells may secrete a glycosaminoglycan-collagen matrix and the result is a structure resembling fibrous connective tissue, which is, therefore, histotypic. This term is not meant to be used along with the word 'culture'. Thus, a tissue culture system demonstrating form and function typical of cells in vivo would be said to be histotypic.

Homokaryon: A cell possessing two or more genetically identical nuclei in a common cytoplasm, derived as a result of cell-to-cell fusion.

Hybrid cell: The term used to describe the mononucleate cell that results from the fusion of two different cells, leading to the formation of a synkaryon.

Hybridoma: The cell that results from the fusion of an antibody-producing tumour cell (myeloma) and an antigenically-stimulated normal plasma cell. Such cells are constructed because they produce a single antibody directed against the antigen epitope that stimulated the plasma cell. This antibody is referred to as a monoclonal antibody.

Immortalization: The attainment by a finite cell culture, whether by perturbation or intrinsically, of the attributes of a continuous cell line. An immortalized cell is not necessarily one that is neoplastically or malignantly transformed.

Immortal cell culture: See 'Continuous cell culture'.

Induction: Initiation of a structure, organ or process in vitro.

In vitro neoplastic transformation: The acquisition, by cultured cells, of the property to form neoplasms, benign or malignant, when inoculated into animals. Many transformed cell populations that arise in vitro intrinsically or through deliberate manipulation by the investigator produce only benign tumours that show no local invasion or metastasis following animal inoculation. If there is supporting evidence, the term in vitro malignant neoplastic transformation or in vitro malignant transformation can be used to indicate that an injected cell line does, indeed, invade or metastasize.

In vitro senescence: In vertebrate cell cultures, the property attributable to finite cell cultures; namely, their inability to grow beyond a finite number of population doublings. Neither invertebrate nor plant cell cultures exhibit this property.

In vitro transformation: A heritable change, occurring in cells in culture, either intrinsically or from treatment with chemical carcinogens, oncogenic viruses, irradiation, transfection with oncogenes, etc., and leading to the acquisition of altered morphological, antigenic, neoplastic, proliferative or other properties. This expression is distinguished from the term in vitro neoplastic transformation in that the alterations occurring in the cell population may not always include the ability of the cells to produce tumours in appropriate hosts. The type of transformation should always be specified in any description.

Karyoplast: A cell nucleus, obtained from the cell by enucleation, surrounded by a narrow rim of cytoplasm and a plasma membrane.

Liposome: A closed lipid vesicle surrounding an aqueous interior; it may be used to encapsulate exogenous materials for their ultimate delivery into cells by fusion with the cell.

Microcell: A cell fragment, containing one to a few chromosomes, formed by the enucleation or disruption of a micronucleated cell.

Micronucleated cell: A cell that has been mitotically arrested and in which small groups of chromosomes function as foci for the reassembly of the nuclear membrane, thus forming micronuclei, the maximum of which would be equal to the total number of chromosomes.

Morphogenesis: (a) The evolution of a structure from an undifferentiated to a differentiated state, (b) The process of growth and development of differentiated structures.

Mutant: A phenotypic variant resulting from a changed or new gene.

Organ culture: The maintenance or growth of organ primordia or the whole or parts of an organ in vitro in a way that may allow differentiation and preservation of the architecture and/or function.

Organogenesis: In animal cell cultures, the evolution, from dissociated cells, of a structure that shows natural organ form or function or both. In plant tissue culture, a process of differentiation by which plant organs are formed de novo or from pre-existing structures. In developmental biology, this term refers to differentiation of an organ system from stem or precursor cells.

Organotypic: Resembling an organ in vivo in three-dimensional form or function or both. For example, a rudimentary organ in culture may differentiate in an organotypic manner, or a population of dispersed cells may become rearranged into an organotypic structure and may also function in an organotypic manner. This term is not meant to be used along with the word 'culture' but is meant to be used as a descriptive term.

Paracrine: In animals, a cell that produces hormones, growth factors or other signalling substances for which the target cells, expressing the corresponding receptors, are located in its vicinity or in a group adjacent to it. See also 'Autocrine' and 'Endocrine'.

Passage: The transfer or transplantation of cells, with or without dilution, from one culture vessel to another. It is understood that any time cells are transferred from one vessel to another, a certain portion of the cells may be lost and, therefore, dilution of cells, whether deliberate or not, may occur. This term is synonymous with the term subculture.

Passage numbers The number of times the cells in the culture have been sub-cultured or passaged. In descriptions of this process, the ratio or dilution of the cells should be stated so that the relative cultural age can be ascertained.

Pathogen free: Free from specific organisms based on specific tests for the designated organisms.

Plating efficiency: This is a term that originally encompassed the terms Attachment (Seeding) efficiency, Cloning efficiency and Colony forming efficiency but is now better described by using one or more of them in its place because the term plating is not sufficiently descriptive of what is taking place. See also 'Attachment', 'Seeding', 'Cloning' and 'Colony forming efficiency'.

Population density: The number of cells per unit area or volume of a culture vessel; also, the number of cells per unit volume of medium in a suspension culture.

Population doubling level: The total number of population doublings of a cell line or strain since its initiation in vitro. A formula to use for the calculation of 'population doublings' in a single passage is:

Number of population doublings =-

where N = number of cells in the growth vessel at the end of a period of growth and N0 = number of cells plates in the growth vessel. It is best to use the number of viable cells or number of attached cells for this determination. Population doubling level is synonymous with the term cumulative population doublings.

Population doubling time: The interval calculated during the logarithmic phase of growth in which, for example, 1.0 x 106 cells increase to 2.0 x 106 cells. This term is not synonymous with the term cell generation time.

Primary cultures A culture started from cells, tissues or organs taken directly from organisms. A primary culture may be regarded as such until it is successfully subcultured for the first time. It then becomes a cell line.

Pseudodiploid: This describes the condition where the number of chromosomes in a cell is diploid but, as a result of chromosomal rearrangements, the karyotype is abnormal and linkage relationships may be disrupted.

Recon: The viable cell reconstructed by the fusion of a karyoplast with a cyto-plast.

Saturation density: The maximum cell number attainable, under specified culture conditions, in a culture vessel. This term is usually expressed as the number of cells per square centimetre in a monolayer culture or the number of cells per cubic centimetre in a suspension culture.

Seeding efficiency: See 'Attachment efficiency'.

Senescence: See '/« vitro senescence'.

Somatic cell genetics: The study of genetic phenomena of somatic cells • The cells under study are most often cells grown in culture.

Somatic cell hybrid: The cell resulting from the fusion of animal cells derived from somatic cells that differ genetically.

Somatic cell hybridization; The in vitro fusion of animal cells derived from somatic cells that differ genetically.

Subcultures See 'Passage'. With plant cultures, this is the process by which the tissue or explant is first subdivided and then transferred into fresh culture medium.

Substrain; A substrain can be derived from a strain by isolating a sing lc cell or groups of cells having properties or markers not shared by all cells of the parent strain.

Suspension culture: A type of culture in which cells, or aggregates of cells, multiply while suspended in liquid medium.

Synkaryon: A hybrid cell that results from the fusion of the nuclei it carries.

Tissue culture; The maintenance or growth of tissues, in vitro, in a way that may allow differentiation and preservation of their architecture and/or function.

Totipotency: A cell characteristic in which the potential for forming all the cell types in the adult organism is retained.

Transfection: The transfer, for the purposes of genomic integration, of naked, foreign DNA into cells in culture. The traditional microbiological usage of this term implied that the DNA being transferred was derived from a virus. The definition as stated here is that which is in use to describe the general transfer of DNA irrespective of its source. See also 'Transformation'.

Transformation: In plant cell culture, the introduction and stable genomic integration of foreign DNA into a plant cell by any means, resulting in a genetic modification. This definition is the traditional microbiological definition. For animal cell culture, see 7« vitro transformation', 'In vitro neoplastic transformation' and 'Transfection'.

Variant; A culture exhibiting a stable phenotypic change, whether genetic or epigenetic in origin.

Virus-free; Free from specified viruses based on tests designed to detect the presence of the organisms in question.

REFERENCES

Committee for a Standardized Karyotype of Rattus norvégiens (1973) Standard karyotype of the Norway rat, Rattus norvégiens. Cytogenetics and Cell Genetics 12:199-205.

Committee on Standardized Genetic Nomenclature for Mice (1972) Standard karyotype of the mouse, Mus musculus. Journal of Heredity 63: 69-72.

Iglewski B (1989) Communicating with the public: a new scientific imperative. ASM News 55(6): 306.

Mueller S, Renfroe M, Schaeffer WI, Shay JW, Vaughn J & Wright M (Tissue Culture Association Terminology Committee

Members) (1990) Terminology associated with cell, tissue and organ culture, molecular biology and molecular genetics. In Vitro Cellular and Developmental Biology 26: 97-101.

Paris Conference (1971), Supplement (1975) Standardization in Human Cytogenetics. Birth Defects: Original Article Series, XI, 9, 1975. The National Foundation, New York. Reprinted in: Cytogenetics and Cell Genetics (1975) 15: 201-238.

Schaeffer WI (1984) In the interest of clear communication. In Vitro Cellular and Developmental Biology 25: 389-390.

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